Particle identification is the process of using information left by a

particle In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

passing through a

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 ionizing elementary particle, particles, such as t ...

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's mass, and therefore its identity.

Specific ionization

A charged particle loses energy in matter by

ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...

at a rate determined in part by its velocity. The energy loss per unit distance is typically called . 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 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, the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

, and thus is efficient only for particles with a small

Lorentz factor The Lorentz factor or Lorentz term (also known as the gamma factor) is a dimensionless quantity expressing how much the measurements of time, length, and other physical properties change for an object while it moves. The expression appears in sev ...

Cherenkov detectors

Cherenkov radiation is emitted by a charged particle when it passes through a material with a speed greater than , where is the index of refraction of the material. The angle of the photons with respect to the charged particle's direction depends on velocity. A number of Cherenkov detector geometries have been used.

Photons

Photon 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 particles that can ...

s are identified because they leave all their energy in a detector's

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

calorimeter A calorimeter is a device used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimeters ...

, but do not appear in the tracking chamber (see, for example, ATLAS Inner Detector) because they are neutral. A neutral

pion In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (letter), pi (), is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the ...

which decays inside the EM calorimeter can replicate this effect.

Electrons

Electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s appear as tracks 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

Muon 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 ''ħ'', but with a much greater mass. It is classified as a ...

s penetrate more material than other charged particles, and can therefore be identified by their presence in the outermost detectors.

Tau particles

Tau Tau (; uppercase Τ, lowercase τ or \boldsymbol\tau; ) is the nineteenth letter of the Greek alphabet, representing the voiceless alveolar plosive, voiceless dental or alveolar plosive . In the system of Greek numerals, it has a value of 300 ...

identification requires differentiating the narrow "jet" produced by the hadronic decay of the tau from ordinary

quark A quark () is a type of 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 nucleus, atomic nuclei ...

jets.

Neutrinos

Neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...

s 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

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

s can sometimes be identified in calorimeters. In particular, antineutrons and Ks can be identified. Neutral hadrons can also be identified at electron-positron colliders in the same way as neutrinos.

Heavy quarks

Quark A quark () is a type of 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 nucleus, atomic nuclei ...

flavor tagging identifies the

flavor Flavour or flavor is either the sensory perception of taste or smell, or a flavoring in food that produces such perception. Flavour or flavor may also refer to: Science * Flavors (programming language), an early object-oriented extension to L ...

of quark that a jet comes from.

B-tagging b-tagging is a method of particle jet, jet flavor tagging used in modern particle physics experiments. It is the identification (or "tagging") of jets originating from bottom quarks (or b quarks, hence the name). Importance b-tagging is important ...

, the identification of

bottom quark The bottom quark, beauty quark, or b quark, is an elementary particle of the third generation. It is a heavy quark with a charge of − ''e''. All quarks are described in a similar way by electroweak interaction and quantum chromodynamic ...

s, is the most important example. B-tagging relies on the bottom quark being the heaviest quark involved in a hadronic decay (

tops Total Operations Processing System (TOPS) is a computer system for managing railway locomotives and rolling stock, known for many years of use in the United Kingdom. TOPS was originally developed between the Southern Pacific Railroad (SP), ...

are heavier, but to have a top in a decay, it is necessary to produce some ''heavier'' particle to have a subsequent decay into a top). This implies that the bottom quark has a short lifetime and it 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 or Charms may refer to: Arts and entertainment * The Charms, an American garage rock band * Otis Williams and the Charms, an American doo-wop group * The Charm (Bubba Sparxxx album), ''The Charm'' (Bubba Sparxxx album), 2006 * Charm (Danny! ...

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.

References

{{DEFAULTSORT:Particle Identification Experimental particle physics