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 to many analyses at particle detectors. 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. A charged particle loses energy in matter by ionization 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 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 light, and thus is efficient only for particles with a small Lorentz factor. 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 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.