Welcome to the LHCb experiment
LHCb delivered and recorded luminosity in 2012, +1.1/fb indicates recorded luminosity in 2010-2011. The number of proton-proton (pp) collisions visible at LHCb, as well as the numbers of cc and bb quark pair produced within LHCb acceptance in 2010-2012 are also shown.
click the image to see the operational plot
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First 13 TeV collisions at #LHCb at #LHC at #CERN, see http://t.co/NkTgNiNaRx Thu 21 May
RT @CERN: Follow @LHCbExperiment physicist Patrick Koppenburg using
beauty quarks to understand antimatter at #13TeV: https://t.co/8QD5EFJD…
Tue 19 May
Observing nearly invisible at #LHCb at #LHC at #CERN, see http://t.co/NkTgNiNaRx Wed 13 May
First 2015 proton-proton collisions at #LHCb at #LHC at #CERN, see http://t.co/NkTgNiNaRx Tue 05 May
Quantum Diaries blogs about LHCb
see film "LHCb - the beauty experiment" at YouTube shortlisted by the NHK Japan Prize Festival 2011 and by the European Science TV and New Media Festival 2012,
for selected physics results click on: φs, B0s →μμ, B0d→K*μμ, ΔACP, ad,ssl, K0S →μμ, B→hhh,
γ angle, B and D oscillations, B0s oscillations, X(3872), BsCP, pA, Λb, Bc, Z(4430), Luminosity, Ξb'-Ξb*-, RK
21 May 2015: First 13 TeV collisions.
Today, proton beams collided at 13 TeV for the first time inside
the LHCb detector. The collisions were part of the ongoing accelerator
tests, and are not useful for physics studies. The collected data are,
however, useful for refining the synchronization of the readout time of
different parts of the calorimeters and muon detectors. The image
displays an event taken today.These collisions were the latest in a sequence of careful steps to prepare the LHC for physics operation at 13 TeV. Proton beams first circulated again after a two year long shutdown on the 5th April, and collided at the relatively low energy of 450 GeV on the 5th May. First collisions for physics studies, in so-called “Stable Beams” conditions, are expected in early June.
Click the image for higher resolution. Read more in the CERN news.
13 May 2015: Observing nearly invisible.
Observation of the rare Bs0→μμ decay from the combined analysis of CMS and LHCb data
[ Branching fraction Bs0→μμ = (2.8+0.7-0.6)x10-9 ; B0→μμ = (3.9+1.6-1.4)x10-10 ]
The CMS and LHCb Collaborations have published today in Nature the first observation of the decay Bs0→μμ, with a statistical significance exceeding six standard deviations, and the best measurement of its branching fraction to date. Furthermore, a three standard deviation evidence for the decay B0→μμ is also obtained. Both measurements are statistically compatible with Standard Model (SM) predictions and allow stringent constraints to be placed on theories beyond the SM. This is one of the most important results obtained by the LHC experiments from the run 1 and tests the SM to the ninth decimal place. The excellent performance of the CMS and LHCb detectors and their data analyses was crucial in obtaining this result as well as the outstanding performance of the LHC itself. The two experiments found a total of about 100 Bs0 and B0 decays into two muons in a sample comprising 1012 beauty hadrons collected during 2011 and 2012.The SM of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic, and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The SM predicts that the Bs0→μμ and B0→μμ decays are very rare, with about four of these decays occurring for every billion Bs0 mesons produced and one decay for every 10 billion B0 mesons. Prior to the start of operation of the LHC, no evidence for either decay mode had been found, despite around 30 years of searching at previous experiments. Upper limits on the branching fractions were an order of magnitude above the SM predictions.
The probabilities, or branching fractions, of the Bs0 and B0 mesons to decay into two oppositely charged muons are very small in the SM and are well predicted. On the other hand a large class of theories that extend the SM, like supersymmetry, allows significant modifications to these branching fractions and therefore an observation of any significant deviation from the SM prediction would indicate a discovery of new effects. The Bs0 and B0 meson decays into a muon pair have long been regarded among the most promising class of measurements where these new effects could show up. Previous LHCb results already severely constrained the type of SM-extension models that are still allowed, as described in the 30 March 2012 news. The results announced in today's publication isolate even more precisely the parameter region in which these new models can exist, and therefore focuses future experimental searches and theoretical attention. All candidate models of physics beyond the Standard Model will have to demonstrate their compatibility with this important result.
The two collaborations first released their individual results for Bs0 meson decay as described in 24 July 2013 news. While the results were in excellent agreement, both fell just below the five sigma statistical precision historically needed to claim an observation. The combined analysis improves the precision of the results and in the same time easily exceeds the 5 sigma requirement, reaching 6.2 sigma. This is the first time that a combined analysis of sets of data from more than one LHC experiment has been performed.
The Bs0 and Bs0 mesons
are produced in the pp collision point and decay into a muon pair after a
distance of the order of 1cm. The left image shows the Bs0 meson decay into two muons presented as green tracks traversing the whole detector. A few other Bs0→μμ decay images can be found at this web page (1, 2, 3). The right image shows one of the μ+μ-
invariant mass spectra released today, in which an excess, attributed
to the signal decays, can be clearly seen above the expected background
shape.The LHCb and CMS experiments will resume data taking in June with proton-proton collisions at a centre-of-mass energy of 13 TeV, which will approximately double the production rates for Bs0 and B0 mesons and lead to further improvements in the precision of these crucial measurements.
Read more in the Nature publication, in the CERN Press Release, in the Nature news and in the CERN news in English and French.
5 May 2015: First 2015 proton-proton collisions.
Today, for the first time in over two years, protons have collided inside the LHCb detector. Each proton beam had an energy of 450 GeV, which is the value they have when injected from the SPS to the LHC.Although these collisions are not at the nominal energy of 13 TeV, and therefore not aimed for physics studies, the collected data are useful for precise synchronization of the readout time of different parts of the calorimeters and muon detectors with the time at which different particles originating from the proton-proton collision point traverse them.
The left image shows LHCb physicists in the LHCb control room. The
right image displays an event taken today. Click the picture and play
with the 3D view of a few recorded events.In a follow up step, most probably tomorrow, LHCb physicists will inject neon gas into the LHC vacuum tube in order to measure the shape of the proton beam, by seeing where the proton-gas interactions occur. This beam-gas imaging method, used only at LHCb, allows the "luminosity" of the colliding beams to be determined, and is described in the news item of 7 October 2014. The luminosity is a vital component in determining how often different physical processes occur in proton-proton collisions. Measurements of these processes at the record proton-proton collision energy of 13 TeV are the among the first physics goals of LHCb at the restart of data taking in June.
5 April 2015: LHC proton beams are circulating again.
Today, for the first time in two years, both proton beams are circulating again in the LHC. The picture shows the LHC operators steering the two beams. The proton beam already traversed the LHCb detector one month ago and then continued through one quarter of the LHC circumference, see 7 March 2015 news.
The two month period of re-commissioning with beam starts now. The
LHC magnets are already “trained” to accept the high current needed to
produce the magnetic field necessary to hold the protons in their orbit
with a record energy of 6.5 TeV inside the LHC ring. Therefore the
prospects for reaching the proton-proton collision energy of 13 TeV at
run 2 are very good.The LHCb detector and its data acquisition system are ready to take data at this highest proton-proton collision energy. LHCb physicists still continue to analyse the run 1 data obtaining exciting results, many of which have been reported on this website. They are looking forward to the first 13 TeV collisions that are expected in June and are convinced that a bright and exciting future lies ahead.
Read more in the CERN Press Release in English and French and see the video from the LHC restart.
20 March 2015: B0→K*μ+μ-: new analysis confirms old puzzle.
Angular analysis of the B0→K*μ+μ- decay.
Today at the 50th Rencontres de Moriond Electroweak, in La Thuile (Italy), the LHCb collaboration presented the result of a first full angular analysis of the B0→K*μμ decay using its full LHC run 1 data sample. A previously published analysis of the experiments 2011 data sample found a deviation with respect to a calculation based on the Standard Model, see 9 August 2013 news. The particle physics community has been eagerly awaiting the results from the full data sample ever since.The analysis of the B0→K*μμ decay is considered as a very promising way to search for effects of yet undiscovered particles, see the 14 June 2013 news for an introduction. Unfortunately, the analysis of this decay is also complicated; the best sensitivity to the new particles comes from the study of the angular distribution of the muons and the kaon and pion from the K* meson decay. Physicists from the LHCb experiment have studied different angular observables as functions of the mass of the muon pair. It was one of these observables, "P5'", that showed a local deviation with respect to the Standard Model calculation (at a level of 3.7σ in the mass squared of the muon pair region q2 from 4.3 to 8.68 Gev2/c4). There is particular interest in these observables because their theoretical prediction is much less dependent on a good understanding of the hadronic physics involved in turning a B meson into a K* meson (so called form-factors). These observables are therefore ideal for searching for the effects of new particles in this decay.
The image shows the distribution of the P5’ observable as functions of the mass squared of the muon pair q2.
The black points show the LHCb results presented for the first time
today. The Standard Model predictions are presented as orange boxes.
These were taken from calculations described in a recent theoretical paper.
The behaviour seen in the 2011 data sample, shown as the blue points
for comparison, is confirmed using the full data set. The measurements
in the q2 region between 4 and 8 GeV2/c4 are both 2.9σ from the Standard Model calculation.The results of the analysis of the B0→K*μμ decay will continue to attract the attention of the particle physics community in the coming years. In the next couple of years, the LHCb collaboration will improve the precision of their analysis with the help of data collected in run 2 of the LHC. It is also anticipated that the theoretical predictions (the orange regions in the image) will improve in precision. Theorists will be busy trying to make sense of this measurement, and seeking for possible associations with other unexpected effects found in similar decays, for example the RK anomaly (see 3 June 2014 news). Stay tuned for future developments on this page.
7 March 2015: The proton beam has traversed the LHCb detector.
The LHCb collaboration is ready to take 13 TeV proton-proton collision data.
This weekend the LHC proton beam has traversed the LHCb detector for the first time in over two years. At the end of November 2014 the proton beam already arrived at a stopper placed in the accelerator known as the “TED”, located at the end of the SPS-LHC transfer line about 300m from the LHCb detector, see 23 November 2014 news.
This weekend the TED exercise was repeated and the LHCb
Collaboration recorded again muons produced by protons absorbed in the
TED. The TED was then opened and the proton beam entered the LHC ring
and was first absorbed in another stopper called the “TDI” located
inside the LHC ring about 50m from the LHCb detector. A very large
number of particles produced during this absorption process traversed
the LHCb detector. Later on the TDI was opened, the proton beam
traversed the LHCb detector inside its beam pipe, then travelled through
one quarter of the LHC circumference and finally arrived to the LHC
beam dump area. The left image shows places, called “hits” by
physicists, at which muons from the proton beam interaction at the TED
traversed different LHCb sub-detectors. Click the picture and play with
the 3D view of these events.
Both proton beams are expected to make the full turn of the LHC
collider by the end of March and the first proton-proton collisions at
the nominal Run 2 energy of 13 TeV are expected by the end of May.The LHCb collaboration is ready to take high energy proton-proton collision data. The two year Long Shutdown (LS1) period offered an opportunity for prolonged access, and hence an extensive programme of consolidation and maintenance work. In summer 2014 a detailed field measurement of the LHCb dipole magnet was performed followed by the re-installation of the beam pipe, see the left image, through which proton beams will circulate in both directions. One section of the beryllium beam pipe was replaced. The new beam pipe support structure is now much lighter and therefore unwanted interactions with it of particles measured inside the LHCb detector are strongly reduced.
3 March 2015: Matter-antimatter trigonometry with LHCb.
Precise measurement of the unitarity triangle angle β.
Today at Les Rencontres de Physique de la Vallée d'Aoste, La Thuile, Italy, the LHCb collaboration presented an important result in our quest to understand the nature and origin of CP violation, which is a difference in behaviour between matter and antimatter. The result, derived from a careful analysis of the full run 1 data sample, is a measurement of the angle β of the ‘unitarity triangle’. This triangle is a geometrical representation of CP violating and associated parameters in the Standard Model. One side is defined to have unit length, the other two sides and three angles can be measured independently in different decays of beauty hadrons. It is the task of experimental physicists to measure these properties and see if they provide a consistent description of the triangle. Any discrepancy would point to signs of New Physics beyond the Standard Model. LHCb has already performed the world’s most precise measurements of γ, one of the other triangle angles, see 11 September 2014 news, and the mixing frequency of Bs mesons, see 7 November 2012 news, which is an essential ingredient for the determination of the side opposite to the angle γ.
The unitarity triangle is shown in the above left figure, with each
experimental input represented by a coloured region. The result for β
is given by the hatched blue diagonal band, marked as sin2βLHCb. The measurement is made from studying the decays of about 41500 B0 and B0 mesons to J/ψ and Ks0 mesons. The B0 mesons can decay to J/ψKs0 in two ways. They can decay directly to J/ψKs0 or they can oscillate, see 7 November 2012 news, into their antimatter partners B0 which in turn decay also to J/ψKs0.
(This possibility of two paths in the decay process is a B physics
analogue of the classical quantum mechanics two-slit experiment, which
is described in this pedagogical video).
The interference between the amplitudes for the two decay paths results
in a time-dependent asymmetry between the decay time distributions of B0 and B0
mesons, as seen in the right image above. The oscillation amplitude
measures sin2β, and gives the magnitude of CP violation present in the
process. A value of sin2β=0 would indicate no CP violation. LHCb
physicists announced today the value of sin2β=0.731±0.035±0.020, which
is consistent with the geometrical expectation provided by the
measurements of the other parameters of the unitarity triangle, and
hence with the predictions of the Standard Model itself.This result also confirms earlier measurements of the same quantity performed by the e+e- collider B factory experiments BaBar and Belle. These experiments were constructed specifically to measure sin2β. Their results were vital in confirming the broad validity of the Standard Model description of CP violation and led to the award of the 2008 Nobel Prize of Physics to the Japanese theorists M.Kobayashi and T.Maskawa, who had been central in developing this description. The solid blue diagonal band shows the new world combination of the B factory measurements with the new LHCb result.
Although the LHCb result for the angle β is not yet as precise as the combined result coming from the average of the B-factory measurements, it is similar in precision to the J/ψKs0 analyses of the individual experiments. It has been long-known that this measurement is a priori more difficult to perform in a hadron collider, such as the LHC, but the new result presented at La Thuile is an emphatic statement that LHCb can significantly contribute to our knowledge of this fundamental parameter. A still more precise result will be achievable with the data to be collected in run 2, which will allow for more stringent tests of the Standard Model.
Read more in the LHCb presentations at La Thuile, in the LHCb publication and in the CERN Courier article.
23 November 2014: The proton beam knocks at the LHC door.
The LHCb collaboration took proton interaction data this weekend.
The proton beam knocked at the LHC's very solid door this weekend and found it still closed, but nonetheless managed to provide the LHCb collaboration with very interesting data. The CERN accelerator system (see video) is now fully operational, except for the LHC collider itself. This past weekend, CERN accelerator system operators tested the two transfer lines between the SPS and LHC. One of these lines ends with a so-called beam stopper known as the "TED", located at the end of the line about 300m from the LHCb detector. The TED is currently closed, and so absorbed the proton beam before it could enter the LHC. However many muons were produced during the absorption process, and these muons passed through the TED and traversed the LHCb detector.
This “beam dump” experiment therefore created an excellent
opportunity for LHCb physicists and engineers to commission the LHCb
detector and data acquisition system. The collected data are also useful
for detector studies and alignment purposes (i.e. determining the
relative geometrical locations of the different sub-detectors with
respect to each other).The image shows the shift leader, run coordinator, spokesperson and sub-detector experts in front of the LHCb data acquisition computer screens.
LHCb took its last collision data on 14th February 2013. The two year Long Shutdown 1 (LS1) period that followed has been used for an extensive program of consolidation and maintenance (see 24 January 2014 “underground” news). Collisions are expected to resume again in Spring 2015.
Click the images for higher resolution and read about the LHC side of the story here.
20 November 2014: LHCb at the Open Data portal.
The LHC experiments have today released physics data at the OpenData.cern.ch portal.The LHCb collaboration has contributed with its International Masterclasses exercise, in which the lifetime of the charm particle called the D0 meson may be measured using real proton-proton collision data recorded by the LHCb experiment during the 2011 data taking period. Details of the LHCb exercise can be found at the LHCb masterclasses web pages.
Read more in the CERN press release in English and French.
19 November 2014: First observation of two new baryonic strange beauty particles.
The LHCb collaboration submitted today a paper reporting the discovery of two new particles. The particles, known as the Ξb'- and Ξb*-, were predicted to exist by the quark model but had never been seen before.Like the protons that the LHC accelerates, the new particles are baryons made from three quarks bound together by the strong force. The types of quarks are different, however: the new Ξb particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton. But the particles are more than just the sum of their parts: their mass also depends on how they are configured. Each of the quarks has an attribute called "spin". In the Ξb'- state, the spins of the two lighter quarks point in the opposite direction to the b quark, whereas in the Ξb*- state they are aligned. This difference makes the Ξb*- a little heavier.
The two new particles are observed through their decay into the ground state Ξb0 and a π-. The image shows the distribution of δm, which is defined as the invariant mass of of the Ξb0π- pair minus the sum of the π- mass and the measured Ξb0 mass. This definition means that the lightest possible mass for the Ξb0π- pair, known as the threshold, is at δm=0. The two peaks are clear observation of the Ξb'- (left) and Ξb*-
(right) baryons above the hatched red histogram representing the
expected background. Both particles have extremely short lifetimes and
do not fly any measurable distance before they decay. But we can tell
that the Ξb*- is the more unstable of the two,
since the peak is wider and the basic rules of quantum mechanics relate
the average lifetime of a particle to the width of its mass peak (after taking into account experimental resolution). This is consistent with the pattern of masses: the Ξb'- mass is just slightly heavier than the threshold, so it is allowed to decay into a Ξb0 and a π- but only barely. The insert shows a zoom in the Ξb'- δm region.Read more in the CERN press release in English and French, in the Research Highlights of Nature, in the Scientic American and in the Physics World news.
15 October 2014: φs: CP violation in the Bs system — looking for a chink in the armour of the Standard Model.
Final run 1 result: φs = - 0.010 ± 0.039 rad
Today, at the workshop “Implications of LHCb measurements and future prospects” at CERN, LHCb physicists have presented their final run 1 results of their analysis of the phase φs (the Bs CP-violating phase, for experts), see 27 August 2012, 5 March 2012 and 3 March 2013 news for introduction. The measurement of φs is one of the most important goals of LHCb experiment.The value of φs is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for Bs mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of New Physics could change its value significantly.
In the CP violation that drives φs the role of Bs oscillations (3 March 2013 news) is very important. Here the Standard Model predicts very small effects, thereby allowing New Physics to manifest itself. This is to be contrasted with other manifestations of CP violation in the Bs system, unaffected by oscillations, where the Standard Model expects large signatures. Such a signature was already observed by LHCb, see 24 April 2013 news.
The 96 000 B0s → J/ψ K+K- decays collected during 2011 and 2012 data taking periods were used in the analysis. The left image shows the J/ψ K+K- invariant mass spectrum. The very clean enhancement at the B0s mass is clearly seen.
The right image shows that the K+K- invariant
mass spectrum is dominated by the presence of the φ meson resonance at a
mass of 1020 MeV. The φ mesons, and similarly the J/ψ and ϒ mesons, are
sometimes called heavy photons since they have the same quantum numbers
as the photon. In their analysis LHCb physicists took into account
different polarization states of the φ meson in analogy to different photon polarization states. For the first time, the phase of φs is measured independently for each polarization state of the K+K- system. No significant difference is observed between the different polarisation states.The result φs = - 0.058 ± 0.049 ± 0.006 rad is the most precise measurement of φs to date. Recently the LHCb Collaboration has reported the final run 1 measurement of the φs using B0s → J/ψ π+π- decays, φππs = + 0.070 ± 0.068 ± 0.008 rad, consistent with the φs measurement reported above. The results from the two analyses have been combined giving the final value of φs = - 0.010 ± 0.039 rad. This is the most precise value of φs and is consistent with the Standard Model prediction of φs = - 0.0363 ± 0.0013 rad. The parameter region in which New Physics could still hide is now even smaller than before.
The LHCb today's result has been included in the HFAG world combination. The φs results of different experiments are shown in the image together with the ΔΓs variable (see 5 March 2012
news for introduction). The green ellipse in the center represents the
LHCb result and the grey ellipse shows the world average. The result of
the first φs measurement in the Bs → Ds+Ds- decays φs = + 0.02 ± 0.17 ± 0.02 rad is also included in the LHCb combination shown in the image.Read more in the LHCb workshop presentation and in the LHCb publication.
click the images for higher resolution
10 October 2014: Celebrating 200th publication.
The LHCb Collaboration has celebrated this week its 200th publication! The paper “Test of lepton universality using B+ → K+l+l- decays“ appeared in Physical Review Letters on October 6th. The American Physical Society, publisher of the Journal, has selected this article to be presented in “spotlighting exceptional research” news.The results published today were already presented at the Large Hadron Collider Physics (LHCP) Conference in New York, see 3 June 2014 news and the Symmetry article.
7 October 2014: How bright is the LHC?
LHCb has the most precise answer.
The LHCb Collaboration has just published the results of the luminosity calibration with a precision of 1.12%. This is the most precise luminosity measurement achieved so far at a bunched-beam hadron collider (see below).At the beginning of operation of a new particle collider, physicists start with measurements of probabilities of different physical processes and compare results with theoretical predictions. Differences may indicate signs for new physics or needs for an improved understanding at higher energies of already known processes. The probabilities are measured as the so called “cross-sections”, σ, in units of area. The number of events N measured in an experiment depends on the value of cross-section σ (given by the nature of the physical interaction) and on a collider parameter luminosity L, N=σL . The luminosity L depends on the number of particles in each collider beam per unit of time and on the size of overlap of both beams at the collision point.
The formula N=σL indicates that the luminosity can be obtained from a measurement of a process for which the cross-section can be calculated with high precision. In this way a precision of few x 10-4 was achieved at the e+ e- collider LEP from the measurement of e+ e- small angle scattering process whose rate was calculated precisely using QED theory. Cross-section predictions with similar or better precision are not available for proton-proton collisions. Therefore, purely experimental methods of luminosity measurements are used at the LHC. The number of particles in each colliding beam per unit of time (so called beam currents) are measured using LHC equipment. Two methods are used to measure the size of overlap of both beams at the collision point.
(1) The van der Meer scans (VDM) method, invented in 1968 by Simon Van der Meer, 1984 Nobel Prize winner, to measure luminosity in the Intersecting Storage Rings (ISR) at CERN, the world's first hadron collider. The idea was to measure the beams' overlap by scanning them across each other and monitoring the interaction rate. This method has been used by all four LHC experiments.
(2) The Beam-Gas Imaging (BGI) method was proposed by LHCb physicist Massimiliano Ferro-Luzzi in 2005. It takes advantage of the excellent precision of the LHCb Vertex Locator (VELO) placed around the proton-proton collision point. The BGI method is based on reconstructing vertices of interactions between beam particles and residual gas nuclei in the beam vacuum in order to measure the angles, positions and shapes of the individual beams without displacing them. This allows one to actually see the trace of the beams. To this date only the LHCb Collaboration is capable of using the BGI method.
The image shows the reconstructed positions of proton-beam-gas
collision points. It is clearly seen that the proton beams are crossing
at an angle. This is intentional since the protons in LHC are not
continuously distributed around the ring but are concentrated in 1380
small regions called bunches (so the LHC is a bunched-beam collider).
Colliding with an angle and not head-on assures that there are no
unwanted collisions between bunches outside the region defined by the
experiment. The luminosity was calibrated during special measurement
periods and then relative changes were tracked by changes of counting
rate of different sub-detectors. The vacuum pressure in the LHC was so
low, that in order to increase the proton-beam-gas collision rate, LHCb
physicists injected neon gas into the LHC vacuum tube during luminosity
calibration periods.Using the beam-gas data, LHCb physicists were able to reveal that a small fraction of the beam charge is spread outside the expected (i.e. "nominal") bunch locations. Since only collisions of protons located in the nominal bunches are included in physics measurements, it was very important to measure which fraction of the total beam current value measured with the LHC equipment participated in the collisions (i.e. contributed to the luminosity). Only LHCb could measure this fraction with sufficient precision and, therefore, the results of LHCb measurements of the charge fraction outside nominal bunch locations, called the "ghost" charge, were also used by the three other LHC experiments.
The LHCb BGI method was so successful that LHC engineers decided to use it also for the beam size measurements necessary for monitoring LHC operation. Dedicated equipment is now being built and will soon be installed in a specially modified region of the LHC ring. The equipment includes also a gas injection system.
The LHCb Collaboration is very well armed to measure precisely cross-sections of different processes at 13 TeV proton-proton collisions after the LHC restart in spring 2015.
Read more in the LHCb publication here and in the CERN Courier article.<>
29 September 2014: LHChamber Music.
Today, CERN, the European Organization for Nuclear Research, is blowing out 60 candles at an event attended by official delegations from 35 countries. Founded in 1954, CERN is today the largest particle physics laboratory in the world and a prime example of international collaboration, bringing together scientists of almost 100 nationalities.To mark the occasion, music-minded physicists have transformed scientific data from the four underground detectors around CERN's Large Hadron Collider into a piece titled LHChamber Music composed by physicist-musician Domenico Vicinanza.
The video, presented during the official ceremony, can be viewed by clicking the arrow above.The left image shows LHCb physicist Paula Collins, in front of the detector, playing music inspired by the data from the first observation of a heavy flavored spin-3 particle (15 July 2014 news).
The LHCb Collaboration has already presented another way of experimental data "sonification" in the 26 August 2013 news "Matter-antimatter quantum music".
11 September 2014: Combination of measurements of the CKM angle γ.
Today at the International Workshop on the CKM Unitarity Triangle, CKM2014, Vienna, Austria, the LHCb collaboration has presented a combination of measurements of the CKM angle γ (in tree decays, see below). For the first time a single experiment has achieved a precision of better than 10 degrees, which is better than the combination of the results of the B-factory experiments, BaBar and Belle. The parameters that describe the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called CKM, or unitarity, triangle. The angles of this triangle are denoted α, β and γ, and of these it is γ that is the least precisely known. A detailed introduction to the CKM angle γ measurement can be found in the 5 October 2012 news and in the CERN Courier article. The measurement of the angle γ in different processes is one of the most important goals of LHCb experiment. The idea is to measure precisely the angle γ in processes in which a contribution from new physics is possible and in processes in which it is not. Comparison between the results of these two categories of measurement is therefore a powerful method to probe for the effects of new physics.
The value of the angle γ = (72.9+9.2-9.9)° presented today was obtained using B(s)→D(s)K(*) decays in the analysis, in which B or Bs meson decays into D or Ds mesons were observed in the full 3 fb-1 2011 and 2012 data set. The image shows a confidence level
(CL) curve that indicates which values of γ describe best the LHCb
data. The 68.3% horizontal line shows how the γ angle uncertainty was
determined. Signs of new physics are not expected to show up in these
decays (the so called tree-level measurements, for experts) and
therefore they will set a base for comparison with the measurements
where observation of new physics effects is possible. The LHCb result will be improved still further before the start of new data taking period in spring 2015 using the already available data, since there are still important analyses to be completed. In addition LHCb has a large set of γ-sensitive observables in B→Dπ decays not discussed in this news. However their sensitivity to γ is suppressed compared to the B→DK-like decays.
9 September 2014: Measurement of the semileptonic asymmetry, adsl.
Today at the International Workshop on the CKM Unitarity Triangle, CKM2014, Vienna, Austria, the LHCb collaboration has presented the results of a measurement of the semileptonic asymmetry, adsl, related to a difference between a probability of a beauty meson, B0, to oscillate into its antimatter partner, B0, and a probability of the reverse process (an introduction to beauty and charm oscillations can be found in the 7 November 2012 news item). Any difference in this probability would be a manifestation of what is called CP-violation. The label "d" indicates decays of B0 mesons composed of anti-beauty b and d quarks while "sl" (semileptonic) indicates that leptons, in this case muons, are present among decay products. When a B0 meson decays semileptonically, the charge of the lepton determines whether it was a decay of a matter B0 or anti-matter B0 meson. On the other hand a presence of a “wrong-sign” lepton in a decay of a B0 meson indicates that a transition to a B0 meson took place before decay. Therefore these "wrong-sign" B0 and B0 decays were used in the analysis. The measurement of the adsl is very interesting since its value is predicted to be very small by the Standard Model and therefore any significant deviation from zero could indicate a (so called virtual) contribution of not yet discovered particles in B0 - B0 oscillations.LHCb physicists presented today a new preliminary value of adsl = (-0.02 ± 0.19 ± 0.30)% using the full 3 fb-1 2011 and 2012 data sample. The LHCb Collaboration has published recently a corresponding value for the strange beauty mesons B0s of assl = (-0.06 ± 0.50 ± 0.36)% using 1 fb-1 of data taken in 2011. Both results indicate no CP-violation to be present within the sensitivity of the measurements and hence are consistent with the very small values predicted by the Standard Model.
The LHCb results are shown in the image together with the Standard Model prediction.The yellow ellipse shows a result from a measurement of the D0 experiment at the Tevatron which is sensitive to both adsl and assl. It lies a significant distance away from the prediction of the Standard Model and hence has excited interest as pointing to a break down in the theory, see 7 July 2012 news for introduction.
The LHCb results are consistent with the Standard Model but do not exclude the D0 result. LHCb will soon update the measurement of assl, which may clarify the situation.
Read more in the LHCb presentation in Vienna and soon in the forthcoming LHCb publication.
click the image for higher resolution
15 July 2014: First observation of a heavy flavored spin-3 particle
Today at the 15th International Conference on B-Physics at Frontier Machines at the University of Edinburgh, Beauty 2014, the LHCb collaboration has presented the results of a study of strange beauty meson B0s decay into an anti-charm meson D0, a K- meson and a π+ meson (B0s → D0K-π+). Previous results indicated the existence of a strange-charm D*sJ(2860)- particle in the D0K- invariant mass spectrum, and the study of the B0s → D0K-π+ decay allows one to study this structure and measure its properties. Today’s LHCb observation shows with 10σ significance that, in fact, this excess seen in the D0K- mass spectrum is composed of two particles with different spins, spin-1 and spin-3. This is the first observation of a heavy flavored spin-3 particle, and the first time that any spin-3 particle has been seen to be produced in B decays.
The B0s → D0K-π+ decay is clearly identified as seen in the left image above. The right image above shows the enhancement at the 2.85 GeV/c2 mass in the D0K-
invariant mass spectrum divided into two components as measured by LHCb
physicists. The wider one corresponds to the spin-1 particle
contribution and the narrow one represents the spin-3 contribution. The
non-peaking distributions show contributions from other resonances that
peak far from the 2.85 Gev/c2 D0K- mass region.The left image shows to experts that the data D0K- angular distribution (black points) is very well described by the presence of both spin-1 and spin-3 particles (solid blue curve). The models with only either a spin-1 (red curve) or a spin-3 (green one) particle are not supported by data.
The D*sJ(2860)- particles are composed of an anti-charm quark c and a strange quark s. The quark-anti-quark pair is bound by strong interactions and can form different quantum states with different values of spin and angular momentum in analogy to the different quantum states of ordinary atoms. The presence of the spin-3 contribution gives a clear signature that both particles are members of the so called 1D family having two units of angular momentum between the quark and the antiquark. This discovery demonstrates that the spectroscopy of the 1D families of heavy flavoured mesons can be studied experimentally. Further insights can be expected with similar analysis of B decays at LHCb and the LHCb upgrade.
Read more in the LHCb presentation, in the CERN Courier article and soon in the LHCb publication.
5 July 2014: First observation of Z boson production in proton-lead collisions
Today at the 37th International Conference on High Energy Physics in Valencia, the LHCb collaboration has presented the first observation of Z boson production in proton-lead collisions at the LHC at a centre-of-mass energy per proton-nucleon pair of 5 TeV.
The main goal of lead-lead collision measurements at the LHC is to
study the possible formation of a quark-gluon plasma state of matter in
which quarks and gluons are freely moving instead of being bound inside
protons and neutrons. According to the Standard Model of Cosmology this
state of matter existed in the universe until 10-5 seconds
after the Big Bang. Although LHCb has not collected any data from
lead-lead collisions, it has recorded data from proton-lead collisions.
Studies performed on this data set make an important contribution to the
interpretation of the possible signals of quark-gluon plasma production
obtained in lead-lead collisions. LHCb has already shown interesting
proton-lead results of this nature, as explained in the 10 May 2013 news.click the image for higher resolution
The LHCb Z boson measurements are valuable for helping to determine the fraction of nucleon momentum carried by colliding partons (quarks and gluons) during the collisions of lead nuclei. The distribution of this fraction, usually termed "x", for partons in nucleons bound inside the atomic nucleus is different from the one for free nucleons. The Z bosons at LHCb are produced by the collisions of partons with small and large values of x allowing information to be learned about the behaviour of partons inside the nuclei both at very low and very large values of x. The capability to explore this kinematical range is an important and unique feature of LHCb experiment.
LHCb is planning to collect proton-lead data for a longer period during the new LHC run, which will allow more precise measurements of Z boson production, and other complementary studies, to be performed.
Read more in the LHCb conference presentation and in the LHCb publication.
3 July 2014: Unpredicted production mechanism of ϒ(3S) bottomium state.
Today at the 37th International Conference on High Energy Physics in Valencia, the LHCb collaboration has presented important results on the production of beauty-anti-beauty quark pair bound states called Upsilon mesons, ϒ, in proton-proton collisions at the Large Hadron Collider LHC. The LHCb results include a precise measurement of the mass of one of these states, and more excitingly reveal the striking observation that about 50% of Υ(3S) mesons observed in LHC collisions are in fact not produced directly, but they originate from the radiative decay of χb(3P) mesons.The beauty-anti-beauty bound state ϒ was discovered in 1977 at Fermilab near Chicago. Just like ordinary atoms bound by the electromagnetic force, the beauty and anti-beauty quarks, bound by the strong force, form different quantum states with different angular momenta and different spin orientations as discussed in the 6 September 2010 news. This atom-like system is called beauty quarkonium (or bottomium).
The different S quantum states (ϒ(1S), ϒ(2S), ϒ(3S)) have beauty
quark and anti-quark spins aligned and no angular momentum L giving the
total spin J=S=1. On the other hand the P states (χb(1P), χb(2P), χb(3P)) have angular momentum L=1. The χb(3P) state was recently observed by the Atlas and D0
collaborations. Today, the LHCb collaboration has presented the most
precise measurement of its mass so far to be performed, and obtained the
result 10511.3 ± 1.7 ± 2.4 MeV/c2 (χb1(3P) mass for experts). The dotted red arrow labelled with "γ" indicates a possible decay path of the χb(3P): it may decay into a photon γ and an ϒ(3S) state. Similar transitions are possible to the ϒ(2S) and ϒ(1S) states.
click the images for higher resolutionTheoretical physicists will need to take into account the new LHCb results in their calculation of bottomium states production in proton-proton collisions at the LHC. The differences in the rates of production of different ϒ states in proton-proton and lead-lead collisions at the LHC have been interpreted as possible evidence for the formation of quark-gluon plasma in lead-lead collision. This interpretation will now have to be reassessed, taking account of the new LHCb results.
Read more in the LHCb conference presentation and in the LHCb publication.
1 July 2014: Guy Wilkinson and Monica Pepe Altarelli – new management for the LHCb Collaboration
Guy Wilkinson from the University of Oxford begins today his 3-year tenure as LHCb spokesperson. He replaces Pierluigi Campana from the Istituto Nazionale di Fisica Nucleare in Frascati. Monica Pepe Altarelli from CERN will play the role of deputy spokesperson during the same period by replacing Roger Forty and Burkhard Schmidt.
click the images for higher resolutionGuy and Monica will face the huge challenges of run 2 and the following long technical shutdown during which LHCb will undergo a major upgrade. In the meantime, the discovery of new physics could be a dream within reach.
Pierluigi, Roger and Burkhard – thank you for your excellent work.
Guy and Monica – good luck.
Read more details at the LHCb Collaboration Web page and in the CERN Bulletin article in English and in French.
17 June 2014: No diploma without LHCb Vertex Locator
The baccalauréat,
often known in France colloquially as le bac, is an academic
qualification which French and international students take at the end of
the lycée (High School) (secondary education). It is the main diploma
required to pursue university studies.Today French students had to pass the examination in physics and chemistry. In the first exercise the French students were calculating collisions at the LHC. Not only was the importance of Higgs boson discovery discussed but also the Beauty meson production inside the LHCb VErtex LOcator (VELO). The VELO detector is composed of two halves placed on both sides of proton beams around the proton-proton collision point. A VELO half is shown in the image.
If you understand French you can try to pass the French baccalauréat exam here.
3 June 2014: An interesting result presented at the LHCP conference.
The LHCb Collaboration has just presented at the Large Hadron Collider Physics (LHCP) Conference in New York an interesting result: the ratio RK of the probability that a B+ meson decays to a K+μ+μ- or a K+e+e- was measured to be different from one with a 2.6σ significance. The RK is the ratio of two rare processes that occur twice in 10 million events (2x10-7) representing b→s processes highly sensitive to the presence of virtual particles that only exist in extensions to the Standard Model.In the Standard Model of particle physics this ratio is expected to be very close to one thanks to the so called lepton universality: leptons (like electrons e and muons μ here) behave in the same way (have the same couplings to the gauge bosons, for experts). The small differences in decay rates, tested so far with high accuracy, related mainly to the differences of their masses, are well understood. In the case of two decays discussed here the difference is expected to be at the per-mille level.
The ratio RK is measured in function of the μ+μ- and e+e- invariant mass squared q2. The LHCb result of RK = 0.745+0.090-0.074 ±0.036 measured in the q2 range between 1 and 6 GeV2
is shown in the image as the black point with error bars together with
the results from other experiments. It is interesting to note that the
result of BaBar Collaboration at low q2 shown by the red left point favors also a value below one. The blue point shows that the result of the RK measurement by the Belle Collaboration is consistent with one in the whole q2 range up to 22 GeV2. The Belle result does not, however, contradict a possible q2 dependence of RK which may be indicated by the BaBar results. As seen in the image the LHCb measurement is the most precise measurement of RK to date.Studies of RK could detect the existence of additional Higgs bosons, new scalar or new pseudo-scalar particles that violate lepton universality, or “heavy Z”, Z’, which was suggested as a possible explanation of deviations observed in the P’5 distribution in the analysis of B0→K*0μ+μ- decays, see 9 August 2013 news.
The pp collision data corresponding to 3 fb-1 of integrated luminosity collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV were used to obtain this result. It will be interesting to follow up with future LHCb measurements of RK at the 13 TeV pp collisions after the LHC restart planned for April 2015.
Read more in the LHCb presentation at the LHCP conference here, at the CERN public page here, in the Symmetry article, in the LHCb publication here and in the APS Physics Viewpoint.
24 April 2014: Studying properties of exotic particles.
The LHCb Collaboration has published today a paper which shows that the f0(500) and the f0(980) mesons, long thought to be four quark states (tetraquarks) made out of the light udud quarks (f0(500)) or susu quarks (f0(980)), are NOT consistent with being tetraquarks. The four quark states cannot be classified within the traditional quark model in which the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). They are therefore called exotic particles.The Collaboration has studied the B0→J/ψπ+π− decays. The left image below shows that the π+π− invariant mass distribution is well understood as a contribution of different resonances and background distribution. The broad contribution of f0(500) is clearly seen in the data. Note that the narrow K0s peak structure is seen in the same mass region. There is no, however, evidence for the f0(980) contribution. The f0(980) production is much smaller than that predicted for tetraquarks and is ruled out at the 8σ level. The tetraquark model predicts a much smaller difference in the production rates of the two f0 mesons. The right image below shows the corresponding π+π− invariant mass distribution for the B0s→J/ψπ+π− decays. The contribution of f0(980) is clearly visible. The absence of f0(980) in B0 decays and its presence in B0s decays as well as the presence of f0(500) in B0 decays and its absence in B0s decays is exactly what is expected if these states are normal qq states. Read more in the LHCb publication here.
click the images for higher resolutionThe precision of the LHCb detector and high production rate at the LHC allowed the LHCb Collaboration to made an important contribution to the understanding of exotic state candidates with heavy charm quarks inside, and to draw conclusions which of them are really exotic and which are not. Recently the Collaboration observed the Z(4430) particle and established its quantum numbers which make it the first confirmed unambiguous tetraquark (cucd), see 9 April 2014 news located just below at this page. This observation has implications to astrophysics and was followed up by an impressive media coverage.
The story of the X(3872) particle is more complicated. It was discovered about 10 years ago by the Belle Collaboration and since then was also studied by other experiments. The mass of 3872 MeV is located in a region where the pairs of charm (D) and anti-charm D mesons can be formed. In this mass region many charm quark-antiquark (cc) states are also present bound by a strong force in an atom-like system called charmonium. In addition the two different forms of exotic particles can be formed in this mass region: 1. About 10 fm large loosely bound DD* molecules in which D and D mesons are bound by a strong force in analogy to the deuterium nucleus composed of a proton and a neutron, and 2. About 1 fm compact four quark qqqq elementary particles, tetraqarks.
The LHCb has unambiguously determined the quantum numbers of X(3872) to be 1++, see 26 February 2013 news . This are the quantum numbers of an as yet unobserved charmonium state called χc1(23P1). However, the charmonium spectrum is very well understood and the mass of the X(3872) makes this assignment very unlikely. The LHCb measurement of the ratio of branching fractions of X(3872) decay into ψ(2S)γ and J/ψγ does not support a pure DD* molecule interpretation of the X(3872), see 24 March 2014 news, and favours a hypothesis for an admixture of cc charmonium state and DD* molecule. However, the relativelly high production cross-section of the X(3872) at the LHC observed by the LHCb Collaboration, see 27 October 2010 news, favours even more an exotic tetraquark compact particle interpretation. More studies of X(3872) production and decay properties are needed to clarify its internal structure.
9 April 2014: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model.
The LHCb Collaboration has published yesterday results of precise measurements of properties of the Z(4430)- particle which allow to determine unambiguously its exotic nature. In the traditional quark model, the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particle physicists were searching since 50 years for the particles, called exotic hadrons, which could not be classified within this scheme. Many candidates have been proposed but up to now there has not been unambiguous proof of their existence.The first evidence for the Z(4430) particle has been presented in 2008 by the Belle Collaboration as narrow peak in the ψ’π- mass distribution in the B → ψ’Kπ- decays. In the latest Belle publication the observation of the Z(4430) particle is confirmed with a significance of 5.2σ and a 3.4σ evidence is presented that the quantum numbers JP = 1+ are favored over the other spin assignments. There are many so called charmonium cc neutral states in this mass region. The fact that the Z(4430) is a charged particle does not allow to classify it as a charmonium state making this particle an excellent exotic candidate. The BaBar collaboration could explain the Z(4430) enhancement in their data by a possible feature of experimental analysis (so called reflections, for experts), not contradicting in the same time the Belle evidence.
The LHCb Collaboration has reported yesterday an analysis of about 25 200 B0 → ψ’Kπ-, ψ’ → μ+μ- decays observed in 3 fb−1 of pp-collision data collected at √s = 7 and 8 TeV. The LHCb data sample exceeds by an order of magnitude that of Belle and BaBar together. The significance of the Z(4430)- signal is overwhelming, at least 13.9σ, confirming the existence of this state. The Z(4430)- quantum numbers are determined to be JP = 1+ by ruling out 0-, 1-, 2+ and 2- assignments at more than 9.7σ, confirming the evidence from Belle. The LHCb analysis establishes the, so called, resonant nature of the observed structure in the data, and in this way proving unambiguously that the Z(4430) is really a particle.
The minimal quark content of the Z(4430) state is ccdu. It is therefore a four quark state or a two-quark plus two-antiquark state.
click the images for higher resolutionThe black points at the left image above show the ψ’π- invariant mass squared distribution of the data. The blue histogram shows the Z(4430) contribution. The right image shows the so called Argand diagram proving to the experts that the Z(4430) structure seen in the data (black points) represents really the resonant particle production and decay, since it follows approximately a circular path (red circle).
Read more in the LHCb publication here, in the CERN Quantum Diaries in English and French, in the CERN Courier article and also in the "Quarks bonding differently at LHCb" blog. See special video at CERN CDS or YouTube.
24 March 2014: New LHCb results at the Rencontres de Moriond.
The LHCb Collaboration has reported new important results at the Rencontres de Moriond EW Interactions and Unified Theories, March 15th - 22nd and QCD and High Energy Interaction, March 22nd – 29th.The run 1 data taking period ended one year ago. The LHCb experiment collected 1fb-1 of data from the pp collisions at 7 TeV in 2011 and additional 2fb-1 at 8 TeV collisions in 2012. The results of analysis of beauty and charm particles decays were already presented at many conferences and reported in the news below. At this year Moriond conference more precise results of different analysis were presented using larger data sample and/or including other decay channels.
The decay of the beauty meson B into an excited K meson K* and a μ+ and μ- pair is considered as an important channel for new physics search, see 9 August 2013, 13 March 2012 and 22 July 2011 news for introduction. Different distributions and branching fractions have been studied for these B meson decays and compared with the Standard Model predictions. The differences in the results of measurements of neutral B meson decays into K*0μ+μ- and charged B+ meson decays into K*+μ+μ- is called an "isospin asymmetry". The Standard Model calculations predict this isospin asymmetry to be small what, in fact, was confirmed by the LHCb analysis of 2011 (1fb-1) data. On the other hand, when the physicists made similar analysis by replacing the excited kaon K* by its ground state K an evidence was obtained for a possible isospin asymmetry (see 25 May 2012 news). The analysis of full 3fb-1 data sample presented at the Rencontres de Moriond gave results consistent with the small asymmetry predicted by the Standard Model in both (K* and K) cases. However, even if the difference between results of measurements of neutral and charged B meson decays is small there is a tendency for differential branching fractions to have lower values than the theoretical predictions as seen in the images below.
click the images for higher resolution
Exotic states, particles which are not composed of quark and
anti-quark pairs (mesons) or three quarks (baryons), have been searched
for since nearly 50 years. One of the most famous candidates for such an
exotic state is called the X(3872). The LHCb Collaboration has
unambiguously determined its quantum numbers JPC to be 1++ (see 26 February 2013 news for introduction). Possible exotic explanations of the X(3872) nature include a DD*
molecules or multi quark anti-quark system such as a
diquark-diantiquark tetraquark or charmonium-molecule mixture. Classical
interpretation of X(3872) as a pure cc charmonium state is not excluded but this assignment is very unlikely (see again 26 February 2013 news).The LHCb Collaboration has presented at the Rencontres de Moriond a result of measurement of ratio of branching fractions of X(3872) decay into ψ(2S)γ and J/ψγ, Rψγ. This ratio is predicted to be different for different natures of X(3872) as seen in the image. The LHCb result 2.46 ± 0.64 ± 0.29 is more precise, but compatible with other experiments. It does not support a pure DD* molecule interpretation.
28 February 2014: First observation of photon polarisation in b→sγ transition.
The LHCb Collaboration has submitted today for publication a paper reporting the first observation of photon polarisation in b→sγ transition. The full 3 fb-1 Run 1 data sample was used to obtain this result. The Collaboration has presented already the first evidence for the photon polarization in this process at the summer 2013 conferences using about 2/3 of the whole data sample, see the news of 19 July 2013 for an introduction.Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photon can have either right or left circular polarization or a superposition of both, read more here.
The beauty particles decay mainly into charm particles, less frequently into strange particles. About once in every 3000 decays into strange particles a photon is emitted. At the underlying quark level a beauty b quark turns into a strange quark s by emitting a photon γ. This famous b→sγ transition is considered as a very interesting process in which signs of new physics could show up. The first evidence for this process was obtained by the CLEO Collaboration in 1993 and since then it was intensively studied in many experiments. This decays occurs only rarely since it requires a quantum fluctuation where a pair of heavy particles (a top quark and a W boson) appear and then rapidly vanish. The interaction between these particles is such that the emitted photon is expected to be almost 100% (left-handed) polarized. However, since the “virtual” top and W particles are not seen in the detector, they could equally well be replaced by other even heavier particles that are predicted in various theories that go beyond the Standard Model. Such theories have been proposed to address important unresolved questions in particle physics, such as the origin of the imbalance between matter and antimatter seen in the Universe. These models generally predict different values for the photon polarisation, and therefore it is seen as one of the most important measurements that can be made with the latest generation of experiments.
click the images for higher resolutionResearchers from the LHCb experiment have now succeeded to observe a non-vanishing value of the polarisation for the first time with a significance of 5.2σ. The analysis is based on nearly 14000 B+→K+π- π+ γ decays, for which the distribution of the γ angle with respect to the normal to the plane defined by the kaon and two pion system is studied in four intervals of the K+π- π+ mass which are shown in the left image. The two curves are fits to the data points, allowing photon polarisation (solid blue curve) or setting it to zero (dashed red curve). The right image shows a typical B+→K+π- π+ γ event.
This investigation is conceptually similar to the historical Wu experiment that discovered parity violation by measuring the asymmetry of the direction of a particle emitted in a weak decay.
Read more in the LHCb publication here, in the CERN Courier article and in the CERN Bulletin article in English and French.
28 January 2014: How long can beauty and charm live together?
[ τ( Bc+) = 509 ± 8 ± 12 fs ]
In the Standard Model of Particle Physics the strongly interacting particles (hadrons) are composed of three quarks (baryons) or quark-antiquark pairs (mesons). There are six types of quarks, three light: up (u), down (d) and strange (s) and three heavy: charm (c), beauty (b) and top (t). The top quark decays so fast that it cannot form bound particle states with other quarks. The heaviest quark which can form baryons or mesons with other quarks is the beauty quark, about five times heavier than proton. The particle physics theory predicts that the lifetime of particles made of the beauty quark and light quarks should be almost identical. The earlier experimental results were not supporting this prediction. Recently the LHCb Collaboration has shown, by measuring precisely the Λb lifetime, that indeed the theoretical predictions are correct, see 15 July 2013 news for details.But what about the lifetime of particles composed of two heavy quarks? The LHCb Collaboration submitted today for publication a result of lifetime measurement of the Bc+ meson, formed of a b and a c quark. The lifetime of the Bc+ meson is measured using semileptonic decays having a J/ψ meson and a muon in the final state. The J/ψ mesons decay in turn into a pair of muons. The measured lifetime is 509 ± 8 ± 12 fs with an uncertainty less than half of that of the combination of results of previous measurements. This lifetime is much shorter than the lifetime of particles composed of a beauty quark and lighter quarks and closer to the lifetime of charmed particles formed by a charm quark and lighter quarks. This precise result is also very interesting for theorists studying decays of this kind of particles. The calculations are challenging since the effects of both strong and weak interactions need to be taken into account.
The image on the left shows the lifetime distribution of the Bc+
candidates, with the fitted components indicated. The analysis is
performed on a data sample of pp collisions at a centre-of-mass energy
of 8 TeV, collected during 2012 and corresponding to an integrated
luminosity of 2 fb-1. Further improvements are expected from the LHCb experiment using Bc+ → J/ψπ+ decays, where systematic uncertainties are expected to be largely uncorrelated with those affecting the present determination.Read more in the LHCb publication and in the CERN Courier article.
click the image for higher resolution
24 January 2014: “Underground” news.
The last run 1 collisions took place at LHCb on February 2013. The proton beams should traverse again LHCb in early 2015. The period in between is called Long Shutdown 1 (LS1). What is happening at LHCb, 100m underground, during this period?”LHCb operated with great success throughout LHC run 1 and has not been subject to any major intervention since its assembly in 2008. The current long shutdown offers a first opportunity for prolonged access, and hence an extensive programme of consolidation and maintenance work has been scheduled. This programme involves all general and detector related services, equipment and safety systems” writes Rolf Lindner, the LHCb Technical Coordinator. Read more details here.
The left image shows the installation of the 30 tons shielding for the LHCb muon detector where 2100 blocks were piled up in a confined space. The right one shows the LHCb dipole consolidation.
click the images for higher resolutionThe LHCb underground cavern is also a very popular scientific “tourist” place. The total number of visitors in 2013 was 4524 for 392 visits guided by members of LHCb Collaboration. In addition about 2100 persons visited the LHCb detector and nearby LHC tunnel during CERN Open Days in September 2013. The CERN Visit Service showed the LHCb surface exhibition to 220 groups with 7935 visitors last year.
If you have not visited LHCb detector you can visit it “virtually” using the links from the LHCb public page (right column). The links include a Google Street View tour.
16 December 2013: LHCb VErtex LOcator VELO.
At LHCb the protons collide inside the VErtex LOcator detector (VELO). The VELO is composed of two halves, each consisting of 21 pairs of back-to-back silicon sensors, whose job is to precisely measure the position at which the charged particles pass (see left image below). You can watch the sophisticated construction process at the video here and can read more details about this detector here. Its sensitive detector elements are held out of harm's way while the beams are being injected and stabilized, but once safer, the silicon elements are moved mechanically in towards the beam to hunt for beauty and charm particles.The LHCb VELO detector plays an essential role in locating precisely the pp collision point as well as the location of beauty and charm particle decays, see, for example, 12 March 2013 news. This detector is so important that the LHCb Collaboration decided to build a second identical Vertex Locator in order to replace the one located around beam should this ever be needed. Since the primary VELO detector is still working perfectly the replacement version is now displayed at the LHCb surface exhibition as seen in the right image below.
The LHCb Collaboration is working towards a major upgrade of the
LHCb experiment for the restart of data-taking in 2019. Most of the
subdetectors and electronics will be replaced so that the experiment can
read out collision events at the full rate of 40 MHz. The upgrade will
also allow LHCb to run at higher luminosity and eventually accumulate an
order of magnitude more data than was foreseen with the current set-up.
The VELO performance will also be strongly improved. Pixel
technology will be used to buid this third version of the detector. The
new detector will contain 40 million pixels, each measuring 55 μm
square. The pixels will form 26 planes arranged perpendicularly to the
LHC beams over a length of 1 m (see image). The sensors will come so
close to the interaction region that the LHC beams will have to thread
their way through an aperture of only 3.5 mm radius.Read more in CERN Courier article and in the LHCb VELO Upgrade Technical Design Report.
29 October 2013: New charm results.
[ x'2 = (5.5 ± 4.9)x10-5 ; y' = (4.8 ± 1.0)x10-3 ]
[ AΓ(KK) = (-0.35 ± 0.62 ± 0.12)x10-3 AΓ(ππ) = (0.33 ± 1.06 ± 0.14)x10-3 ]
The LHCb Collaboration has reported recently new important results on charm physics.(1) Ten months ago, the LHCb Collaboration presented the first observation of the D0-D0 oscillations in which the D0 matter mesons turn into their antimatter partners. Contrary to the B0-B0 and B0s-B0s oscillations in which the mesons turn into their antimatter partners many times during their lifetime, the D0-D0 oscillations are very slow, over one hundred times the average lifetime (see 7 November 2012 news for introduction). LHCb has now updated this result using the full 2011 and 2012 data set of 3 fb-1. The new result is 2.5 times more precise. The values parameterizing the oscillations, the so-called mixing parameters y' and x'2, are shown above.
By now, CP violation, differences in the behaviour of matter and antimatter, has been observed in all oscillating neutral-meson (K0, B0, B0s) systems apart from the charm system. First evidence for charm CP violation (see 14 November 2011 news) has not been unambiguously confirmed to date (see 12 March 2013 news). The D0 mesons are the only mesons containing up-type quarks which undergo matter anti-matter oscillations (called also mixing) and therefore provide unique access to effects from physics beyond the Standard Model.
As part of the new analysis, LHCb has investigated whether there is a CP violating contribution to the oscillations, in contrast to the Standard Model expectation. This is done by investigating whether the oscillation parameters for mesons produced as D0 and D0 differ. Studying the D0 and D0 decays separately shows no evidence for CP violation and provides the most stringent bounds on the parameters (AD and |q/p| for experts) describing this violation from a single experiment.
(2) LHCb physicists measured the asymmetry AΓ of the inverse of effective lifetimes in decays of D0 and D0 mesons to the K- K+ and π-π+ final states. The measured values of the parameter AΓ
shown above represent the world’s best measurements of this quantity,
and are the first searches for CP violation in charm oscillations with
sensitivity better than 10-3. They do not indicate CP violation, and show no difference in AΓ between the two final states.The results of other experiments combined by the Heavy Flavor Averaging Group indicated a hint for possible non-zero values of the CP violation parameters (|q/p| and φ for experts). Both LHCb results presented above do not support this indication as seen in the image. The size of the contour with the new LHCb results is about a factor of two smaller in each of |q/p| and φ. They provide very stringent limits on the underlying parameters, thus constraining the room for physics beyond the Standard Model.
click the image for higher resolution
8 October: 2013 Nobel prize in Physics.
The Nobel Prize committee of the Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2013 to François Englert and Peter Higgs for "the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider".
.Congratulations to François Englert and Peter Higgs.
The image shows LHCb spokesperson Pierluigi Campana showing the LHCb detector to Peter Higgs on December 18th, 2012.
.
click the image for higher resolution
28-29 September 2013: CERN Open Day at LHCb.
see CERN Open Day web page here for details.2 September 2013: Excellent performance of LHCb Ring Imaging Cherenkov (RICH) detectors.
The physics results presented on this web page were obtained thanks to the excellent LHC collider performance, to the excellent LHCb data acquisition and analysis, and certainly also to the excellent quality of LHCb detector. As an example the exceptional precision of the particle identification achieved by one of the two RICH detectors is shown in the left image below. RICH detectors work by measuring the emission of Cherenkov radiation. This phenomenon occurs when a charged particle passes through a certain medium faster than light does. As it travels, a cone of light is emitted, which the RICH detectors reflect onto an array of sensors using mirrors. The shape of the cone of light depends on the particle’s velocity, enabling the detector to determine its speed. Scientists can then combine this information with a record of its trajectory (collected using the tracking system and a magnetic field) to calculate its mass, charge, and therefore its identity.But why the speed of light is lower in the medium? - see an explanation here.
click the images for higher resolutionThe LHCb Collaboration has recently published a paper in the European Physics Journal C describing the performance of the LHCb RICH detectors. The Journal has honoured the excellent quality of LHCb detector by placing the RICH performance image on its cover page as seen in the right image above.
Read more about the LHCb detector at this web site here.
26 August 2013: Matter-antimatter quantum music.
A fascinating feature of quantum mechanics, in which the B0s, B0 and D0 particles turn into their antimatter partners and back, has been discussed already few times at this page, see 3 March 2013 and 7 November 2012 news. This feature is called oscillations or mixing. The B0s mesons oscillate with by far the highest frequency of about 3 million million times per second (3*1012), on average about 9 times during their lifetime. The B0 mesons oscillate about 37 times slower with a frequency of about 80 thousand million times per second (8*1010). A musical tone is defined by its frequency. LHCb physicists tried to hear this beautiful (involving beauty quarks) matter-antimatter quantum music.The left video image above shows the last stage of the event filtering process. Two accumulations of events are clearly visible allowing to select the B0 and B0s particles. As the blue box moves through the image we are able to hear the background noise, then the loud tone of B0-B0 oscillations, the background noise again and then the tone of the B0s-B0s oscillations. The higher frequency B0s-B0s oscillations are experimentally more difficult to observe and therefore their tone is weaker. The very high-pitched quantum oscillation frequencies were reduced by millions of times in order to fit into the range that can be heard by humans. Additional explanations can be found in the right hand side video above.
Read also the CERN public page news. Experts can read more about the LHCb analysis, event selection and oscillation parameter measurements in a recent publication here.
9 August 2013: LHCb results hint at new physics?
The LHCb Collaboration has just published the results of a new analysis of the B0→K*0μ+μ- decay, with K*0→K+π-. These results were presented three weeks ago at the European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, and triggered very interesting discussions. The analysis of the B0→K*μμ decay is considered as a very promising channel to search for new physics effects, see the 14 June 2013 news for an introduction. A contribution from new physics particles could modify the angular distributions of the decay products. LHCb physicists have studied different variables related to these angular distributions as functions of the μ+μ- invariant mass squared. In previously published results, no significant deviation from the Standard Model prediction has been found, see the 13 March 2012 news. In order to increase sensitivity to new physics effects LHCb physicists started to analyse additional observables (the so called Pi' observables) which are considered theoretically clean. This means that they are less sensitive than other observables to some theoretical parameters that are not precisely known (form-factors for experts). Four such observables, labelled P4', P5', P6' and P8', have been studied.
The image shows the distribution of the P5' observable as a function of the μ+μ- invariant mass squared q2.
The black data points are compared with the Standard Model prediction. A
3.7σ deviation of data above the prediction is observed for the third
bin corresponding to q2 between 4.3 and 8.68 GeV2/c4. Taking into account that this deviation is observed in one out of 24 bins investigated in this work (the so-called look-elsewhere effect), the significance of the deviation becomes 2.8σ..
click the image for higher resolution
The results presented so far are based on 1fb-1 of data recorded from pp collisions at 7 TeV in 2011. Particle physicists are impatiently waiting for result of analysis of additional 2fb-1 of data taken at 8 TeV in 2012.
Read more in the LHCb presentation in Stockholm and in the LHCb paper here and in the CERN Courier article.
24 July 2013: The B0s→μμ decay is observed.
The CMS and the LHCb Collaborations have announced today at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, that the B0s→μμ decay is observed. The LHCb Collaboration presented already at this conference the measurement of the B0s→μμ branching fraction of (2.9+1.1-1.0)x10-9 with a significance of 4.0σ (see the 19 July 2013 news). The CMS Collaboration presented the same day a similar result giving a branching fraction of (3.0+1.0-0.9)x10-9 with a significance of 4.3σ (see CMS public page article). The results of both experiments are compatible and, therefore, a decision was taken to combine them.
The CMS and the LHCb Collaborations have obtained a combined preliminary value of the B0s→μμ branching fraction of (2.9±0.7)x10-9. Although a thorough evaluation of the combined significance has not been performed, it is clear that the B0s→μμ
decay is observed (with a significance above 5σ). The result is in
agreement with the Standard Model prediction of (3.56±0.30)x10-9.
The image shows the CMS and LHCb results and their combination together
with the results of the CDF Collaboration as well as the D0 and ATLAS
Collaborations 95% CL limits which are not included in the combination. click the image for higher resolution
Read more in the plenary presentation in Stockholm, in the LHCb paper and also in the CERN Quantum Diaries in English and French.
19 July 2013: First evidence of photon polarisation in b→sγ transition.
The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, a first significant non-zero measurement of an observable proportional to the photon polarisation in b→sγ transition. The transition of a b-quark to an s-quark by emission of a photon (γ) is considered a very important process to investigate possible manifestation of new physics. This decay process is forbidden in the first approximation in the Standard Model (SM) of particle physics and moreover in the second-order processes that govern the process in the SM the emitted photon is expected to be strongly polarised. Therefore it is very sensitive to new physics effects arising from the exchange of new heavy particles in electroweak penguin diagrams (see 14 June 2013 news). Indeed, several models of new physics predict that the emitted photon should be less polarised than in the SM. Up to now different experiments have measured the decay rate of this process, ruling out significant deviations of the rate from the SM prediction and strongly reducing the allowed parameter space of new physics models. The photon polarisation was, however, never previously observed.
Free quarks are not observed in nature. Therefore physicists
measure the b→sγ transition in decays of particles containing a b quark,
like B mesons, into strange particles containing a s quark, like K
mesons. The LHCb physicists have used the process B+→Kresγ where excited K meson states, Kres, decay in turn into three particles, K+, π- and π+.
The red distribution in the image shows the contribution of more than
8000 signal events reconstructed and selected in the 2012 data sample,
corresponding to an integrated luminosity of about 2 fb-1
collected in pp collisions at 8 TeV. The other distributions show
different background contributions. An angular analysis of the B+
decay products has allowed to obtain first evidence, with 4.6σ
significance, for photon polarization in the b→sγ transition with
respect to the no-polarization scenario. Further theoretical analysis
is, however, needed to obtain a numerical value for this polarization.Read more in the LHCb presentation in Stockholm and in the LHCb conference note here.
19 July 2013: Measurement of the B0s→μμ branching fraction and search for B0→μμ decays at the LHCb experiment.
The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, improved measurements for the rare decays B0s→μμ and B0→μμ. Last year at the Hadron Collider Particle Symposium in Kyoto, the Collaboration presented the first evidence, with 3.5σ significance, for the B0s→μμ decay using the total 1.0 fb-1 of data taken in 2011 and 1.1 fb-1 of the data accumulated in 2012, see 12 November 2012 news. The full LHCb data sample of 3.0 fb-1 was used to obtain today's result. The analysis strategy is very similar to that reported in November 2012 with an improved event selection algorithm (BDT). The significance of the result has been improved to 4.0σ making the evidence even stronger. A branching fraction of (2.9+1.1-1.0)x10-9 is obtained. The result is in agreement with the Standard Model prediction of (3.56±0.29)x10-9. It puts very strong constraints on the parameters of different models of new physics and squeezes even more than previous results the parameters of supersymmetric extensions of the Standard Model (SUSY) - see 30 March 2012 news for introduction.
The μ+μ- invariant mass
spectrum for the BDT selection algorithm bins with the smallest
background contribution is shown in the left image. The solid blue line
shows that the data distribution presented as black dots is well
understood and can be separated into different components presented with
the help of different colour lines. The dashed red narrow distribution
shows the B0s →μμ contribution around the B0s mass of 5372 MeV/c2. The green dashed line shows a possible B0 contribution. The B0 decay yield is not significant yet and an improved limit on the B0→μμ branching fraction of 7.4x10-10 at 95% CL is obtained..
click the images for higher resolution
Read more in the LHCb presentation in Stockholm and in the LHCb paper and in the CERN Press Release.18 July 2013: Observation of unexpected resonant structure in B→Kμμ decays.
The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, a first observation of unexpected resonant structure in B+→K+μ+μ- decays. Precise study of this decay could uncover a possible contribution from new physics. This contribution could, however, hide behind the dominant B+→K+μ+μ- decay modes which proceed through the decay of the B+ to a cc resonance (charmonium) and a K+ meson, followed by the decay of the resonance to a μ+μ- pair. To probe for physics beyond the Standard Model it is necessary to remove regions of μ+μ- mass dominated by the resonances. Up to now only the J/ψ and ψ(2S) resonances were taken into account because contributions from resonances with masses above 3900 MeV, where the kaon has a low recoil against the dimuon pair, were thought to be negligible.
The image shows the μ+μ- mass distribution in
the low recoil region. What was expected is a smoothly falling
distribution, dominated by the non-resonant decay. However, two peaks
are clearly visible, one at the low edge corresponding to the decay
ψ(3770)→μ+μ- and a wide peak at a higher mass. The mean and width of the wider peak are 4191+9-8 MeV/c2 and 65+22-16 MeV/c2,
which are compatible with the so-called ψ(4160) resonance (the name
ψ(4160) is misleading, first measurements of the mass of this state gave
values lower than it is now known to be). First observations of both
the decay B+→ψ(4160)K+ and the subsequent decay ψ(4160)→μ+μ- are reported with statistical significance exceeding six standard deviations.
click the image for higher resolution
Read more in the LHCb presentation in Stockholm and in the LHCb paper here.
15 July 2013: A mystery of the beauty baryon lifetime resolved.
[ τ(Λb)/ τ(B0) = 0.976±0.012±0.006 ]
[ τ((Λb) = 1.482±0.018±0.012 ps ]
The LHCb Collaboration has just published an important precise measurement of the Λb beauty baryon lifetime. The Λb
is a particle composed of the up (u), down (d) and beauty (b) quarks,
so it can be understood as being like a neutron (composed of udd quarks)
in which one of the d quarks has been replaced by the beauty (b) quark.
Therefore the Λb baryon is about 6 times heavier than the neutron. The lifetime of the Λb baryon was first measured by experiments which took data at the electron-positron collider LEP in the 1990s. The results of the measurements were puzzling. It was a real nightmare for theoretical physicists.
In fact, the calculations they were using, the Heavy Quark Expansion HQE, predicted that the Λb lifetime should be very similar to that of the B0 meson, but the LEP experiments found the Λb lifetime to be about 20% shorter than the B0 lifetime.
The LHCb Collaboration has recently discovered a new decay mode Λb → J/ψpK-, J/ψ→μμ. The image shows the signal yield of more than 15,000 Λb decays in 1.0 fb-1 of LHCb data. This decay allows the precise measurement of the Λb decay point from the intersection of four charged tracks. The B0 decay point was also measured precisely using four charged tracks of the B0 decay into J/ψK*0, K*0→K+π-. In this way the LHCb collaboration made the most precise measurement of the Λb to B0 lifetime ratio to be 0.976±0.012±0.006, close to 1 and in agreement with the original HQE prediction. The mystery of the Λb lifetime is now resolved. Using previous determinations of the B0 meson lifetime, the Λb lifetime was found to be 1.482±0.018±0.012 ps.
Read more in the LHCb publication, in the CERN Courier article and in the CERN Quantum Diaries in English and French.
14 June 2013: LHCb: a place to find penguins.
How "penguins" can help LHCb to chase for new physics? - read explanations in the June issue of CERN Courier and then follow links to the LHCb physics results announced in the 13 March 2012 and 25 May 2012 news at this page.The name of penguin appeared in particle physics in 1977 following a bet lost by CERN theoretical physicist John Ellis, see also explanations in the CERN Courier article. The original penguin diagram for b quark to s quark decay, where a gluon produces an ss pair, is shown in the image at the right hand side.
LHCb is investigating decays where a μ+μ-
pair is produced from a photon or from a Z boson as seen in the diagram
on the left named as an "electroweak penguin"; the diagram on the right
is a "box" diagram.See also the hangout video,
click the images for higher resolution
6 June 2013: LHCb PhD student wins the Swiss FameLab competition.
Donal Hill, LHCb PhD student, was nominated as the best Swiss FameLabber 2013 during the Swiss FameLab final in Zurich. Nine candidates selected in a previous heat in Geneva were evaluated by a judging panel and by the audience. Donal will represent Switzerland at the Cheltenham Science Festival in the UK in June 2013.FameLab is an exciting competition for young researchers. It encourages scientists to inspire and excite public imagination with a vision of the 21st century of science.
Congratulations to Donal!
See Donal's presentation here, as well as the CERN bulletin article in English and French.
17 May 2013: LHCb physicist rewarded by the European Physical Society.
The High Energy Physics Division of the European Physical Society announced today the winners of its 2013 prizes, which will be awarded at the Europhysics Conference on High-Energy Physics (EPS-HEP 2013), Stockholm (Sweden) 18-24 July 2013. The 2013 Young Experimental Physicist Prize, for outstanding work by one or more young physicists in the field of Particle Physics and/or Particle Astrophysics, was awarded to Diego Martinez Santos “for his outstanding contributions to the trigger and commissioning of the LHCb experiment, and the analyses leading to first evidence for the rare decay B0s →μμ”.Congratulations to Diego!
The European Physical Society recognized in this way the quality and the excellence of the LHCb experiment, and of the whole collaboration who built and is operating it.
Read more on the analysis of B0s →μμ data in the 12 November 2012 news.
10 May 2013: First and important results from the proton with lead ion collision 2013 run.
The LHCb Collaboration has just presented at the Workshop on proton-nucleus collisions at the LHC, Trento, Italy, the first results from the analysis of proton with lead ion collision run data taken in January-February 2013. Already these first results made an important contribution to the understanding of heavy ion collisions.In the Standard Model of Cosmology quarks and gluons were freely moving in a state called a quark-gluon plasma until < 10-5 seconds after the Big Bang. As the Universe cooled, they became confined inside protons and neutrons. The theory of quark-gluon interactions, the strong force interaction theory, QCD, predicts that the state of quark-gluon plasma can also exists in high temperature matter created by high energy collisions between large atomic nuclei, called by physicists heavy ion collisions. But how to prove that the quark-gluon plasma is really formed? A reduced rate of J/ψ particle production in heavy ion collisions was considered as a "smoking gun" argument in favour of quark-gluon plasma formation by physicists analysing results of measurements performed in the CERN Super Proton Synchrotron (SPS) after 1986 and more recently in the Brookhaven RHIC collider. Profound analysis has shown, however, that reality is more complicated. In some models, for example, the J/ψ particle could also be regenerated in nuclear matter, partons (quark, gluons) could be saturated and/or lose energy, etc. in normal (so called cold) nuclear matter.
Data recording collisions of protons with lead ions were collected in the LHC experiments in January-February this year. In such collisions, formation of a quark-gluon plasma is not expected, and therefore measurements based on these data allow the study of interactions in cold nuclear matter. The analysis of J/ψ production was of particular interest.
The LHCb results are shown in the image at the left hand side. A reduced value of the nuclear attenuation factor RpA,
the ratio of the J/ψ production in the proton with lead ion (pA)
collisions to that in proton-proton collisions as a function of the rapidity
y is clearly seen. The rapidity variable is related to the J/ψ
production angle with respect to the incoming proton direction. The
experimentally measured points (triangles with error bars) in the image
show that the largest suppression is in the forward direction.The colored distributions show theoretical predictions of RpA calculated by François Arleo from LAPTH, Annecy and Stéphane Peigné from Subatech, Nantes, taking into account the J/ψ energy loss (E. loss) in cold nuclear matter with and without the parton saturation effects. The LHCb results are in agreement with these predictions.
Two sets of data were taken: pA and Ap, where in the second case the direction of the proton and lead ion beams were reversed. This allowed the LHCb detector, recording the particles only on one side of the interaction point, to make measurements in both forward and backward directions with respect to the proton beam (positive and negative rapidity).
Read more in the LHCb presentation in Trento and in the LHCb conference note.
24 April 2013: First observation of CP violation in the decays of B0s mesons.
[ ACP(B0s→K-π+) = +0.27 ± 0.04 ± 0.01 ]
[ ACP(B0→K+π-) = -0.080 ± 0.007 ± 0.003 ]
The LHCb Collaboration has just submitted for publication a paper
which sets an important milestone in the history of particle physics. A
difference between properties of matter and antimatter, named CP violation by particle physicists, was discovered in 1964 in the decays of neutral K mesons and was rewarded with the 1980 Nobel Prize in Physics for James Cronin and Val Fitch.
M. Kobayashi and T. Maskawa proposed in 1973 a mechanism which could
incorporate CP violation within the Standard Model with not less than 6
quarks. In 2001, CP violation was observed in the decay of so-called
Beauty Particles, the B0 mesons composed an anti-quark b
and a quark d. The Standard model mechanism of CP violation was
confirmed and therefore Kobayashi and Maskawa were rewarded with the 2008 Nobel Prize in Physics. In March 2012 the LHCb Collaboration reported an observation of CP violation in charged B± meson decays into DK±. Today, the LHCb Collaboration has announced an observation of CP violation in the decays of Strange Beauty particles, the B0s mesons composed of a beauty antiquark b bound with a strange quark s.
The B0 and B0s meson decays into K and π mesons were studied.
The decay into a negative K meson red track and a positive π green track is show in the event displays above.click the images for higher resolution
The full 1.0 fb-1 data sample collected in 2011 was used to obtain these results, the precision will be further improved using the total dataset available which has more than tripled thanks to the excellent 2012 data taking period.
More details can be found in the LHCb paper here. Read also the CERN Press Release in English and French, the CERN Bulletin article in English and French as well as CERN Courier article.
20 March 2013: LHCb 100th publication.
The LHCb Collaboration has submitted its 100th publication! It is signed by 621 authors from 63 different universities and laboratories from 17 countries. The paper "Search for direct CP violation in D0→h-h+ modes using semileptonic B decays" has been presented at the Rencontres de Moriond QCD, La Thuile, Italy, and is described in the 12 March 2013 news as the second independent analysis. You can celebrate this important moment in the life of our Collaboration with the help of a poster available in the LHCb secretariat or printing it directly from a file. The poster shows an event, described in the 12 March 2013 news, and used in the analysis.The LHCb papers have made very important contributions to particle physics as described in other items on this page. However, even more important contributions are expected in the near future. In fact, most results presented in LHCb papers to date used the full 1.0 fb-1 data sample collected in 2011. The total dataset available for future analysis has more than tripled thanks to the excellent 2012 data taking period. The only published paper using the 2012 data is the analysis presented in the "First evidence for the B0s →μμ decay" paper, see 12 November 2012 news, in which already half of 2012 data sample was used.
The highlights of recent LHCb results showing the presentations at different conferences, conference contributions and papers can be found here.
12 March 2013: Improved search for CP violation in charm decays.
[ ΔACP = (−0.34 ± 0.15 ± 0.10 )%, pion tagged ]
[ ΔACP = (+0.49 ± 0.30 ± 0.14)%, muon tagged ]
The LHCb Collaboration presented today at the Rencontres de Moriond QCD,
La Thuile, Italy, results of an improved search for the difference
between properties of matter and antimatter, CP violation, in charm
decays, see 14 November 2011 news for introduction. The difference (Δ) of CP asymmetry (ACP) between the decay rates of D (matter) and D (antimatter) mesons
into K+K– pairs and into π+π- pairs was measured. The results presented today profited from three improvements to the previous analysis: the full 1.0 fb-1 data sample collected in 2011 was used instead of 0.6 fb-1,
the analysis technique was improved and also in addition another
independent method was used to select matter D and antimatter D particle decays.In the Standard Model CP violation was expected to be very small in the charm sector, whereas new physics effects could generate enhancements. Therefore the 14 November 2011 announcement by the LHCb Collaboration of 3.5σ evidence of CP violation in charm sector, ΔACP = (-0.82 ± 0.21 ± 0.11)%, triggered intensive theoretical activity with conclusions that some special Standard Model effects could generate CP violation effects even as big as about 1%. This interesting LHCb result was later confirmed by the CDF and Belle collaborations. The new improved LHCb result presented today, ΔACP = (−0.34 ± 0.15 ± 0.10 )%, is more precise thanks to the larger data sample and several improvements resulting in better background suppression by a factor of 2.5. The central value is, however, closer to zero than in the previous measurement, which it supersedes.
In the measurement presented above the D (matter) and D (antimatter) mesons were selected using the D* meson decays, D*+(-) → π+(-)D(D), which means that the presence of π+ in the decay identified matter D meson production while π- accompanied antimatter D production. LHCb physicists presented today also results of a second independent analysis in which the D and D mesons were selected using so called semileptonic beauty B decays, for example B+(-) → μ+(-)νD(D). In the second analysis, the positive charge of μ+ identified the D meson, while the negative one, μ-, the D production. The image at the left hand side shows a selected event. A zoom around the pp interaction point shows a B+ meson decay point located at the distance of 17 mm from the pp collision point and the D meson decay place still 9 mm further away. The second analysis also measures a value that is consistent with zero: ΔACP = (+0.49 ± 0.30 ± 0.14)%. A combination of the two LHCb results gives ΔACP = (-0.15 ± 0.16)%.
The image at the left hand side shows a comparison of different measurements of ΔACP.
The previous LHCb result is shown as the shaded grey point. A naive
world average is shown as the yellow band. As shown in the image, the
two new LHCb results are consistent with each other and with other
results at the 2σ level, but do not confirm the previous evidence of CP
violation in the charm sector. Theoretical work has shown that several well-motivated models could induce large CP violation effects in the charm sector. These new results constrain the parameter space of such models. Further update of this and related measurements will be needed to discover if – and at what level – nature distinguishes between charm and anticharm.
More details can be found in the LHCb presentation in La Thuile, in CERN seminar, in the LHCb paper and conference contribution here. Read also the CERN Bulletin article here and the CERN Courier article.
3 March 2013: Precise search for new physics.
[ Δms = 17.768 ± 0.023 ± 0.006 ps-1 ]
[ φs = 0.01 ± 0.07 ± 0.01 rad ]
ΔΓs = 0.106 ± 0.011 ± 0.007 ps-1 ]
The LHCb Collaboration presented today at the Rencontres de Moriond EW, La Thuile, Italy, three important results of their more and more precise search for new physics. The 1 fb-1 data sample collected in 2011 was used to obtain these results.(1) A fascinating feature of quantum mechanics, in which the B0s, B0 and D0 particles turn into their antimatter partners, has been discussed already at this page, see 15 March 2011 and 7 November 2012 news. This feature is called oscillations (mixing). The B0s mesons oscillate with by far the highest frequency of about 3 million million times per second (3*1012), on average about 9 times during their lifetime.
The B0s meson decays into D-sπ+ were used in this analysis with D-s decays into five different channels. The image at the left hand side illustrates the B0s-B0s
oscillations in a spectacular way, showing how the matter turns into
antimatter and back over many oscillation periods. The frequency of
oscillation is defined by the Δms parameter. The value of this parameter as measured by the LHCb collaboration is shown above..
click the image for higher resolution
click the images for higher resolution(3) Finally, LHCb physicists have opened a door for important future measurements by presenting the results of a first study of the time-dependent CP-violating asymmetry in hadronic B0s meson decays into an φφ pair. Both φ mesons decayed in turn into a K+K- pair. The invariant mass spectrum for the four kaons is presented in the right image above showing a clear accumulation of B0s → φφ decays at the B0s mass. Using about 880 events the CP-violating phase φs was measured to be in the interval of [-2.46,-0.76] rad at 68% confidence level.
The results presented in this news represent the most precise measurements to date. They are in agreement with the Standard Model prediction. The parameter region in which the signs of new physics can still hide is significantly reduced. More details can be found in the LHCb presentations in La Thuile and in the LHCb papers here. Read also the CERN Courier article.
26 February 2013: X(3872) looks more and more exotic.
[ X(3872) JPC = 1++ ]
The LHCb Collaboration presented today at the Rencontres de Physique de la Vallée d'Aoste, La Thuile, Italy, an important result which makes the exotic nature of the X(3872) particle very probable.In the quark model of particle physics proposed in 1964 by Murray Gell-Mann and George Zweig mesons, like the π (pion), are formed from quark and anti-quark pairs and baryons (like the proton) from three quarks. This model is very successful. Particles which cannot be described in this model, known as exotic states, have been searched for since nearly 50 years ago. Their existence has not yet been firmly established (except of a special case of pionium). One of the most famous candidates for such an exotic state is called the X(3872). It was discovered in B+ meson decay into an X(3872) and a K+ meson by the BELLE collaboration almost 10 years ago. Its existence was confirmed later by the CDF, D0 and BaBar experiments. LHCb has previously reported studies of the X(3872) in the data sample taken in 2010, see 27 October 2010 news. Particles are classified according to their quantum numbers JPC. An analysis by the CDF experiment has limited the possible values of X(3872) JPC to either 1++ or 2-+.
LHCb physicists have observed the X(3872) particle in the decay of a B+ meson into an X(3872) and a K+ meson. The X(3872) was observed in the invariant mass of a J/ψ particle and a π+π- pair, while the J/ψ was identified from its decay into μ+μ- pair. The image at the left hand side shows the difference between the invariant mass of the π+π-J/ψ
combination and the J/ψ showing clearly the X(3872) and ψ(2S)
enhancements over the smooth background distribution (the ψ(2S) particle
decays also into a π+, π-, and J/ψ)..
click the image for higher resolution
The exotic nature of the X(3872) would be unambiguously determined if its quantum numbers could not be described by the quark-antiquark combination. However, this is not the case. In fact, the mass of 3872 MeV is located in a region in which many charm quark-antiquark states are present bound by a strong force in an atom-like system called charmonium. The X(3872) has the quantum numbers of an as yet unobserved charmonium state called χc1(23P1). However, the charmonium spectrum is very well understood and the mass of the X(3872) makes this assignment very unlikely. Possible exotic explanations of the X(3872) nature include a DD* or multi quark anti-quark system such as a diquark-diantiquark tetraquark or charmonium-molecule mixture.
More details can be found in the LHCb presentation in La Thuile and in the LHCb paper here. Read also the CERN Courier article and the CERN Bulletin article in English and French.
14 February 2013: End of 2013 data taking period.
Today at 7:25 LHCb physicists have observed the last collision at the LHC. After a very succesfull period of proton with lead ion collisions the last few days of data taking were reserved for proton-proton collisions at 2.76 TeV. The two year shut-down period, called LS1, will start two days later in order to set-up LHC for doubling the proton-proton collision energy to 13 TeV at March 2015. LHCb Collaboration congratulates and thanks LHC team for excellent performance.Read the CERN Bulletin article in English and French.
20 January 2013: Start of 2013 data taking period.
Today at 15:11 LHCb physicists have observed the first 2013 collisions of protons with lead ions in the LHCb detector. The 2013 data taking period has started. The proton - lead ion collisions were alredy observed by the LHCb on 13 September 2012 during the short test run.15 November 2012: LHCb thanks LHC.
LHC collider has delivered at 9:50 today 2 fb-1 of
luminosity to LHCb this year corresponding to about 100 million million
of proton-proton collisions visible at LHCb. This was possible thanks to
an exceptional performance of the machine and an impressive commitment
from everybody involved in the operation of LHC in this very long
running year. The image shows how the LHCb Collaboration thanks the LHC
operation team on the LHC status screens which are visible in the
control rooms all over CERN and on the Web. The collider is currently
running with very high efficiency. This can be seen in the image where
very long proton-proton collisions periods are interleaved by short
set-up intervals.12 November 2012: First evidence for the B0s →μμ decay.
[ Branching ratio B0s →μμ = (3.2+1.5-1.2)x10-9 ]
The LHCb Collaboration has presented today at the Hadron Collider Particle Symposium in Kyoto the result of the branching ratio measurement of the B0s meson decay into μ+μ- pair to be (3.2+1.5-1.2)x10-9. Both experimental and theoretical physicists were impatiently waiting for this result, an important milestone of the LHCb program. The significance of the measurement is 3.5σ and therefore is classified as the first evidence for the B0s →μμ decay. The result is in agreement with the Standard Model prediction of (3.54±0.30)x10-9 (for experts: this number takes into account a correction to the value (3.23±0.27)x10-9 due to the finite width difference of the B0s system). LHCb physicists had previously presented 15 March this year the lowest published limit of 4.5x10-9 for this decay, which allowed to squeeze strongly the parameters of supersymmetric extensions of the Standard Model (SUSY) - see 30 March 2012 news. The measurement presented today squeezes the parameter space even more.
The total 1.0 fb-1 of data taken in 2011 and 1.1 fb-1
of the data accumulated this year were used to obtain this result. A
special event selection (BDT for experts) was used to classify data into
bins with different ratios of B0s →μμ decays and background contributions. The μ+μ- invariant mass
spectrum for the bins with the smallest background contribution is
shown in the left image. The solid blue line shows that the data
distribution presented as black dots is well understood and can be
separated into different components presented with the help of different
colour lines. The dashed red narrow distribution shows the B0s →μμ contribution around the B0s mass of 5366 MeV/c2.
click the image for higher resolutionA typical B0s →μμ decay candidate event is shown above. The two muon tracks from B0s decay are seen as a pair of purple tracks traversing the whole detector in the left image above. The right image shows the zoom around the proton-proton collision point, origin of many particle tracks. The two muon purple tracks originate from the B0s decay point located 14 mm from the proton-proton collision.
The precision of these results will be improved using an additional 1 fb-1 of data or more that will be available by the end of this year thanks to the strong and continuous support from the LHC operations team for the LHCb physics program.
More details can be found in the LHCb presentation in Kyoto and in the LHCb paper here and also in the CERN seminar. Read also the CERN Bulletin article in English and French, the CERN Courier article and also the CERN Quantum Diaries blog in English and French.
7 November 2012: Oscillating charm and beauty.
[ Δmd = 0.5156 ± 0.0051 ± 0.0033 ps-1 ]
[ x'2 = (-0.9 ± 1.3)x10-4 ; y' = (7.2 ± 2.4)x10-3 ]
A fascinating feature of quantum mechanics has been reported 15 March 2011 on this page. The strange beauty particle (matter) B0s composed of a beauty antiquark (b) bound with a strange quark s turns into its antimatter partner composed of a b quark and an s antiquark (s) with a frequency of about 3 million million times per second (3*1012).
This feature is called "oscillations" or "mixing". The LHCb
Collaboration has just published the first observation of similar
oscillations of charm mesons D0 composed of a charm quark and an anti-up quark (D0-D0 oscillations) and the most precise measurement of a parameter defining the frequency of the oscillations of beauty mesons B0 composed of a beauty antiquark (b) bound with a d quark (B0-B0 oscillations).The B0 decays into D+π- and J/ψK*0 were used to study B0-B0 oscillations. The images below show the asymmetry which is proportional to the difference between the number of events in which the matter (or antimatter) B0 particle decayed with the same flavour identity with which it was produced, and the number of events in which it did not, as a function of its lifetime. The B0-B0 oscillations are clearly visible. LHCb physicists have parametrized them with a value Δmd = 0.5156 ± 0.0051 ± 0.0033 ps-1 corresponding to the frequency of about 80 thousand million times per second (8*1010), about 37 times slower than B0s-B0s oscillations. The B0-B0 oscillations have been previously measured at LEP, Tevatron and B factories. The LHCb result is currently the most precise measurement of the Δmd parameter.
click the image for higher resolutionD0-D0 oscillations are very slow, over one hundred times the average lifetime of a D0 meson, so the full oscillation period cannot be observed. Instead, it is necessary to look for small changes in the flavour mixture (matter or antimatter) of the D0 mesons as a function of the time at which they decay. One of the best channels to search for this mixing is the D0 decay to the Kπ final state. The initial matter-antimatter identity can be identified by the charge of the accompanying pion in the decay D*+→D0π+ or D*-→D0π-. The mixing effect (oscillation) appears as a decay-time dependence of the ratio R between the number of reconstructed “wrong-sign” (WS; D0→K+π-) and “right-sign” (RS; D0→K-π+) processes. In the absence of mixing, R is predicted to be constant as a function of the D0 decay time t, while, in case of mixing, it is predicted to be an approximately parabolic function of t. The left image below shows the WS over RS ratio R, as a function of decay time, from a total of 36 thousand WS and 8.4 million RS decays selected from the 1.0 fb-1 of data recorded in 2011. The horizontal dashed line shows the no-mixing hypothesis, the solid line is the best fit to data when mixing is allowed. The clear time dependence observed excludes the no-mixing hypothesis by 9.1 σ.
click the image for higher resolutionComparison of values parameterizing the D0-D0 oscillations, y' and x'2, obtained by different experiments, are presented in right image above. The LHCb results exclude the no-mixing hypothesis by more than 5 σ for the first time and therefore can be classified as the first observation of this effect.
Since the Standard Model predictions for the mixing parameters have large uncertainties, the next step will be to focus on cleaner observables to search for possible new physics contributions. In particular, LHCb is now well placed to investigate whether there is a CP violating contribution to the oscillations, in contrast to the Standard Model expectation. This will be achieved by studying charm mixing in this and other decay channels and exploiting the large increase in available statistics following the successful 2012 LHC run.
Read more in the LHCb publications: D0-D0 and B0-B0 oscillations, in the CERN Courier article, and also in the USLHC Quantum Diaries blog.
5 October 2012: Measurement of the γ angle.
[ γ = (71.1+16.6-15.7)° only B±→DK decays ]
[ γ = 85.1° uncertainty regions [61.8,67.8]° and [77.9,92.4]°, B±→DK and B±→Dπ decays ]
LHCb is an experiment set up to explore what happened after the
Big Bang that allowed matter to survive and build the Universe we
inhabit today. Therefore LHCb physicists are measuring differences
between properties of matter and antimatter, called CP violation by
experts.
CP violation was discovered experimentally in K meson decays in 1964.
M. Kobayashi and T. Maskawa proposed in 1973 a mechanism which could
incorporate CP violation within the Standard Model with not less than 6
quarks; they were awarded the Nobel prize of Physics in 2008 for this idea.
The size of this violation is set by the parameter η, which is shown as the y axis in the figures below. Constraints on η and the related parameter ρ (the x-axis) are measured in various ways in different experiments as shown in the compilation made by the CKMfitter group in the left image below.
The constraints show that in fact the values of ρ and η
within the small colored region in the center of the images are
compatible with the experimental results and confirm in this way the
Kobayashi and Maskawa Standard Model mechanism of CP violation.
However, since this mechanism does not explain the large quantity of
matter observed in the Universe physicists are searching for other
sources of CP violation outside the Standard Model.An interesting possibility is to measure precisely the angle γ of the triangle shown in the right image below in processes in which the new physics contribution is possible and in processes in which it is not. Differences between measurements in these two cases would be a sign of new physics. The measurement of the angle γ in different processes is one of most important goals of the LHCb experiment. The value of this angle is known up to now with precision of only about 10 or 12°, as seen in the right image below, using the combination of results from other experiments.
click the image for higher resolution
LHCb physicists have just presented at the 7th International Workshop on the CKM Unitarity Triangle,
Cincinnati, Ohio, USA, the measurement of the angle γ in processes
where the contribution of new physics is not expected. These
measurements will set a base for comparison with the measurement where
observation of new physics effects is possible. The B±→DK and B±→Dπ decays were used with D mesons decays into KK, ππ, K0Sππ, K0SKK or Kπππ. The value of the angle γ = (71.1+16.6-15.7)° was obtained using only B±→DK decay results in the analysis.
The image at the left hand side shows the confidence level of the signal as a function of angle γ, for the combination of B±→DK
modes. The peak of the distribution gives the measured central value,
and the width gives the error.
The best value of the angle γ = 85.1° was obtained with the two
corresponding uncertainty regions [61.8,67.8]° and [77.9,92.4]° (at 68%
CL) when the B±→Dπ decays were included in addition. More details can be found in the LHCb presentation in Cincinnati and in the LHCb Conference Contribution here. Read also the CERN Courier article and the CERN Bulletin article in English and French.
3 October 2012: Matter antimatter asymmetry in three-body charmless B decays becomes more and more interesting.
[ ACP(B±→π±π+π-) = +0.120 ± 0.020 ± 0.019 ± 0.007 ]
[ ACP(B±→π±K+K-) = -0.153 ± 0.046 ± 0.019 ± 0.007 ]
Earlier this year, LHCb physicists reported at the ICHEP2012 Conference (see 7 July 2012 (2) news)
the first evidence of inclusive CP asymmetry (differences between
properties of matter and antimatter) in the charmless three-body B meson
decays B±→K±π+π- and B±→K±K+K- in which the b-quark decays into a u,d or s-quark instead of its dominant decay into a charm c-quark.
It was interesting to note that much larger asymmetries were observed in some small special regions, like invariant mass squared of the π+π- pair in the K±π+π- decay lower than 1 GeV2, or of the K+K- pair in the K±K+K- final state between 1.2 and 2 GeV2 (7 July 2012 (2) news).
click the image for higher resolutionThese measurements have now, at the 7th International Workshop on the CKM Unitarity Triangle, Cincinnati, Ohio, USA, been complemented with results from the rarer B±→π±π+π- and B±→π±K+K- decays, finding evidence of even larger CP violation. A remarkable feature of the new LHCb results is that the CP violation effects appear to arise in some special kinematical regions that are not dominated by contributions from narrow resonances. For example, in B±→π±K+K- decays a broad feature at low K+K- invariant mass that was previously observed by the BaBar collaboration [PRL 99 (2007) 221801] appears to be present only in B+ decays, as shown by the filled triangles distribution in the left figure above, and not in the B- distribution (open triangles). This points to some interesting hadronic dynamics that could generate the observed direct CP violation. LHCb is now starting detailed studies of these channels, that can also exploit the three times larger data sample available after 2012 running, to further understand these effects. The results of these analysis will also establish whether the observed CP violation is consistent with the Standard Model expectation or has a more exotic origin.
The middle and right images above show the π±K+K- invariant mass distribution for B- and B+ decays with m2K+K- < 1.5 GeV2/c4. The difference between properties of matter and antimatter, the CP violation, is clearly seen as a difference between the height of two peaks located at the B mass (at 9σ level for experts).
More details can be found in the CERN Courier article, in the LHCb presentations in Cincinnati and in the CERN seminar here.
25 September 2012: Searching for new physics in rare kaon decays.
[ Branching ratio K0S →μμ < 9x10-9 at 90% CL ]
Having previously set the world's most restrictive limits on the dimuon decays of D0, B0 and B0s mesons, LHCb physicists have turned their attention to the search for similar decays of another member of the particle zoo, the K0S meson. Using the 2011 data sample, LHCb has set a limit on the branching ratio B(K0S→μ+μ-) to be less than 9x10-9, a factor of 30 improvement over the previous most restrictive limit measured in 1973. [For experts: the limit is at 90% confidence level.] While several techniques used were common with the search for B0s→μ+μ- (see news items of 8 April and 22 July 2011 and 5 March 2012 for more details), the challenge of this measurement lies in the specificity of kaon decays, which are very different from B decays for which LHCb detector was optimized.The image below shows the invariant mass distribution of selected μ+μ- pairs, candidates for K0S decay. The dashed lines indicate the signal region around the K0S mass, where no significant signal is seen.
click the image for higher resolutionK meson decays into pairs of muons played a very important role in the history of particle physics. There are two types of K mesons: the short-lived, K0S ("K-short") and the long-lived, K0L ("K-long"). The results of branching ratio measurements of the K-long decay into muon pairs in the early 1970s disagreed strongly with the predictions of particle physics theory based on existence of three quarks u, d and s. The calculated branching ratios were of the order of 10-4 while the experimental limits were about 4 orders of magnitude lower. In order to solve this problem Glashow, Iliopoulos and Maiani proposed the existence of an additional quark, called a charm quark – a 1970s version of a new physics model. In the mechanism they proposed (GIM mechanism) a destructive virtual contribution of this new quark reduced very strongly the K-long decay rate into muon pairs. The discovery of the J/ψ state in November 1974 gave the first evidence for the existence of charm quark and at the same time confirmation of the GIM mechanism.
40 years later LHCb physicists are searching for K-short decays into muon pairs, again on the look-out for new physics. The branching ratio is calculated to be 5x10-12 within the framework of the Standard Model of particle physics with 6 quarks. Although the new limit 9x10-9 is still three orders of magnitude above the prediction, it starts to approach the level where new physics effects might begin to appear. Moreover, the data collected by LHCb in 2012 already exceed the sample from 2011 and by the end of the year the total dataset should have more than tripled.
Read more in the CERN Courier article, the CERN Quantum Diaries and LHCb publication.
13 September 2012: First proton-lead ion collisions at LHCb.
During the night, at 1:30 am, LHCb recorded the first proton-lead ion collisions at the LHC. The proton-lead physics data taking is planned to take place in January and February 2013. Today the LHC operational team made tests of collisions in order to prepare the set-up of the LHC collider and the LHC experiments for next year. Note that collisions of protons with lead ions are more difficult than proton-proton or lead-lead collisions. In fact the speed of protons and lead ions is slightly different, even though it is close to the speed of light at LHC. The LHC operators succeeded in making the proton path length inside LHC ring slightly longer than the lead-ion path length in order to compensate for this difference.
click the image for higher resolutionA typical proton-lead collision event at LHCb is shown in the left image above. Note that the lead ions arrive at the LHCb collision point from the right hand side and the protons from the left hand side. The right image shows the invariant mass spectrum of the decay products of Λ and Λ particles indicating excellent prospects for physics analysis of p-lead collision data early next year.
Read also the CERN Courier article.
21 August 2012: LHCb has doubled its recorded luminosity.
The data sample used to obtain the LHCb results presented on this page was obtained using the integrated luminosity of 1.11 fb-1 recorded by LHCb during the 2010-2011 data taking period. The same inegrated luminosity has just been recorded this year, which corresponds to doubling the total available integrated luminosity. The data sample which can be used for physics analysis has more than doubled in the same time, since it is expected that the cross-sections for bb and cc quark-antiquark pair production have increased by 15% and 12%, respectively, at the increased pp collision energy of 8 TeV this year. LHCb physicists expect to more than triple their data set this year by the end of pp collision run at December, which should allow them to obtain even more interesting physics results.You can follow live the progress of delivered and recorded luminosity at LHCb, the number of proton-proton collisions visible at LHCb, as well as the number of bb and cc quark pairs produced within the LHCb acceptance at the image above on this page.
3 August 2012: 1 fb-1 of luminosity has been delivered to LHCb this year.
"This is great achievement considering that it comes about two months earlier than last year. Once more, the excellent performances of the machine, the skill and the commitment of the whole LHC team, made possible this result. We have also the exciting perspective of getting another 1 fb-1, or more, this year. This increased sample will allow us to push further our knowledge of Standard Model, and find finally where new physics is hiding so well." - said Pierluigi Campana, LHCb spokesperson.12 July 2012: LHCb film shortlisted by the European Science TV and New Media Festival 2012.
"LHCb - A Beauty Experiment", a short documentary on LHCb, has been shortlisted by the European Science TV and New Media Festival 2012. The festival takes place in association with the ESOF 2012 Science Congress in Dublin at Trinity College on 13-15th July 2012. The goal of the festival is to help writers to develop TV drama that involves science and technology. It is interesting to note that the LHCb film was already shortlisted by the NHK Japan Prize Festival in October 2011. Watch the film on YouTube in different resolutions and with subtitles in 15 different languages.
Screen shots from the film show the LHCb control room on 23 November, 2009 (left) and on 30 March, 2010 (right).7 July 2012 (1): Measurement of the flavour-specific matter antimatter asymmetry.
[ assl = (-0.24 ± 0.54 ± 0.33)% ]
Physicists from the LHCb experiment have today released results that help to shed light on one of the most significant experimental discrepancies with the Standard Model of particle physics. In 2010, and with an update in 2011, the D0 experiment, analyzing data taken at the proton-antiproton collider Tevatron at Fermilab, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons, see 2010 Fermilab Press Release. The observed difference was close to 1%, measured with almost the full D0 data sample of 9 fb-1. Like-signed dimuons can be produced from the decay of particles containing the b quark, which can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons would be an indication of CP violation. The D0 result differs by 3.9σ from the tiny value predicted in the framework of the Standard Model and could indicate the presence of a new physics contribution. This difference can be expressed as an asymmetry, Absl, where the label "b" indicates decay of particles containing b-quarks and "sl" (semileptonic) indicates that the decay involves leptons, in this case muons. The decaying matter (antimatter) B particles are composed of b-antiquarks(quarks) and d- or s-quarks(antiquarks). D0 physicists could not distinguish which type of decaying particles is at the origin of the measured asymmetry, therefore they present the measured asymmetry Absl in the image below as an inclined band across the plane of individual asymmetries in the decays of Bd and Bs mesons, labelled adsl and assl, respectively. The vertical band shows the measurement of the adsl asymmetry by the BaBar and Belle collaborations working at the Υ(4S) resonance, which is in agreement with the Standard Model calculations shown as the SM point in the image.
click the image for higher resolutionLHCb physicists have presented today at the ICHEP2012 Conference in Melbourne the most precise determination to date of the corresponding asymmetry for the Bs meson, assl. The LHCb result assl = (-0.24 ± 0.54 ± 0.33)% is shown as the horizontal blue band in the image above. The B0s semileptonic decays into D±sμ∓ final states were studied to obtain this result. The charmed D±s mesons were reconstructed in the φπ± mode. More details can be found in the LHCb presentation in Melbourne and in the LHCb Conference Contribution here. The LHCb result is consistent with the Standard Model prediction and does not confirm the deviation from the Standard Model reported by the D0 experiment. The D0 experiment previously published a result using Dsμ events, shown as the horizontal yellow band.
The full 2011 LHCb data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.
Read also the CERN Bulletin article in English and French, in the CERN Courier article and the CERN Quantum Diaries.
7 July 2012 (2): Evidence for matter antimatter asymmetry in three-body charmless B decays.
[ ACP(B±→K±π+π-) = +0.034 ± 0.009 ± 0.004 ± 0.007 ]
[ ACP(B±→K±K+K-) = -0.046 ± 0.009 ± 0.005 ± 0.007 ]
LHCb physicists have presented today at the ICHEP2012 Conference in Melbourne
measurements of differences between properties of matter and antimatter
(CP violation asymmetry) in the charmless three-body B meson decays B±→K±π+π- and B±→K±K+K-.
The dominant B meson decays involve a beauty b-quark decay into a charm
c-quark. In the rarer charmless decays (without charmed mesons)
discussed here the b-quark decays into a u,d or s-quark. LHCb physicists
have
measured the charge asymmetry ACP obtained from the
difference between negative and positive B event rates. The numerical
values of measured asymmetries are shown above. The significance is 2.8σ
for the B±→K±π+π- decay and 3.7σ for the B±→K±K+K- channel. The latter is the first evidence of inclusive CP asymmetry in charmless three-body B± decays.
click the image for higher resolutionThe results presented here are obtained by integrating (summing) the asymmetries over all kinematical variables of particles observed in the decays of B mesons. It is interesting to note that much larger asymmetries are observed in some small special regions, like invariant mass squared of the π+π- pair in the K±π+π- decay lower than 1 GeV2, or of the K+K- pair in the K±K+K- final state between 1.2 and 2 GeV2, as seen in the images above. Note that the value plotted in the figures is the raw asymmetry ARAWCP, and does not include all the small corrections that are included in the numerical results. LHCb physicists are planning further study of this intriguing feature.
The full 2011 data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.
More details can be found in the LHCb presentation in Melbourne and in the LHCb Conference Contribution here. Read also the CERN Bulletin article in English and French and the CERN Quantum Diaries.
25 May 2012: Puzzling asymmetries.
The decay of the beauty meson B into an excited K meson K* and a μ+ and μ- pair is considered as an important channel for new physics search, see 13 March 2012 and 22 July 2011 news. Different distributions and branching fractions have been studied for these B meson decays and compared with the Standard Model predictions. Recently LHCb physicists have studied in addition the differences in the results of measurements of neutral B meson decays into K*0μ+μ- and charged B+ meson decays into K*+μ+μ-. Theoretical uncertainties of the Standard Model calculations are strongly reduced in these differences. Physicists call these differences "asymmetries" and since this case involves differences between the decays of particles with different charges, it is called an "isospin asymmetry" AI. The Standard Model calculations predict this isospin asymmetry to be small, as shown with the help of the color line in the left image below, where the prediction is presented as a function of the square of the di-muon invariant mass (q2). The experimentally measured distribution, shown by points with error bars, is consistent with this prediction.
click the image for higher resolutionA surprise came when the physicists made similar analysis by replacing the excited kaon K* by its ground state K. The negative asymmetry is clearly visible in the right image above (4.4σ different from zero after integration (summation) over the whole q2 region). The Standard Model calculation is not yet available for this asymmetry, but is, as in the Bs→K*μμ mode, expected to be very close to zero.
The images shown above were obtained with the full 2011 data sample. These results have been made possible by the strong and continuous support from the LHC operations team for the LHCb physics program, and the data sample is expected to more than double by the end of this year. In the meantime theorists will analyze this puzzling result in order to establish whether this effect can be accommodated in the framework of the Standard Model, or whether a new physics explanation is required.
Read more in CERN Bulletin article in English and French, in the CERN Courier article and LHCb publication.
16 May 2012: First observation of two excited states of Λb.
The quark model, independently proposed by physicists Murray Gell-Mann and George Zweig in 1964 to classify the strongly interacting particles called hadrons, is very successful. In this model baryons are composed of three quarks and mesons are composed of quark-antiquark pairs. The simplest baryon, the proton, which is the nucleus of the hydrogen atom, is composed of three light quarks uud while its neutral partner the neutron is composes of udd quarks. By replacing one of the d quarks by a heavier strange quark s we obtain a Λ particle composed of uds quarks. Furthermore by replacing in the Λ the s quark by a charm quark c or a beauty quark b we obtain a Λc or a Λb particle.The three quarks forming the Λ, Λc and Λb are in their lowest quantum mechanical state. Like electrons in atoms quarks can form excited states with different values of angular momentum and quark spin orientation. These excited states were previously observed for the Λ and Λc particles. They were, however, never observed for the Λb particle.
The LHCb collaboration has made first observations of two excited states of Λb.
click the image for higher resolutionThe Λb excited states have been reconstructed in three steps. In the first step the Λc+ particles were reconstructed through their decay into a proton p, a negative K- meson and a positive π+ meson. In the second step the Λc particles were combined with negative π- mesons in order to form the Λb particles. The Λb signal is clearly seen as the enhancement in the left image above showing the Λc+π- invariant mass spectrum. Finally the Λb particles have been combined with a pair of opposite sign pions π+π-. In the right image above two enhancements are clearly seen corresponding to the two Λb excited states with masses of 5912 and 5920 MeV, about 6 times the proton mass.
LHCb physicists have observed about 16 Λb(5912)→Λbπ+π- decays (4.9σ significance) and about 50 Λb(5920)→Λbπ+π- decays (10.1σ) among about 60 million million (6*1013) pp collisions seen by the LHCb detector at LHC during the 2011 data taking period.
Read more in CERN Bulletin article in English and French, in the CERN Courier article and LHCb publication.
27 April 2012: The rarest B decay ever observed.
The LHCb collaboration has made the first observation of the decay B+ → π+μ+μ-. With a branching ratio of about 2 per 100 million decays, this is the rarest decay of a B hadron ever observed, see CERN Courier article.
click the image for higher resolutionThe image shows the invariant-mass distribution of selected π+μ+μ- combinations, showing the clear peak corresponding to B+ decays (green long-dash). Also shown are the components in the fit from partially reconstructed decays (red dotted) and misidentified K+μ+μ- (black dashed) and the total (blue solid line). Candidates for which the μ+μ- pair is consistent with coming from a J/ψ or ψ(2S) decay have been excluded.
6 April 2012: LHCb looks forward to electroweak physics.
Measurements of the production of W and Z bosons, see 10 June and 22 July 2010 news, allowed LHCb physicists to explore electroweak physics, see CERN Courier article.
click the image for higher resolutionThe two images above show the complementarity of measurements made by different LHC experiments. The left image shows the x-Q2 kinematic region explored by the LHCb experiment (pink), compared with ATLAS and CMS (light green), as well as previous experiments. The right image shows LHCb's measurement of W charge asymmetry, shown as a function of lepton pseudorapidity, compared with theoretical predictions, and with results from the ATLAS and CMS experiments. More details and explanations can be found in the CERN Courier article.
30 March 2012: LHCb strongly squeezes SUSY parameter space.
Results presented by the LHCb Collaboration at the Rencontres de Moriond EW and QCD conferences allowed theorists to squeeze strongly the parameters of supersymmetric extensions of the Standard Model (SUSY), the most popular new physics model. The simplest version of this model, called the Minimal Supersymmetric Standard Model (MSSM), predicted the frequencies with which Bs and Bd mesons decay into pairs of oppositely charged muons to have values significantly different from the Standard Model (SM) prediction. This is shown in the left image below, which was presented by David Straub (SNS and INFN, Pisa) at the Moriond EW conference. The predictions for both frequencies (branching ratios BR) depend on different parameters of the MSSM and cover nearly all of the left image surface. The LHCb results, see 5 March 2012 news, limit the predictions that are still allowed to a small region around the SM expected value. It is interesting to note that certain combinations of MSSM parameters allow lower BR values than those predicted by the SM. The LHCb measurements of the parameter φs, which sets the scale for the difference between properties of matter and antimatter for the strange beauty Bs mesons, see 5 March 2012 news, also strongly limits the SUSY parameter space that is still allowed, as shown by the vertical lines on the right image below.
SUSY contributions to observables that can be measured in
experiments depend, in general, on more than 100 free parameters.
Therefore in order to be able to analyse experimental data physicists
are using a simplified model, the Constrained MSSM (CMSSM), with 5
parameters m0, m1/2, A0, tan β and μ/|μ|.
Nazila Mahmoudi (Clermont-Ferrand and CERN) presented the left image
below at the Moriond QCD conference. The parameter space below and to
the left of the red line is excluded by the results of searches for
direct production of SUSY particles at the CMS experiment, while the
large yellow region shows the parameter space excluded by the analysis
of Bs →μμ decays at LHCb.
The image is made for a relatively high value of the parameter tan
β=50. The LHCb exclusion region is smaller at lower values of tan β as
can be seen in the right image below, made with tan β=35. The green
regions on both images are still not excluded by LHCb's measurements.
click the image for higher resolutionThe images above illustrate that direct and indirect SUSY searches at LHC are complementary and show that the allowed parameter space is characterized by low tan β and high m0 and m1/2 within the CMSSM. LHCb's results have ruled out large deviations in several observables, but now LHCb physicists can start to probe the most interesting parameter region of new physics.
13 March 2012 (1): B0d →K*μμ, a first measurement of the zero-crossing point.
[ Zero crossing point B0d →K*μμ = 4.9+1.1-1.3 GeV2 ]
LHCb physicists have continued their search for physics beyond the Standard Model using the B0d decay into a K* meson (an excited kaon), and a μ+ and μ-. New physics contributions can change various distributions that describe the decay process. For example, the number of decays as a function of the square of the di-muon invariant mass (q2) and the di-muon forward-backward asymmetry (AFB) can both be affected in many new physics scenarios. The variable AFB indicates whether more or fewer muons of one sign are observed in the same direction as the K* than opposite to it. The distribution of AFB in function of q2 is shown below. The results have been announced today at the Rencontres de Moriond QCD conference.
click the image for higher resolutionThe point at which the AFB distribution is crossing zero is well predicted within the Standard Model, and any deviation could indicate a possible contribution from new physics. LHCb physicists have already presented at the 2011 summer conferences a first indication that the asymmetry is changing sign. Today, using three times higher statistics, they presented the first measurement of the zero-crossing point of 4.9+1.1-1.3 GeV2. This value, showed by the hatched vertical region in the Figure, is in agreement with the Standard Model prediction showed by the colored line.
The LHCb Collaboration aims to more than double its data set this year. With the new data, the zero-crossing point will be measured with higher precision, and a possible difference with the Standards Model prediction may be discovered.
Read more in the LHCb staff page.
13 March 2012 (2): A first measurement of the CP asymmetry in the decay B0s →K+K-.
In a fascinating world of quantum mechanics the strange beauty particle (matter) B0s turns into its antimatter partner about 3 million million times per second (3*1012), see 15 March 2011 news. Therefore CP violation effects, which are differences between the properties of matter and antimatter, can appear as variations with decay time. A name "time-dependent CP violation" is therefore used.
click the image for higher resolutionThe left figure shows the CP asymmetry in the B0d decay into π+π- pair and the right figure shows the asymmetry in the B0s decay into K+K- pair. It can be clearly seen, by comparing time scales of both figures, that the B0d mesons oscillate much slower than the B0s mesons. A new analysis from LHCb has measured the CP asymmetry separated into two components: the difference in the decay rate of B meson and anti-B meson (physicists call it "direct CP violation") and the quantum-mechanical phase difference between the B meson and anti-B meson decay ("mixing-induced"). These two components appear as a cosine-like and as a sine-like oscillation in the asymmetry plots above.
The results, announced today at the Rencontres de Moriond QCD conference, for the B0d → π+π- decay are Adirπ+π- = 0.11 ± 0.21 ± 0.03 and Amixπ+π- = -0.56 ± 0.17 ± 0.03, meaning the oscillation appears to come mainly from the sine-like component. These are the first measurements of these quantities at a hadron collider, and are consistent with previously published results from other experiments. The results for the B0s → K+K- decay - which are measured for the first time ever - are AdirK+K- = 0.02 ± 0.18 ± 0.04 and AmixK+K- = 0.17 ± 0.18 ± 0.05.
About 2/3 of data taken in 2011 were used in this analysis. The LHCb Collaboration aims to analyse three times more data by the end of this year. This will allow to see if CP violation occurs in the B0s → K+K- decay, and see if the amounts of direct and mixing-induced CP violation are as expected by the Standard Model.
Read more in the LHCb staff page.
5 March 2012 (1): Search for New Physics, an important milestone
[ Branching ratio B0s →μμ < 4.5x10-9 at 95% CL ]
The LHCb collaboration has announced today at the Rencontres de Moriond EW conference one of the most hotly anticipated results from the LHC. LHCb has shown that the frequency with which a Bs meson decays into a pair of oppositely charged muons is not larger than 4.5 times out of one billion decays. Theorists have calculated that, in the Standard Model, this decay should occur about 3 times in every billion, but that if new particles predicted by theories such as supersymmetry exist, the decay could occur much more often (see news items of 8 April and 22 July 2011 for more details). The new results represent a milestone in the search for "new physics" beyond the Standard Model.
click the image for higher resolutionThe left figure shows the mass calculated from the two muons for events that survive all selection requirements. The blue area shows the shape of random combinations of muons, while the red area shows the shape expected for real Bs decays (with the hatched area indicating the uncertainty on the sum of the two contributions). The data are seen to be consistent with a small excess over the background-only hypothesis. This excess is slightly less than, but consistent with, the Standard Model expectation, as shown in the right figure. The blue points show how likely the data distribution is for a given rate of Bs decay. The black dashed line shows the expected shape of the curve for the Standard Model rate, with the green band indicating the uncertainty. The horizontal lines allow to set limits with different degrees of certainty: for experts, the solid red line gives the 95% C.L.
Measuring the rate of this Bs decay has been a major goal of particle physics experiments in the past decade, with the limit on its decay rate being gradually improved by CDF, D0, LHCb and CMS experiments. The latest results by LHCb set the tightest limits to date. More data is needed to finally discover if the decay occurs at a rate above, below, or at that predicted by the Standard Model. LHCb aims to more than double the size of its data set in 2012, which could be enough to finally answer this question.
Read more in CERN Press Release CERN Bulletin article, in the CERN Quantum Diaries blog and also in the LHCb staff page.
5 March 2012 (2): Heavier strange-beauty lives longer and improved φs measurements.
[ φs = -0.002 ± 0.083 ± 0.027 rad]
LHCb physicists have reported today at the Rencontres de Moriond EW conference important progress in measurement of the difference between properties of matter and antimatter for the strange beauty Bs mesons. The new results improve on those presented at the summer 2011 conferences. The size of this difference is controlled by the parameter φs, which is predicted to be small in the Standard Model. However, effects of new particles not predicted by the Standard Model can make the measured value much larger. The progress is shown in the image below: the remaining allowed region is shown in yellow and compared to the previous results from LHCb in blue, and from CDF and D0 experiments in green and red, respectively.
click the image for higher resolutionIn the mysterious world of quantum mechanics the strange-beauty meson Bs matter antimatter system is alternatively described as a heavy and light mass Bs meson system. The mass difference Δms determinates the matter antimatter oscillation frequency, see 15 march 2011 news. The difference of their lifetimes was measured together with the value of φs. Since it was not previously know if the heavier or lighter Bs mesons live longer, LHCb physicists have obtained two possible values for φs corresponding to two blue regions in the image above. (The axis label ΔΓs corresponds to the difference of inverse lifetimes between the heavy and light Bs meson, physicists call it width (Γ) difference.) Recently LHCb physicists have succeeded to measure that the heavier strange-beauty Bs mesons live longer and in this way eliminated one of two blue regions in the image. Sophisticated quantum mechanical interference effects were used in this measurement. Experts can read details of the analysis in the published article.
The full sample of data collected in 2011, three times larger than that used in summer 2011, was used to obtain the result
φs = -0.001 ± 0.101 ± 0.027 rad using Bs decays into into a J/ψ meson and a φ meson; combining it with the measurement of the Bs decay into J/ψ and f0(980) LHCb physicists have obtained the final result φs = -0.002 ± 0.083 ± 0.027 rad.
2012 data taking period will start soon. Search for new physics in the small yellow region will continue.
Read more in CERN Press Release CERN Bulletin article, in the CERN Courier article, in the CERN Quantum Diaries blog and also in the LHCb staff page.
5 March 2012 (3): First evidence for CP violation in the decays of Bs mesons.
Measurement of CP violation, which describes differences between the properties of matter and antimatter, is a very important goal of LHCb. CP violation is well established in the K0 and B0 meson systems. Recent results from the LHCb collaboration have also provided evidence for CP violation in the decays of D0 mesons. Consequently, there now remains only one neutral heavy meson system, the B0s, where CP violation has not been seen.
click the image for higher resolutionThe LHCb collaboration has recently submitted for publication the first evidence for CP violation in the decays of B0s mesons. The paper (available here for experts), submitted to Physical Review Letters, confirms the preliminary results shown at the EPS-HEP meeting in Grenoble, see 22 July 2011 news. The B meson decays into K and π mesons were studied. The decay into a positive K meson red track and a negative π green track is show in the event display above. The four plots on the right hand side show the Kπ invariant mass distribution divided into different components as shown by the legend in the top-right figure. The different charge combination of K and π indicates if the decaying B particle is a matter or an antimatter particle. The two upper plots show that the decay rates of B0d mesons are different, as known from previous experiments. The lower two plots show that the difference is also visible for the B0s mesons. The measured CP asymmetry for the B0d mesons ACP = -0.088 ± 0.011 ± 0.008 constitutes the most precise measurement available to date and the first observation (6σ) at a hadron collider. The measured CP asymmetry for the B0s mesons ACP = 0.27 ± 0.08 ± 0.02 is the first evidence (3.3σ) for CP violation in the decays of these mesons.
LHCb physicists have used 1/3 of data collected in 2011 in this analysis and expect to have ten times more data by the end of this year.
Read more in CERN Press Release CERN Bulletin article, in the CERN Quantum Diaries blog and also in the LHCb staff page.
1 December 2011: LHCb looks to the future.
After a very succesful 2011 data taking period the LHCb Collaboration is preparing next year's operation. The first 2012 collisions should be observed in April. At the same time LHCb physicists are also actively working on the longer term future in which data will be taken at a much higher rate, see CERN Courier article.14 November 2011: CP violation in charm decays.
[ ΔACP = (-0.82 ± 0.21 ± 0.11)% ]
The LHCb Collaboration has presented today at the Hadron Collider Particle Symposium in Paris possible first evidence for CP violation, the difference between behaviour of matter (particles) and antimatter (antiparticles), in charm decays. The study of CP violation in both charm and beauty particle decays is central to the LHCb physics programme. In the Standard Model CP violation is expected to be very small in the charm sector, whereas new physics effects could generate enhancements.In this new analysis the LHCb physicists have used data collected in the first half of the 2011 run to study the differences in decay rates of neutral D meson particles composed of a charm quark c bound with an up antiquark (u) and D meson antiparticles (D) composed of a charm antiquark (c) bound with an up quark (u). The decays of D*+ mesons into D mesons and π+, and D*- mesons into D mesons and π- were used to select the D and D mesons. In the next step of the analysis the difference (asymmetry ACP) between the decay rates of D and D mesons into K+K– pairs as well as into π+π- pairs was measured. By determining the difference, ΔACP, in CP asymmetries for the K+K- and π+π- decays, the analysis strongly suppresses possible measurement biases which could arise through effects related to particle production, selection etc. The following preliminary result is obtained:
ΔACP = (-0.82 ± 0.21 (stat.) ± 0.11 (sys.) )% [ 3.5 sigma significance for experts ]
A very interesting period now begins. LHCb physicists are analysing the remainder of the data collected in 2011. If the result is confirmed theoretical work will be required to establish whether this effect can be accommodated in the framework of the Standard Model, or whether a new physics explanation is required.
click the image for higher resolutionThe figures show the invariant mass distribution of the K-K+ (around 1.4 million) and π-π+ (around 0.4 million) pairs. The distributions are centered at the D meson mass of 1865 MeV.
Read more in CERN Bulletin article in English and French, in the CERN Courier, in the CERN Quantum Diaries blog in English and French and also in the LHCb staff page.
30 October 2011: End of 2011 proton-proton collision data taking period.
The 2011 proton-proton collision data taking period has ended today. LHC collider and LHCb experiment have been working extremaly well. LHCb has recorded all in all an impressive 1.1 fb-1 out of a 1.22 fb-1 delivered at 3.5 TeV.“We’ve got from the LHC the amount of data we dreamt of at the beginning of the year and our results are putting the Standard Model of particle physics through a very tough test ” said LHCb Spokesperson Pierluigi Campana. “So far, it has come through with flying colours, but thanks to the great performance of the LHC, we are reaching levels of sensitivity where we can see beyond the Standard Model. The researchers, especially the young ones, are experiencing great excitement, looking forward to new physics.”
Read more in CERN Press release in English and French.
3 October 2011: 1 fb-1 of luminosity has been delivered to LHCb.
This is a very important milestone for LHCb which will allow LHCb physicists to reach an unprecedented accuracy in most of the core physics processes that are under study.Read more in CERN Bulletin article in English and French.
click the image for higher resolutionThe left hand image shows Mike Lamont (Operations Group Leader), Pierluigi Campana (LHCb Spokesperson), Steve Myers (Director for Accelerators and Technology), and Paul Collier (Head of the Beams Department) celebrate the LHCb milestone. The right hand image shows the zoom on the computer screen located above them and showing the LHC screen congratulating LHCb for its new record.
3 October 2011: LHCb film shortlisted by Japan Prize Festival
"LHCb - A Beauty Experiment", a short documentary on LHCb, has been shortlisted by the NHK Japan Prize Festival. Every year, the festival awards the very best in global educational media. Watch the film at YouTube in different resolutions and with subtitles.27 August 2011: φs: different properties of matter and antimatter for Bs mesons
[ φs = 0.03 ± 0.16 ± 0.07 ]
LHCb physicists have presented today the most precise measurement of φs (the Bs mixing phase, for experts) at the Lepton Photon conference in Mumbai (India). The value of φs is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for Bs mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of new physics could change its value significantly - see the analogy in the 8 April news.The decay of the strange-beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark (s), into a J/ψ meson and a φ meson was used for this measurement. The J/ψ meson decays in turn into a μ+μ- pair, and the φ decays to K+K– pair. In order to make this difficult measurement LHCb physicists had to analyse the Bs decay particles in 3 dimensions as well as to measure precisely the fast oscillations of strange beauty - see 15 March news.
The "artist's view" below shows the result of the φs measurement in a plane together with the correlated measurement of another value, ΔΓs. The results of the measurement favour two regions, one of which is located around φs = -0.036 ± 0.002 rad, the Standard Model prediction. The LHCb measurement is in agreement with the Standard Model prediction but the shaded region representing the LHCb result indicates that there is still room for a new physics contribution. The hints for a larger contribution of new physics suggested by the CDF and D0 experiments at Fermilab, also shown in the figure, are not confirmed.
click the image for higher resolutionIn February the LHCb Collaboration made first observation of the Bs decay into J/ψ f0(980) - see 27 February news. This decay contributed to the φs measurement.
Using both Bs decays LHCb physicists have obtained the value φs = 0.03 ± 0.16 ± 0.07.
Read also CERN Press Release in English and French.
Read also CERN Bulletin article in English and French.
Read also CERN Courier article.
22 July 2011 (1): Hunting for New Physics continues
[ Branching ratio B0s →μμ < 1.2x10-8 at 90% CL and 1.5x10-8 at 95% CL ]
LHCb physicists continue their search for new physics, see 8 April 2011 news for introduction. They have presented updated results during the International Europhysics Conference on High Energy Physics (EPS-HEP) at Grenoble this week. The LHCb physicists have succeeded in setting the limit for an enhanced decay rate of the strange beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark s, into a μ+ and μ- pair, as low as about 4 times the rate calculated within the Standard Model (limits 1.2x10-8 at 90% CL and 1.5x10-8 at 95% CL for experts). This result was obtained from the analysis of about 8 times more data than in the previous analysis.
click the image for higher resolutionThe computer reconstructed images above show the most significant event compatible with the strange beauty B0s decay into muon pair seen as a pair of purple tracks traversing the whole detector. The right hand image shows a close-up around the proton-proton interaction point from which many tracks originate. The B0s decays about 1 cm from the proton-proton collision point into two muons (purple tracks). The invariant mass of muon pair corresponds to the B0s mass. The number of observed B0s candidates is slightly above the background predictions and compatible with the expected signal predicted by the Standard Model (SM) theory. This analysis is much more sensitive than previous experiments, and although large deviations (by more than a factor 4) from the SM are excluded, there is still plenty of room for new physics contributions. LHCb physicists are expecting to be able to analyse about three times as many events by the end of 2011, and about ten times more by the end of 2012, to give a final answer for the possibility of a new physics contribution to this interesting rare decay.
22 July 2011 (2): Hunting more for New Physics
LHCb Physicists have also presented results of their search for new physics using the B0d (composed of a beauty antiquark (b) bound with a down quark d) decay into an excited K meson, K*, and a μ+ and μ-. The partial rate as a function of the square of the di-muon invariant mass (q2) and the di-muon forward-backward asymmetry (AFB) can both be affected in many new physics scenarios. The variable AFB indicates whether more or fewer muons of one sign are observed in the forward direction than in the backward direction. The distribution of AFB in function of q2 is shown below.
click the image for higher resolutionThe LHCb precision measurements shown in black are in agreement with the Standard Model (SM) theory and indicate for the first time that the asymmetry is changing sign as predicted by the SM. The results of measurements by other experiments are presented in the figure on the right hand side.
22 July 2011 (3): Different properties of matter and antimatter
An important part of the LHCb physics programme is reserved for studying differences between the properties of matter and antimatter (CP violation for experts). At the EPS-HEP meeting in Grenoble this week the LHCb physicists have presented distributions in which these differences can be clearly seen.
click the image for higher resolutionThe blue line follows the measured points. After background subtraction (slightly sloping black line from left to right) the Kπ invariant mass distribution is divided into different components shown in different colours. The different charge combination of K and π indicates if the decaying B particle is a matter or an antimatter particle. The red curve in the upper plots shows the decay rate of B0d (matter, left plot) and anti-B0d (antimatter, right plot). The horizontal red dotted line helps to show that the decay rates of matter and antimatter B particles to Kπ mesons are different. The green distributions in lower plots show that the difference between matter and antimatter decay rate is also observed in B0s decays to Kπ.
1 June 2011: Pierluigi Campana - new spokesperson for the LHCb collaboration
Pierluigi Campana from the Istituto Nazionale di Fisica Nucleare in Frascati, begins his 3-year tenure as LHCb spokesperson this June. He replaces Andrei Golutvin, from Imperial College London and Russia’s Institute for Theoretical and Experimental Physics. As the new voice for the collaboration, Campana will lead the experiment through what should prove to be a very exciting phase.Read more details at the LHCb Collaboration Web page and in the CERN Bulletin article in English and in French.
8 April 2011: Hunting for New Physics
Quantum mechanics allows energy non-conservation during a very short time, typical in particle collisions. This opens up the possibility to study signal for the existence of particles for which there is insufficient energy to produce them directly. This feature is used by LHCb physicists to search for heavy particles expected in new physics models - models that describe physics outside the Standard Model (SM) of particle physics. Since the effects of new physics are expected to be very small, LHCb physicists are looking for modifications of the properties of very rare SM processes which can be calculated with high precision. The rare decay of the strange beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark s, into a μ+ and μ- pair is an excellent candidate. Only one out of 3*109 B0s mesons should decay into a μ+μ- pair according to precise SM calcutions. This rate could be higher if new physics particles, such as those in models with an extended Higgs sector, for example, were to influence this decay.LHCb physicists have not seen these decays in the data taken during the 2010 run and were able to set a limit which is about 19 times higher than the SM prediction, that is, the limit close to the one set by the Fermilab experiments CDF and D0 after many years of data taking. The figure below was used to set this limit at different levels of statistical probability.
click in the image to get it in higher resolutionLHCb physicists expect to observe the B0s mesons decay into μ+μ- pairs with the rate calculated within the SM using data taken this year and in 2012. An earlier observation of a higher rate will give interestng evidence for new physics.
Read more details in the CERN Courier article.
25 March 2011: LHCb movie
LHCb live and LHCb physics are presented in a short movie (about 15 min) "LHCb - the beauty experiment", available at YouTube and CDS. The YouTube version allows you to choose subtitle language (cc just below the movie window), as well as the image resolution including High Definition version 1080p.
15 March 2011: Oscillating Strange Beauty
or how matter turns into antimatter and back
Using data collected in proton–proton collisions at the LHC
at a centre-of-mass energy of 7 TeV LHCb has observed a fascinating
feature of quantum mechanics.
The strange beauty particle (matter) B0s composed of a beauty antiquark (b) bound with a strange quark
s turns into its antimatter partner composed of a b quark and an s antiquark (s) about 3 million million times per second (3*1012).
The B0s particles have been identified through their decay into strange charm Ds particles (composed of a charm quark c bound with a strange s
antiquark) and one or three πs.
Of course, LHCb observes B0s particles and antiparticles only during their short lifetime in which
they travel about 1 cm in the LHCb detector.The plot shows the observation of these oscillations when all data have been folded into one oscillation period. The variable Amix is proportional to the difference between the number of events in which the produced matter(antimatter) B0s particle had the same identity during its decay, and the number of events in which it had not, as a function of its lifetime.
click in the image to get it in higher resolutionThe B0s oscillations were observed for the first time by the Fermilab experiments CDF and D0 in 2006, see Press Release article. The oscillation parameters measured by the LHCb Collaboration show agreement with those measured at Fermilab.
27 February 2011: LHCb makes first observations of interesting B0s decays
Using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 7 TeV, the LHCb experiment has observed two new rare decay modes of B0s mesons for the first time. The decay B0s → J/ψ f0(980) will be important for studying differences between properties of matter and anti-matter (CP violation for experts) in the B0s system, while the decay B0s → D*–s2Xμ+ν will be valuable for testing predictions of strong interaction (QCD) theory.The first new decay mode observed is of the decay B0s → J/ψ f0(980). The B0s consists of a b antiquark (b) bound with an s quark, and can decay to a J/ψ (cc) together with an ss state, which can be a φ or, more rarely, an f0. While the φ decays to K+K–, the f0 decays to π+π–. The figure shows the enhancement in the π+π– invariant mass distribution in the region of 980 MeV indicating an observation of f0(980).
click in the image to get it in higher resolutionThe observation of these two new decay modes demonstrates that the LHCb experiment is already competitive in the field of heavy flavour physics. Great progress is expected with the larger data sample due from the coming run, with the potential to constrain, or even observe, new physics.
Read more details in the CERN Courier article, which also includes description of the second decay.
Read more details in the National Science Foundation article.
27 October 2010: Exotic mesons
In 1964 Murray Gell-Mann and George Zweig proposed the quark model (QM) in which mesons, like π mesons, are formed from quark and anti-quark pairs and baryons (like protons) from three quarks. The model is very succesful, but recently particles which could not be classified in this model have been discovered. LHCb has observed one of these exotic state candidates called X(3872) using its decay into a J/ψ meson (see 6 September news) and a π+ π- pair. The J/ψ decays in turn into a μ+ and μ- pair. The invariant mass of J/ψ π+ π- is shown in the figure below. The left enhancement at the mass of 3686 GeV is consistent with the QM bound state ψ' of charm and anti-charm quarks, the right one at the mass of 3872 GeV has properties that are very difficult to reconcile with the Gell-Mann Zweig QM. Possible explanations include a meson-meson molecule (DD* for experts) or multi quark anti-quark system (diquark-diantiquark tetraquark meson for experts).
this plot was made using all data taken in 2010, click in the image to get it in higher resolution, click here to see original plot.The particle with the mass of 3872 Gev was first observed by the BELLE collaboration in 2003 and is called X(3872). The observation of this particle at this early stage of data taking by LHCb confirms the excellent performance of the LHCb detector and data analysis.
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13 October 2010: LHCb control room in action
LHCb physicists discussing data taking.
click in the image to see other photos.-----------------------------------------------------------------------------------------------------------
24 September 2010: Young scientists at LHCb
The young scientists took part in the LHCb shifts during the European researchers’ night on Friday 24 September.
click in the image to see other photos.Read CERN Bulletin article in English and French, as well as the one in the Symmetry Breaking.
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6 September 2010: Beautiful atoms
The LHCb has observed beautiful atoms. The atoms are bound states of the beauty quark and anti-beauty quark. The atoms are bound by the strong force, the force which also binds quarks inside proton. The beautiful atom is 10 times heavier than the proton (yes, we can create mass from energy using famous Einstein formulae E=mc2), has a size sligtly smaller than the size of the proton but about 100 000 times smaller than the size of the hydrogen atom which is composed of a proton and an electron and is bound by the electromagnetic force. Just like ordinary atoms beauty and anti-beauty quarks form different quantum states with different angular momenta and different spin orientations (see figure below right). Only the states marked 1S, 2S and 3S are observed at LHCb by detecting their decay into a μ+ and μ- pair (left).
The figures above show the invariant mass of μ+ and μ- particles (left) and the schematic view of the beautifull atom quantum states (right),
click in images to get them in higher resolution. The invariant mass plot was made using all data taken in 2010, click here to see original plot.The beauty-anti-beauty atom, called "Upsilon" was discovered in 1977 at Fermilab near Chicago.
The states 1S, 2S and 3S do not decay into Beauty Particles since their mass is lower than the sum of masses of Beauty and anti-Beauty particles (BB threshold in the figure). On the other hand the state 4S does decay. This feature is used by the experiments BABAR and BELLE producing the 4S state at e+e- colliders as a source of Beauty and anti-Beauty particles.
The charm and anti_charm quarks form two bound atom states 1S and 2S called J/ψ and ψ' observed at LHCb through their decay into a μ+ and μ- pair (left) and a e+ and e- pair (right).
click in images to get them in higher resolution.-----------------------------------------------------------------------------------------------------------
22 July 2010: From a B to Z, LHCb explores the particle alphabet
LHCb has unveiled pictures of a Z boson inside the experiment. This boson is one of the best understood of all particle species. It shows us how the forces of electricity, magnetism and radiation are connected inside the Standard Model, our theory of particle physics. Measurements of how often we see Z bosons inside LHCb will provide a sensitive test of how well our theory describes this particle at the record breaking energies of the LHC.
click in images to get them in higher resolutionIn this picture the Z boson has decayed immediately to two muons μ, shown by the thick white lines which point to the green muon chamber hits in the outer circle of the Eolas display (described in 10 June 2010 News). Not much else happens inside LHCb when a Z is at work – only a few other particles are visible – and this makes it an easy particle to find. We’re looking forward to collecting more of them now, and really testing how well the Standard Model performs for us.
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12 July 2010: LHCb is younger!
The average age of the LHCb Collaboration members has strongly decreased after arrival of the LHCb summer students seen below in front of the Globe of Science and Innovation. The Summer Students follow the lectures given at the first floor of the Globe in the morning. During the breaks they can visit the new CERN exhibition "Universe of Particles" located on the ground floor and the rest of the time they make an important contribution to the LHCb data taking and data analysis.
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10 June 2010: The W boson in action
LHCb has taken its first snapshots of the W boson in action. This particle conveys the weak force, which makes certain forms of radioactivity possible. It is shown here having decayed to a muon μ (shown as a straight white line, pointing to the filled green muon detector hit circles in the 2D picture, and as a red line pointing to blue muon hits in the 3D picture), which we see, and a neutrino ν, which we don't, with very little else around it.
click in images to get them in higher resolutionEolas (gaelic for 'knowledge') is a 2D view of a collision inside LHCb. It is a transmogrified view, chosen to illustrate where particles deposit energy as they fly outwards from the collision point. The radius represents flight through the detector along the beam direction - through the tracking detectors, then the first muon chamber, then the electromagnetic and hadronic calorimeters, and finally the last four muon chambers. The φ angle represents the angle in the x,y direction perpendicular to the beam. Information is colour coded. Particle tracks are shown by the dashed lines. The transverse momentum of the particle is shown by the solid white long along this path - the higher this is, the longer the solid white bar is. Yellow bars show energy deposited in the electromagnetic calorimeter, cyan energy deposited in the hadronic calorimeter. Deposits in muon chambers are illustrated by green circles. If these are filled, they are associated with a particle track passing through them.
We will use samples of W bosons to test our theory of particle physics, the Standard Model, to high precision. This is exciting because we don't know yet if our theory holds at LHC energies - if it doesn't, if there are new particles to find in nature, we'll see W bosons behaving in a way we don't expect. With these first snapshots taken, we're on our way to finding out.
The W boson was discovered in 1983 at CERN by the UA1 and UA2 experiments giving the Nobel Prize to Carlo Rubbia and Simon van der Meer.
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7 May 2010: Strange Beauty and Charm
LHCb has reconstructed an event having all characteristics of a Strange Beauty Particle decay! A computer view of this event is shown below. The Strange Beauty Particle (called Bs) is composed of an anti-quark b (b is for beauty) and a quark s (s is for strange). It is produced by the collision of two 3.5 TeV protons from the LHC at a location marked as "PV" (Primary Vertex), together with many other particles (not shown). The Bs decays after travelling about 1.5 mm into three particles called μ-, Ds+ and neutrino ν at a place marked "SV" (Secondary Vertex). The ν is not detected since it can even traverse the whole Earth without any interaction. The Charm Particle Ds+ is composed of a c quark (c is for charm) and anti-quark s. The Ds+ particle decays in turn after travelling 6.5 mm into three long lived particles K+, K- and π+ in a place called "TV" (Tertiary Vertex). The K+, K- and π+ are traversing the LHCb detector where the tracking system is used to reconstruct their trajectories with such a very high precision that it is clear that the particles come from three different places called vertices.
click in image to get it in higher resolution-----------------------------------------------------------------------------------------------------------
21 April 2010: First reconstructed Beauty Particle
LHCb has reconstructed its first Beauty Particle! You can see below a computer view of this event in two projections (images on the left hand side). The Beauty Particle (called B+) is composed of an anti-quark b (that has a very short lifetime of 1.5 thousandth of a nanosecond!) and a quark u. It is produced by the collision of two very high energy protons from the LHC at a location marked as "Primary vertex", together with many other particles (shown in black). The B+ decays after travelling about 2mm into two particles (called J/ψ and K+) at a place marked "B decay vertex". The J/ψ particle decays in turn immediately into two long lived particles called μ+ and μ-. The μ+ , μ- and K+ are traversing the LHCb detector where the tracking system is used to reconstruct their trajectories with such a very high precision, that it is clear they do not come from the primary vertex. The fact that the reconstructed tracks do not cross exactly in two points reflects experimental precision of computer reconstruction. The real particle tracks originate at the two vertices. The images on the right hand side show the same event when the tracks from the "Primary vertex" are forced to come from the "Primary vertex".
click in images to get them in higher resolution
The LHCb physicists have collected about 10 million
proton-proton collisions in order to find this first Beauty Particle.
The reconstruction of each event is not easy, there are about 100
particle tracks reconstructed in this event, see full event display
below.
More details: LHCb physicists have calculated invariant mass of μ+ and μ- particles from the "B decay vertex" and found that it correspond to the J/ψ mass, see below invariant mass distribution
of all μ+ and μ- pairs with the peak corresponding to the J/ψ decays. The reconstructed invariant mass of J/ψ and K+ is 5.32 GeV, in agreement with to the known B+
mass, 5.5 times higher than the colliding proton mass
but 650 times smaller than the colliding proton energy (yes, we
can create mass from energy using famous Einstein formulae
E=mc2).
see comments in articles: NewScientist internet, magazine and ZDNet.
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