Tracking (particle physics)

In particle physics, tracking is the process of reconstructing the trajectory (or track) of electrically charged particles in a particle detector known as a tracker. The particles entering such a tracker leave a precise record of their passage through the device, by interaction with suitably constructed components and materials. The presence of a calibrated magnetic field, in all or part of the tracker, allows the local momentum of the charged particle to be directly determined from the reconstructed local curvature of the trajectory for known (or assumed) electric charge of the particle.

Generally, track reconstruction is divided into two stages. First, track finding needs to be performed where a cluster of detector hits believed to originate from the same track are grouped together. Second, a track fitting is performed. Track fitting is the procedure of mathematically fitting a curve to the found hits and from this fit the momentum is obtained.

Identification and reconstruction of trajectories from the digitised output of a modern tracker can, in the simplest cases, in the absence of a magnetic field and absorbing/scattering material, be achieved via straight-line segment fits. A simple helical model, to determine momentum in the presence of a magnetic field, might be sufficient in less simple cases, through to a complete (e.g.) Kalman Filter process, to provide a detailed reconstructed local model throughout the complete track in the most complex cases.

This reconstruction of trajectory plus momentum allows projection to/through other detectors, which measure other important properties of the particle such as energy or particle type (Calorimeter, Cherenkov Detector). These reconstructed charged particles can be used to identify and reconstruct secondary decays, including those arising from 'unseen' neutral particles, as can be done for B-tagging (in experiments like CDF or at the LHC) and to fully reconstruct events (as in many current particle physics experiments, such as ATLAS, BaBar, Belle and CMS).

In particle physics there have been many devices used for tracking. These include cloud chambers (1920–1950), nuclear emulsion plates (1937–), bubble chambers (1952–), spark chambers (1954-), multi wire proportional chambers (1968–) and drift chambers (1971–), including time projection chambers (1974–). With the advent of semiconductors plus modern photolithography, solid state trackers, also called silicon trackers (1980–), are used in experiments requiring compact, high-precision, fast-readout tracking; for example, close to the primary interaction point in a collider like the LHC.