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Large Hadron Collider
(LHC)
Plan of the LHC experiments and the preaccelerators.
LHC experiments
ATLASA Toroidal LHC Apparatus
CMSCompact Muon Solenoid
LHCbLHC-beauty
ALICEA Large Ion Collider Experiment
TOTEMTotal Cross Section, Elastic Scattering and Diffraction Dissociation
LHCfLHC-forward
MoEDALMonopole and Exotics Detector At the LHC
FASERForwArd Search ExpeRiment
SNDScattering and Neutrino Detector
LHC preaccelerators
p and PbLinear accelerators for protons (Linac 4) and lead (Linac 3)
(not marked)Proton Synchrotron Booster
PSProton Synchrotron
SPSSuper Proton Synchrotron

The LHCb (Large Hadron Collider beauty) experiment is a particle physics detector experiment collecting data at the Large Hadron Collider at CERN.[1] LHCb is a specialized b-physics experiment, designed primarily to measure the parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark). Such studies can help to explain the matter-antimatter asymmetry of the Universe. The detector is also able to perform measurements of production cross sections, exotic hadron spectroscopy, charm physics and electroweak physics in the forward region. The LHCb collaborators, who built, operate and analyse data from the experiment, are composed of approximately 1650 people from 98 scientific institutes, representing 22 countries.[2] Vincenzo Vagnoni[3] succeeded on July 1, 2023 as spokesperson for the collaboration from Chris Parkes (spokesperson 2020–2023).[4] The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The (small) MoEDAL experiment shares the same cavern.

Physics goals

The experiment has wide physics program covering many important aspects of heavy flavour (both beauty and charm), electroweak and quantum chromodynamics (QCD) physics. Six key measurements have been identified involving B mesons. These are described in a roadmap document[5] that formed the core physics programme for the first high energy LHC running in 2010–2012. They include:

  • Measuring the branching ratio of the rare Bs → μ+ μ− decay.
  • Measuring the forward-backward asymmetry of the muon pair in the flavour-changing neutral current Bd → K* μ+ μ− decay. Such a flavour changing neutral current cannot occur at tree-level in the Standard Model of Particle Physics, and only occurs through box and loop Feynman diagrams; properties of the decay can be strongly modified by new physics.
  • Measuring the CP violating phase in the decay Bs → J/ψ φ, caused by interference between the decays with and without Bs oscillations. This phase is one of the CP observables with the smallest theoretical uncertainty in the Standard Model, and can be significantly modified by new physics.
  • Measuring properties of radiative B decays, i.e. B meson decays with photons in the final states. Specifically, these are again flavour-changing neutral current decays.
  • Tree-level determination of the unitarity triangle angle γ.
  • Charmless charged two-body B decays.

The LHCb detector

The fact that the two b-hadrons are predominantly produced in the same forward cone is exploited in the layout of the LHCb detector. The LHCb detector is a single arm forward spectrometer with a polar angular coverage from 10 to 300 milliradians (mrad) in the horizontal and 250 mrad in the vertical plane. The asymmetry between the horizontal and vertical plane is determined by a large dipole magnet with the main field component in the vertical direction.

The LHCb collaboration's logo

LHCb detector along the bending plane

Subsystems

The Vertex Locator (VELO) is built around the proton interaction region.[6][7] It is used to measure the particle trajectories close to the interaction point in order to precisely separate primary and secondary vertices.

The detector operates at 7 millimetres (0.28 in) from the LHC beam. This implies an enormous flux of particles; VELO has been designed to withstand integrated fluences of more than 1014 p/cm2 per year for a period of about three years. The detector operates in vacuum and is cooled to approximately −25 °C (−13 °F) using a biphase CO2 system. The data of the VELO detector are amplified and read out by the Beetle ASIC.

VELO
  • The vertex locator (VELO) during construction.
    The vertex locator (VELO) during construction.
  • One element of VELO
    One element of VELO

The RICH-1 detector (Ring imaging Cherenkov detector) is located directly after the vertex detector. It is used for particle identification of low-momentum tracks.

The main tracking system is placed before and after the dipole magnet. It is used to reconstruct the trajectories of charged particles and to measure their momenta. The tracker consists of three subdetectors:

  • The Tracker Turicensis, a silicon strip detector located before the LHCb dipole magnet
  • The Outer Tracker. A straw-tube based detector located after the dipole magnet covering the outer part of the detector acceptance
  • The Inner Tracker, silicon strip based detector located after the dipole magnet covering the inner part of the detector acceptance

Following the tracking system is RICH-2. It allows the identification of the particle type of high-momentum tracks.

The electromagnetic and hadronic calorimeters provide measurements of the energy of electrons, photons, and hadrons. These measurements are used at trigger level to identify the particles with large transverse momentum (high-Pt particles).

The muon system is used to identify and trigger on muons in the events.

LHCb upgrade (2019–2021)

At the end of 2018, the LHC was shut down for upgrades, with a restart currently planned for early 2022. For the LHCb detector, almost all subdetectors are to be modernised or replaced.[8] It will get a fully new tracking system composed of a modernised vertex locator, upstream tracker (UT) and scintillator fibre tracker (SciFi). The RICH detectors will also be updated, as well as the whole detector electronics. However, the most important change is the switch to the fully software trigger of the experiment, which means that every recorded collision will be analysed by sophisticated software programmes without an intermediate hardware filtering step (which was found to be a bottleneck in the past).[9]

Results

During the 2011 proton-proton run, LHCb recorded an integrated luminosity of 1 fb−1 at a collision energy of 7 TeV. In 2012, about 2 fb−1 was collected at an energy of 8 TeV.[10] During 2015–2018 (Run 2 of the LHC), about 6 fb−1 was collected at a center-of-mass energy of 13 TeV. In addition, small samples were collected in proton-lead, lead-lead, and xenon-xenon collisions. The LHCb design also allowed the study of collisions of particle beams with a gas (helium or neon) injected inside the VELO volume, making it similar to a fixed-target experiment; this setup is usually referred to as "SMOG".[11] These datasets allow the collaboration to carry out the physics programme of precision Standard Model tests with many additional measurements. As of 2021, LHCb has published more than 500 scientific papers.[12]

Hadron spectroscopy

LHCb is designed to study beauty and charm hadrons. In addition to precision studies of the known particles such as mysterious X(3872), a number of new hadrons have been discovered by the experiment. As of 2021, all four LHC experiments have discovered about 60 new hadrons in total, vast majority of which by LHCb.[13] In 2015, analysis of the decay of bottom lambda baryons (Λ0
b) in the LHCb experiment revealed the apparent existence of pentaquarks,[14][15] in what was described as an "accidental" discovery.[16] Other notable discoveries are those of the "doubly charmed" baryon in 2017, being a first known baryon with two heavy quarks; and of the fully-charmed tetraquark in 2020, made of two charm quarks and two charm antiquarks.

Hadrons discovered at LHCb.[17][18] The term 'excited' for baryons and mesons means existence of a state of lower mass with the same quark content and isospin.
Quark content[i] Particle name Type Year of discovery
1 Excited baryon 2012
2 Excited baryon 2012
3 Excited meson 2013
4 Excited meson 2013
5 Excited meson 2013
6 Excited meson 2013
7 Excited meson 2013
8 Excited meson 2013
9 Excited meson 2014
10 Excited baryon 2014
11 Excited baryon 2014
12 Excited meson 2015
13 Excited meson 2015
14 Excited meson 2015
15 Excited meson 2015
16[ii] Pentaquark 2015
17 Tetraquark 2016
18 Tetraquark 2016
19 Tetraquark 2016
20 Excited meson 2016
21 Excited baryon 2017
22 Excited baryon 2017
23 Excited baryon 2017
24 Excited baryon 2017
25 Excited baryon 2017
26 Excited baryon 2017
27[iii] Baryon 2017
28 Excited baryon 2018
29 Excited baryon 2018
30 Excited baryon 2018
31 [19] Excited meson 2019
32 Pentaquark 2019
33 Pentaquark 2019
34 Pentaquark 2019
35 Excited baryon 2019
36 Excited baryon 2019
37 Excited baryon 2020
38 Excited baryon 2020
39[iv] Excited baryon 2020
40 Excited baryon 2020
41 Excited baryon 2020
42[v] Tetraquark 2020
43[vi] Tetraquark 2020
44 Tetraquark 2020
45 Excited baryon 2020
46 Excited meson 2020
47 Excited meson 2020
48 Excited meson 2020
49 Tetraquark 2021
50 Tetraquark 2021
51 Tetraquark 2021
52 Tetraquark 2021
  1. ^ Abbreviations are the first letter of the quark name (up='u', down='d', top='t', bottom='b', charmed='c', strange='s'). Antiquarks have overbars.
  2. ^ Previously unknown combination of quarks
  3. ^ Previously unknown combination of quarks; first baryon with two charm quarks, and the only weakly-decaying particle discovered so far at the LHC.
  4. ^ Simultaneous with CMS; CMS had not enough data to claim the discovery.
  5. ^ Previously unknown combination of quarks; first tetraquark made exclusively of charm quarks
  6. ^ Previously unknown combination of quarks; first tetraquark with all quarks being different

CP violation and mixing

Studies of charge-parity (CP) violation in B-meson decays is the primary design goal of the LHCb experiment. As of 2021, LHCb measurements confirm with a remarkable precision the picture described by the CKM unitarity triangle. The angle of the unitarity triangle is now known to about 4°, and is in agreement with indirect determinations.[20]

In 2019, LHCb announced discovery of CP violation in decays of charm mesons.[21] This is the first time CP violation is seen in decays of particles other than kaons or B mesons. The rate of the observed CP asymmetry is at the upper edge of existing theoretical predictions, which triggered some interest among particle theorists regarding possible impact of physics beyond the Standard Model.[22]

In 2020, LHCb announced discovery of time-dependent CP violation in decays of Bs mesons.[23] The oscillation frequency of Bs mesons to its antiparticle and vice versa was measured to a great precision in 2021.

Rare decays

Rare decays are the decay modes harshly suppressed in the Standard Model, which makes them sensitive to potential effects from yet unknown physics mechanisms.

In 2014, LHCb and CMS experiments published a joint paper in Nature announcing the discovery of the very rare decay , rate of which was found close to the Standard Model predictions.[24] This measurement has harshly limited the possible parameter space of supersymmetry theories, which have predicted a large enhancement in rate. Since then, LHCb has published several papers with more precise measurements in this decay mode.

Anomalies were found in several rare decays of B mesons. The most famous example in the so-called angular observable was found in the decay , where the deviation between the data and theoretical prediction has persisted for years.[25] The decay rates of several rare decays also differ from the theoretical predictions, though the latter have sizeable uncertainties.

Lepton flavour universality

See also: Lepton § Universality

In the Standard Model, couplings of charged leptons (electron, muon and tau lepton) to the gauge bosons are expected to be identical, with the only difference emerging from the lepton masses. This postulate is referred to as "lepton flavour universality". As a consequence, in decays of b hadrons, electrons and muons should be produced at similar rates, and the small difference due to the lepton masses is precisely calculable.

LHCb has found deviations from this predictions by comparing the rate of the decay to that of ,[26] and in similar processes.[27][28] However, as the decays in question are very rare, a larger dataset needs to be analysed in order to make definitive conclusions.

In March 2021, LHCb announced that the anomaly in lepton universality crossed the "3 sigma" statistical significance threshold, which translates to a p-value of 0.1%.[29] The measured value of , where symbol denotes probability of a given decay to happen, was found to be while the Standard Model predicts it to be very close to unity.[30] In December 2022 improved measurements discarded this anomaly.[31][32][33]

In August 2023 joined searches in leptonic decays by the LHCb and semileptonic decays by Belle II (with ) set new limits for universality violations. [31][32][34][35]

Other measurements

LHCb has contributed to studies of quantum chromodynamics, electroweak physics, and provided cross-section measurements for astroparticle physics.[36]

See also

References

  1. ^ Belyaev, I.; Carboni, G.; Harnew, N.; Teubert, C. Matteuzzi F. (2021-01-13). "The history of LHCB". The European Physical Journal H. 46 (1): 3. arXiv:2101.05331. Bibcode:2021EPJH...46....3B. doi:10.1140/epjh/s13129-021-00002-z. S2CID 231603240.
  2. ^ "LHCb Organization".
  3. ^ LHCb collaboration (2023-07-05). "New management for the LHCb collaboration in 2023". CERN. Retrieved 2024-02-05.
  4. ^ "New spokesperson for the LHCb collaboration". LHCb, CERN. Retrieved 2024-02-05.
  5. ^ B. Adeva et al (LHCb collaboration) (2009). "Roadmap for selected key measurements of LHCb". arXiv:0912.4179 [hep-ex].
  6. ^ [1] Archived 2016-03-03 at the Wayback Machine, The LHCb VELO (from the VELO group)
  7. ^ [2], VELO Public Pages
  8. ^ "Transforming LHCb: What's in store for the next two years?". CERN. Retrieved 2021-03-21.
  9. ^ "Allen initiative – supported by CERN openlab – key to LHCb trigger upgrade". CERN. Retrieved 2021-03-21.
  10. ^ "Luminosities Run1". Retrieved 14 Dec 2017., 2012 LHC Luminosity Plots
  11. ^ "New SMOG on the horizon". CERN Courier. 2020-05-08. Retrieved 2021-03-21.
  12. ^ "LHCb - Large Hadron Collider beauty experiment". lhcb-public.web.cern.ch. Retrieved 2021-03-21.
  13. ^ "59 new hadrons and counting". CERN. Retrieved 2021-03-21.
  14. ^ "Observation of particles composed of five quarks, pentaquark-charmonium states, seen in Λ0
    b    →J/ψpK− decays"    . CERN/LHCb. 14 July 2015. Retrieved 2015-07-14.
  15. ^ R. Aaij et al. (LHCb collaboration) (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ0
    b    →J/ψK−p decays". Physical Review Letters. 115 (7): 072001. arXiv:1507.03414. Bibcode:2015PhRvL.115g2001A. doi:10.1103/PhysRevLett.115.072001. PMID 26317714. S2CID 119204136.
  16. ^ G. Amit (14 July 2015). "Pentaquark discovery at LHC shows long-sought new form of matter". New Scientist. Retrieved 2015-07-14.
  17. ^ "New particles discovered at the LHC". www.nikhef.nl. Retrieved 2021-03-21.
  18. ^ "Observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner".
  19. ^ "pdgLive". pdglive.lbl.gov. Retrieved 2021-03-21.
  20. ^ The LHCb Collaboration, ed. (2020). Updated LHCb combination of the CKM angle γ.
  21. ^ "LHCb observes CP violation in charm decays". CERN Courier. 2019-05-07. Retrieved 2021-03-21.
  22. ^ Dery, Avital; Nir, Yosef (December 2019). "Implications of the LHCb discovery of CP violation in charm decays". Journal of High Energy Physics. 2019 (12): 104. arXiv:1909.11242. Bibcode:2019JHEP...12..104D. doi:10.1007/JHEP12(2019)104. ISSN 1029-8479. S2CID 202750063.
  23. ^ "LHCb sees new form of matter–antimatter asymmetry in strange beauty particles". CERN. Retrieved 2021-03-21.
  24. ^ Khachatryan, V.; Sirunyan, A.M.; Tumasyan, A.; Adam, W.; Bergauer, T.; Dragicevic, M.; Erö, J.; Friedl, M.; Frühwirth, R.; Ghete, V.M.; Hartl, C. (June 2015). "Observation of the rare B s 0 → μ + μ − decay from the combined analysis of CMS and LHCb data". Nature. 522 (7554): 68–72. doi:10.1038/nature14474. hdl:2445/195036. ISSN 1476-4687. PMID 26047778. S2CID 4394036.
  25. ^ "New LHCb analysis still sees previous intriguing results". CERN. Retrieved 2021-03-21.
  26. ^ "How universal is (lepton) universality?". CERN. Retrieved 2021-03-21.
  27. ^ "LHCb explores the beauty of lepton universality". CERN. Retrieved 2021-03-21.
  28. ^ "LHCb tests lepton universality in new channels". CERN Courier. 2021-10-19. Retrieved 2021-10-27.
  29. ^ "Intriguing new result from the LHCb experiment at CERN". CERN. Retrieved 2021-03-23.
  30. ^ LHCb collaboration; Aaij, R.; Beteta, C. Abellán; Ackernley, T.; Adeva, B.; Adinolfi, M.; Afsharnia, H.; Aidala, C. A.; Aiola, S.; Ajaltouni, Z.; Akar, S. (22 March 2022). "Test of lepton universality in beauty-quark decays". Nature Physics. 18 (3): 277–282. arXiv:2103.11769. Bibcode:2022NatPh..18..277L. doi:10.1038/s41567-021-01478-8. ISSN 1745-2473. S2CID 232307581.
  31. ^ a b LHCb collaboration (2023). "Test of Lepton Universality in b → s ℓ+ ℓ− decays". Physical Review Letters. 131 (5): 051803. arXiv:2212.09152. doi:10.1103/PhysRevLett.131.051803. PMID 37595222. S2CID 254854814.
  32. ^ a b LHCb collaboration (2023). "Measurement of lepton universality parameters in B+ → K+ ℓ+ ℓ− and B0→K∗0ℓ+ℓ− decays". Physical Review D. 108 (3): 032002. arXiv:2212.09153. doi:10.1103/PhysRevD.108.032002. S2CID 254853936.
  33. ^ "Improved lepton universality measurements show agreement with the Standard Model". Retrieved 2023-01-08.
  34. ^ Belle II Collaboration; Aggarwal, L.; Ahmed, H.; Aihara, H.; Akopov, N.; Aloisio, A.; Anh Ky, N.; Asner, D. M.; Atmacan, H.; Aushev, T.; Aushev, V.; Bae, H.; Bahinipati, S.; Bambade, P.; Banerjee, Sw. (2023-08-02). "Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic $B$-Meson Decays at Belle II". Physical Review Letters. 131 (5): 051804. arXiv:2301.08266. Bibcode:2023PhRvL.131e1804A. doi:10.1103/PhysRevLett.131.051804. PMID 37595249. S2CID 256080428.
  35. ^ Wright, Katherine (2023-08-02). "Standard Model Stays Strong for Leptons". Physics. 16 (5): s91. arXiv:2301.08266. Bibcode:2023PhRvL.131e1804A. doi:10.1103/PhysRevLett.131.051804. PMID 37595249. S2CID 256080428.
  36. ^ Fontana, Marianna (2017-10-19). "LHCb inputs to astroparticle physics". Proceedings of the European Physical Society Conference on High Energy Physics. Vol. 314. Venice, Italy: Sissa Medialab. p. 832. doi:10.22323/1.314.0832.
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