LHCb

Large Hadron Collider
(LHC)
	Plan of the LHC experiments and the preaccelerators.
LHC experiments
ATLAS	A Toroidal LHC Apparatus
CMS	Compact Muon Solenoid
LHCb	LHC-beauty
ALICE	A Large Ion Collider Experiment
TOTEM	Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf	LHC-forward
MoEDAL	Monopole and Exotics Detector At the LHC
FASER	ForwArd Search ExpeRiment
SND	Scattering and Neutrino Detector
LHC preaccelerators
p and Pb	Linear accelerators for protons (Linac 4) and lead (Linac 3)
(not marked)	Proton Synchrotron Booster
PS	Proton Synchrotron
SPS	Super Proton Synchrotron

46°14′28″N 06°05′49″E / 46.24111°N 6.09694°E / 46.24111; 6.09694

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 B_s → μ⁺ μ⁻ decay.
Measuring the forward-backward asymmetry of the muon pair in the flavour-changing neutral current B_d → 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 B_s → J/ψ φ, caused by interference between the decays with and without B_s 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

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.
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 $\Xi _{\rm {cc}}^{++}$ in 2017, being a first known baryon with two heavy quarks; and of the fully-charmed tetraquark $\mathrm {T} _{\rm {cccc}}$ 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	${\rm {bud}}$	$\Lambda _{\rm {b}}(5912)^{0}$	Excited baryon	2012
2	${\rm {bud}}$	$\Lambda _{\rm {b}}(5920)^{0}$	Excited baryon	2012
3	${\rm {c{\bar {u}}}}$	${\rm {D_{J}(2580)^{0}}}$	Excited meson	2013
4	${\rm {c{\bar {u}}}}$	${\rm {D_{J}(2740)^{0}}}$	Excited meson	2013
5	${\rm {c{\bar {d}}}}$	${\rm {D_{J}^{*}(2760)^{+}}}$	Excited meson	2013
6	${\rm {c{\bar {u}}}}$	Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle \rm D_J(3000)^0}	Excited meson	2013
7	${\rm {c{\bar {u}}}}$	${\rm {D_{J}^{*}(3000)^{0}}}$	Excited meson	2013
8	${\rm {c{\bar {d}}}}$	${\rm {D_{J}^{*}(3000)^{+}}}$	Excited meson	2013
9	${\rm {c{\bar {s}}}}$	${\rm {D_{s1}^{*}(2860)^{+}}}$	Excited meson	2014
10	${\rm {bsd}}$	$\Xi _{\rm {b}}^{'-}$	Excited baryon	2014
11	${\rm {bsd}}$	$\Xi _{\rm {b}}^{*-}$	Excited baryon	2014
12	${\rm {{\bar {b}}u}}$	${\rm {B_{J}(5840)^{+}}}$

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