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.

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 figure also shows SPD/PS components which were actually removed during upgrade 1 along with M1.

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]

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 ${\displaystyle \Xi _{\rm {cc}}^{++}}$ in 2017, being a first known baryon with two heavy quarks; and of the fully-charmed tetraquark ${\displaystyle \mathrm {T} _{\rm {cccc}}}$ 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	${\displaystyle {\rm {bud}}}$	${\displaystyle \Lambda _{\rm {b}}(5912)^{0}}$	Excited baryon	2012
2	${\displaystyle {\rm {bud}}}$	${\displaystyle \Lambda _{\rm {b}}(5920)^{0}}$	Excited baryon	2012
3	${\displaystyle {\rm {c{\bar {u}}}}}$	${\displaystyle {\rm {D_{J}(2580)^{0}}}}$	Excited meson	2013
4	${\displaystyle {\rm {c{\bar {u}}}}}$	${\displaystyle {\rm {D_{J}(2740)^{0}}}}$	Excited meson	2013
5	${\displaystyle {\rm {c{\bar {d}}}}}$	${\displaystyle {\rm {D_{J}^{*}(2760)^{+}}}}$	Excited meson	2013
6	${\displaystyle {\rm {c{\bar {u}}}}}$	${\displaystyle {\rm {D_{J}(3000)^{0}}}}$	Excited meson	2013
7	${\displaystyle {\rm {c{\bar {u}}}}}$	${\displaystyle {\rm {D_{J}^{*}(3000)^{0}}}}$	Excited meson	2013
8	${\displaystyle {\rm {c{\bar {d}}}}}$	${\displaystyle {\rm {D_{J}^{*}(3000)^{+}}}}$	Excited meson	2013
9	${\displaystyle {\rm {c{\bar {s}}}}}$	${\displaystyle {\rm {D_{s1}^{*}(2860)^{+}}}}$	Excited meson	2014
10	${\displaystyle {\rm {bsd}}}$	${\displaystyle \Xi _{\rm {b}}^{'-}}$	Excited baryon	2014
11	${\displaystyle {\rm {bsd}}}$	${\displaystyle \Xi _{\rm {b}}^{*-}}$	Excited baryon	2014
12	${\displaystyle {\rm {{\bar {b}}u}}}$	${\displaystyle {\rm {B_{J}(5840)^{+}}}}$	Excited meson	2015
13	${\displaystyle {\rm {{\bar {b}}d}}}$	${\displaystyle {\rm {B_{J}(5840)^{0}}}}$	Excited meson	2015
14	${\displaystyle {\rm {{\bar {b}}u}}}$	${\displaystyle {\rm {B_{J}(5970)^{+}}}}$	Excited meson	2015
15	${\displaystyle {\rm {{\bar {b}}d}}}$	${\displaystyle {\rm {B_{J}(5970)^{+}}}}$	Excited meson	2015
16[ii]	${\displaystyle {\rm {c{\bar {c}}uud}}}$	${\displaystyle {\rm {P_{c}(4380)^{+}}}}$	Pentaquark	2015
17	${\displaystyle {\rm {c{\bar {c}}s{\bar {s}}}}}$	${\displaystyle {\rm {X(4274)}}}$	Tetraquark	2016
18	${\displaystyle {\rm {c{\bar {c}}s{\bar {s}}}}}$	${\displaystyle {\rm {X(4500)}}}$	Tetraquark	2016
19	${\displaystyle {\rm {c{\bar {c}}s{\bar {s}}}}}$	${\displaystyle {\rm {X(4700)}}}$	Tetraquark	2016
20	${\displaystyle {\rm {c{\bar {u}}}}}$	${\displaystyle {\rm {D_{3}^{*}(2760)^{0}}}}$	Excited meson	2016
21	${\displaystyle {\rm {cud}}}$	${\displaystyle \Lambda _{\rm {c}}(2860)^{+}}$	Excited baryon	2017
22	${\displaystyle {\rm {css}}}$	${\displaystyle \Omega _{\rm {c}}(3000)^{0}}$	Excited baryon	2017
23	${\displaystyle {\rm {css}}}$	${\displaystyle \Omega _{\rm {c}}(3050)^{0}}$	Excited baryon	2017
24	${\displaystyle {\rm {css}}}$	${\displaystyle \Omega _{\rm {c}}(3066)^{0}}$	Excited baryon	2017
25	${\displaystyle {\rm {css}}}$	${\displaystyle \Omega _{\rm {c}}(3090)^{0}}$	Excited baryon	2017
26	${\displaystyle {\rm {css}}}$	${\displaystyle \Omega _{\rm {c}}(3119)^{0}}$	Excited baryon	2017
27[iii]	${\displaystyle {\rm {ccu}}}$	${\displaystyle \Xi _{\rm {cc}}^{++}}$	Baryon	2017
28	${\displaystyle {\rm {bsd}}}$	${\displaystyle \Xi _{\rm {b}}(6227)^{-}}$	Excited baryon	2018
29	${\displaystyle {\rm {buu}}}$	${\displaystyle \Sigma _{\rm {b}}(6097)^{+}}$	Excited baryon	2018
30	${\displaystyle {\rm {bdd}}}$	${\displaystyle \Sigma _{\rm {b}}(6097)^{-}}$	Excited baryon	2018
31	${\displaystyle {\rm {c{\bar {c}}}}}$	${\displaystyle \psi _{3}(3842)}$[19]	Excited meson	2019
32	${\displaystyle {\rm {c{\bar {c}}uud}}}$	${\displaystyle {\rm {P_{c}(4312)^{+}}}}$	Pentaquark	2019
33	${\displaystyle {\rm {c{\bar {c}}uud}}}$	${\displaystyle {\rm {P_{c}(4440)^{+}}}}$	Pentaquark	2019
34	${\displaystyle {\rm {c{\bar {c}}uud}}}$	${\displaystyle {\rm {P_{c}(4457)^{+}}}}$	Pentaquark	2019
35	${\displaystyle {\rm {bud}}}$	${\displaystyle \Lambda _{\rm {b}}(6146)^{0}}$	Excited baryon	2019
36	${\displaystyle {\rm {bud}}}$	${\displaystyle \Lambda _{\rm {b}}(6152)^{0}}$	Excited baryon	2019
37	${\displaystyle {\rm {bss}}}$	${\displaystyle \Omega _{\rm {b}}(6340)^{-}}$	Excited baryon	2020
38	${\displaystyle {\rm {bss}}}$	${\displaystyle \Omega _{\rm {b}}(6350)^{-}}$	Excited baryon	2020
39[iv]	${\displaystyle {\rm {bud}}}$	${\displaystyle \Lambda _{\rm {b}}(6070)^{0}}$	Excited baryon	2020
40	${\displaystyle {\rm {csd}}}$	${\displaystyle \Xi _{\rm {c}}(2923)^{0}}$	Excited baryon	2020
41	${\displaystyle {\rm {csd}}}$	${\displaystyle \Xi _{\rm {c}}(2939)^{0}}$	Excited baryon	2020
42[v]	${\displaystyle {\rm {cc{\bar {c}}{\bar {c}}}}}$	${\displaystyle {\rm {T_{cccc}}}}$	Tetraquark	2020
43[vi]	${\displaystyle {\rm {{\bar {c}}d{\bar {s}}u}}}$	${\displaystyle {\rm {X_{0}(2900)}}}$	Tetraquark	2020
44	${\displaystyle {\rm {{\bar {c}}d{\bar {s}}u}}}$	${\displaystyle {\rm {X_{1}(2900)}}}$	Tetraquark	2020
45	${\displaystyle {\rm {bsu}}}$	${\displaystyle \Xi _{\rm {b}}(6227)^{0}}$	Excited baryon	2020
46	${\displaystyle {\rm {{\bar {b}}s}}}$	${\displaystyle {\rm {B_{s}(6063)^{0}}}}$	Excited meson	2020
47	${\displaystyle {\rm {{\bar {b}}s}}}$	${\displaystyle {\rm {B_{s}(6114)^{0}}}}$	Excited meson	2020
48	${\displaystyle {\rm {c{\bar {s}}}}}$	${\displaystyle {\rm {D_{s0}(2590)^{+}}}}$	Excited meson	2020
49	${\displaystyle {\rm {c{\bar {c}}s{\bar {s}}}}}$	${\displaystyle {\rm {X(4630)}}}$	Tetraquark	2021
50	${\displaystyle {\rm {c{\bar {c}}s{\bar {s}}}}}$	${\displaystyle {\rm {X(4685)}}}$	Tetraquark	2021
51	${\displaystyle {\rm {c{\bar {c}}u{\bar {s}}}}}$	${\displaystyle {\rm {Z_{cs}(4000)^{+}}}}$	Tetraquark	2021
52	${\displaystyle {\rm {c{\bar {c}}u{\bar {s}}}}}$	${\displaystyle {\rm {Z_{cs}(4220)^{+}}}}$	Tetraquark	2021

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 ${\displaystyle \gamma \,\,(\alpha _{3})}$ 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 B_s mesons.^[23] The oscillation frequency of B_s 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 ${\displaystyle \mathrm {B} _{\rm {s}}^{0}\to \mu ^{+}\mu ^{-}}$, 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 ${\displaystyle \mathrm {P} _{5}^{'}}$ angular observable was found in the decay ${\displaystyle \mathrm {B} ^{0}\to \mathrm {K} ^{*0}\mu ^{+}\mu ^{-}}$, 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

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 ${\displaystyle \mathrm {B} ^{+}\to \mathrm {K} ^{+}\mu ^{+}\mu ^{-}}$ to that of ${\displaystyle \mathrm {B} ^{+}\to \mathrm {K} ^{+}\mathrm {e} ^{+}\mathrm {e} ^{-}}$,^[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 ${\displaystyle R_{\rm {K}}={\frac {{\mathcal {B}}(\mathrm {B} ^{+}\to \mathrm {K} ^{+}\mu ^{+}\mu ^{-})}{{\mathcal {B}}(\mathrm {B} ^{+}\to \mathrm {K} ^{+}\mathrm {e} ^{+}\mathrm {e} ^{-})}}}$, where symbol ${\displaystyle {\mathcal {B}}}$ denotes probability of a given decay to happen, was found to be ${\displaystyle 0.846_{-0.041}^{+0.044}}$ 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 ${\displaystyle b\rightarrow s\ell ^{+}\ell ^{-}}$ by the LHCb and semileptonic decays ${\displaystyle b\rightarrow s\ell \nu }$ by Belle II (with ${\displaystyle \ell =e,\mu }$) 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]