The LHCb (Large Hadron Collider beauty) experiment is one of eight particle physics detector experiments collecting data at the
Large Hadron Collider
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundr ...
at
CERN
The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gen ...
. LHCb is a specialized
b-physics experiment, designed primarily to measure the parameters of
CP violation
In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics should be t ...
in the interactions of b-
hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
s (heavy particles containing a
bottom quark
The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of − ''e''.
All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exce ...
). 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
Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just tw ...
spectroscopy,
charm
Charm may refer to:
Social science
* Charisma, a person or thing's pronounced ability to attract others
* Superficial charm, flattery, telling people what they want to hear
Science and technology
* Charm quark, a type of elementary particle
* Ch ...
physics and
electroweak
In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
physics in the forward region. The LHCb collaboration, who built, operate and analyse data from the experiment, is composed of approximately 1260 people from 74 scientific institutes, representing 16 countries.
Chris Parkes succeeded on July 1, 2020 as spokesperson for the collaboration to
Giovanni Passaleva (spokesperson 2017-2020). The experiment is located at point 8 on the LHC tunnel close to
Ferney-Voltaire
Ferney-Voltaire () is a commune in the Ain department in the Auvergne-Rhône-Alpes region of eastern France. It lies between the Jura Mountains and the Swiss border; it forms part of the metropolitan area of Geneva.
History
Ferney was first no ...
,
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
just over the border from
Geneva
Geneva ( ; french: Genève ) frp, Genèva ; german: link=no, Genf ; it, Ginevra ; rm, Genevra is the second-most populous city in Switzerland (after Zürich) and the most populous city of Romandy, the French-speaking part of Switzerland. Situa ...
. The (small)
MoEDAL experiment
MoEDAL (Monopole and Exotics Detector at the LHC) is a particle physics experiment at the Large Hadron Collider (LHC).
Experiment
MoEDAL shares the cavern at Point 8 with LHCb, and its prime goal is to directly search for the magnetic monopole ( ...
shares the same cavern.
Physics goals
The experiment has wide physics program covering many important aspects of heavy flavour (both
beauty
Beauty is commonly described as a feature of objects that makes these objects pleasurable to perceive. Such objects include landscapes, sunsets, humans and works of art. Beauty, together with art and taste, is the main subject of aesthetics, o ...
and charm), electroweak and
quantum chromodynamics
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...
(QCD) physics. Six key measurements have been identified involving B mesons. These are described in a roadmap document 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
The Standard Model of particle physics is the theory describing three of the four known fundamental forces ( electromagnetic, weak and strong interactions - excluding gravity) in the universe and classifying all known elementary particles. It ...
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
Bs oscillations. This phase is one of the CP observables with the smallest theoretical uncertainty in the
Standard Model
The Standard Model of particle physics is the theory describing three of the four known fundamental forces ( electromagnetic, weak and strong interactions - excluding gravity) in the universe and classifying all known elementary particles. It ...
, 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
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
with a polar angular coverage from 10 to 300
milliradian
A milliradian ( SI-symbol mrad, sometimes also abbreviated mil) is an SI derived unit for angular measurement which is defined as a thousandth of a radian (0.001 radian). Milliradians are used in adjustment of firearm sights by adjusting t ...
s (mrad) in the horizontal and 250 mrad in the vertical plane. The
asymmetry
Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
between the horizontal and vertical plane is determined by a large
dipole magnet
A dipole magnet is the simplest type of magnet. It has two poles, one north and one south. Its magnetic field lines form simple closed loops which emerge from the north pole, re-enter at the south pole, then pass through the body of the magnet. ...
with the main field component in the vertical direction.
Subsystems
The Vertex Locator (VELO) is built around the proton interaction region. 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 from the LHC beam. This implies an enormous flux of particles; VELO has been designed to withstand integrated fluences of more than 10
14 p/cm
2 per year for a period of about three years. The detector operates in
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
and is cooled to approximately 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 The ring-imaging Cherenkov, or RICH, detector is a device for identifying the type of an electrically charged subatomic particle of known momentum, that traverses a transparent refractive medium, by measurement of the presence and characteristics ...
) is located directly after the vertex detector. It is used for
particle identification
Particle identification is the process of using information left by a particle passing through a particle detector to identify the type of particle. Particle identification reduces backgrounds and improves measurement resolutions, and is essential ...
of low-
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...
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
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
and
hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
ic
calorimeters provide measurements of the
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
of
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
,
photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...
, and
hadrons
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...
. 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
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
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. 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).
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. 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". 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.
Hadron spectroscopy
LHCb is designed to study beauty and charm
hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
s. In addition to precision studies of the known particles such as mysterious
X(3872) The X(3872) is an exotic meson candidate with a mass of 3871.68 MeV/c2 which does not fit into the quark model because of its quantum numbers. It was first discovered in 2003 by the Belle experiment in Japan and later confirmed by several other ex ...
, 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. In 2015, analysis of the decay of
bottom lambda baryon
The lambda baryons (Λ) are a family of subatomic hadron particles containing one up quark, one down quark, and a third quark from a higher flavour generation, in a combination where the quantum wave function changes sign upon the flavour of an ...
s (Λ) in the LHCb experiment revealed the apparent existence of
pentaquark
A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them.
As quarks have a baryo ...
s,
[
][
] in what was described as an "accidental" discovery.
[
] Other notable discoveries are those of the "doubly charmed" baryon
in 2017, being a first known
baryon
In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classifie ...
with two heavy quarks; and of the fully-charmed tetraquark
in 2020, made of two charm quarks and two charm antiquarks.
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.
In 2019, LHCb announced discovery of CP violation in decays of charm mesons. 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.
In 2020, LHCb announced discovery of time-dependent CP violation in decays of B
s mesons. 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
CMS may refer to:
Computing
* Call management system
* CMS-2 (programming language), used by the United States Navy
* Code Morphing Software, a technology used by Transmeta
* Collection management system for a museum collection
* Color manage ...
experiments published a joint paper in
Nature
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
announcing the discovery of the very rare decay
, rate of which was found close to the Standard Model predictions. 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. 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
lepton
In particle physics, a lepton is an elementary particle of half-integer spin (spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...
s (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
, and in similar processes. 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
Sigma (; uppercase Σ, lowercase σ, lowercase in word-final position ς; grc-gre, σίγμα) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used a ...
"
statistical significance
In statistical hypothesis testing, a result has statistical significance when it is very unlikely to have occurred given the null hypothesis (simply by chance alone). More precisely, a study's defined significance level, denoted by \alpha, is the p ...
threshold, which translates to a
p-value
In null-hypothesis significance testing, the ''p''-value is the probability of obtaining test results at least as extreme as the result actually observed, under the assumption that the null hypothesis is correct. A very small ''p''-value means ...
of 0.1%. 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.
Other measurements
LHCb has contributed to studies of quantum chromodynamics, electroweak physics, and provided cross-section measurements for astroparticle physics.
See also
*
B-factory In particle physics, a B-factory, or sometimes a beauty factory, is a particle collider experiment designed to produce and detect a large number of B mesons so that their properties and behavior can be measured with small statistical uncertainty. T ...
*
Belle II experiment
The BelleII experiment is a particle physics experiment designed to study the properties of B mesons (heavy particles containing a beauty quark) and other particles. BelleII is the successor to the Belle experiment, and commissioned at the Super ...
References
External links
*
LHCb Public WebpageLHCb section from US/LHC Website* (Full design documentation)
LHCb experimentrecord on
INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 19 ...
{{CERN
CERN experiments
Particle experiments
Large Hadron Collider
B physics