Particle identification
Encyclopedia
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 to many analyses at particle detectors.
at a rate determined in part by its velocity. The energy loss per unit distance is typically called dE/dx. The energy loss is measured either in dedicated detectors, or in tracking chambers designed to also measure energy loss. The energy lost in a thin layer of material is subject to large fluctuations, and therefore accurate dE/dx determination requires a large number of measurements. Individual measurements in the low and high energy tails are excluded.
, and thus is efficient only for particles with a small Lorentz factor
.
s are identified because they leave all their energy in a detector's electromagnetic
calorimeter, but do not appear in the tracking chamber (see, for example, ATLAS Inner Detector) because they are neutral. A neutral pion
which decays inside the EM calorimeter can replicate this effect.
s appear as a track in the inner detector and deposit all their energy in the electromagnetic calorimeter. The energy deposited in the calorimeter must match the momentum measured in the tracking chamber.
jets.
flavor tagging identifies the flavor
of quark a jet
comes from. B-tagging
, the identification of bottom quark
s, is the most important example.
B-tagging relies on the b quark being the heaviest quark involved in a hadronic decay (tops are heavier but to have a top in a decay is necessary to produce some heavier particle to have a subsequent decay into a top). This implies that the b quark has a short lifetime and is possible to look for its decay vertex in the inner tracker. Additionally, its decay products are transversal to the beam, resulting in a high jet multiplicity. Charm
tagging using similar techniques is also possible, but extremely difficult due to the lower mass.
Tagging jets from lighter quarks is simply impossible, due to QCD background there are simply too many indistinguishable jets.
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
passing through a particle detector
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...
to identify the type of particle. Particle identification reduces backgrounds and improves measurement resolutions, and is essential to many analyses at particle detectors.
Charged particles
Charged particles have been identified using a variety of techniques. All methods rely on a measurement of the momentum in a tracking chamber combined with a measurement of the velocity to determine the charged particle mass, and therefore its identity.Specific ionization
A charged particle loses energy in matter by ionizationIonization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
at a rate determined in part by its velocity. The energy loss per unit distance is typically called dE/dx. The energy loss is measured either in dedicated detectors, or in tracking chambers designed to also measure energy loss. The energy lost in a thin layer of material is subject to large fluctuations, and therefore accurate dE/dx determination requires a large number of measurements. Individual measurements in the low and high energy tails are excluded.
Time of flight
Time of flight detectors determine charged particle velocity by measuring the time required to travel from the interaction point to the time of flight detector, or between two detectors. The ability to distinguish particle types diminishes as the particle velocity approaches its maximum allowed value, speed of lightSpeed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, and thus is efficient only for particles with a small Lorentz factor
Lorentz factor
The Lorentz factor or Lorentz term appears in several equations in special relativity, including time dilation, length contraction, and the relativistic mass formula. Because of its ubiquity, physicists generally represent it with the shorthand symbol γ . It gets its name from its earlier...
.
Cherenkov detectors
Cherenkov radiation is emitted by a charged particle when it passes through a material with a speed greater than c/n, where n is the index of refraction of the material. The angle of the photons with respect to the charged particle direction depends on velocity. A number of Cherenkov detector geometries have been used.Photons
PhotonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s are identified because they leave all their energy in a detector's electromagnetic
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
calorimeter, but do not appear in the tracking chamber (see, for example, ATLAS Inner Detector) because they are neutral. A neutral pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
which decays inside the EM calorimeter can replicate this effect.
Electrons
ElectronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s appear as a track in the inner detector and deposit all their energy in the electromagnetic calorimeter. The energy deposited in the calorimeter must match the momentum measured in the tracking chamber.
Muons
Muons penetrate more material than other charged particles, and can therefore be identified by their presence in the outermost detectors.Tau particles
Tau identification requires differentiating the narrow "jet" produced by the hadronic decay of the tau from ordinary quarkQuark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
jets.
Neutrinos
Neutrinos do not interact in particle detectors, and therefore escape undetected. Their presence can be inferred by the momentum imbalance of the visible particles in an event. In electron-positron colliders, both the neutrino momentum in all three dimensions and the neutrino energy can be reconstructed. Neutrino energy reconstruction requires accurate charged particle identification. In colliders using hadrons, only the momentum transverse to the beam direction can be determined.Neutral hadrons
Neutral hadrons can sometimes be identified in calorimeters. In particular, antineutrons and KL0s can be identified. Neutral hadrons can also be identified at electron-positron colliders in the same way as neutrinos.Heavy quarks
QuarkQuark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
flavor tagging identifies the flavor
Flavour (particle physics)
In particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics, flavour is a global symmetry...
of quark a jet
Jet (particle physics)
A jet is a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics or heavy ion experiment. Because of QCD confinement, particles carrying a color charge, such as quarks, cannot exist in free form. Therefore they fragment into hadrons...
comes from. B-tagging
B-tagging
b-tagging is an example of a jet flavor tagging method used in modern high-energy particle physics experiments. It is the identification of jets originating from bottom quarks .-Importance:...
, the identification of bottom quark
Bottom quark
The bottom quark, also known as the beauty quark, is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass , combined with low values of the CKM matrix elements Vub and Vcb, gives it a...
s, is the most important example.
B-tagging relies on the b quark being the heaviest quark involved in a hadronic decay (tops are heavier but to have a top in a decay is necessary to produce some heavier particle to have a subsequent decay into a top). This implies that the b quark has a short lifetime and is possible to look for its decay vertex in the inner tracker. Additionally, its decay products are transversal to the beam, resulting in a high jet multiplicity. Charm
Charm quark
The charm quark or c quark is the third most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks...
tagging using similar techniques is also possible, but extremely difficult due to the lower mass.
Tagging jets from lighter quarks is simply impossible, due to QCD background there are simply too many indistinguishable jets.