Tracking (particle physics)

In <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a>, tracking
 is the process of reconstructing the trajectory (or track) of <a href="/facts/Charged_particle/2zqBJGse">electrically charged particles</a> in a <a href="/facts/Particle_detector/rYj0qKtA">particle detector</a> 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 <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a>, 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) <a href="/facts/Electric_charge/xxsBfmlB">electric charge</a> 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 <a href="/facts/Curve_fitting/zrLKw6Xj">fitting a curve</a> 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.) <a href="/facts/Kalman_Filter/wNK7rnbk">Kalman Filter</a> 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 (<a href="/facts/Calorimeter_(particle_physics)/FCA2evfS">Calorimeter</a>, <a href="/facts/Cherenkov_Detector/ef5z6j24">Cherenkov Detector</a>). These reconstructed charged particles can be used to identify and reconstruct secondary <a href="/facts/Particle_decay/3koAz34P">decays</a>, including those arising from 'unseen' neutral particles, as can be done for <a href="/facts/B-tagging/P36ynqcd">B-tagging</a> (in experiments like <a href="/facts/Collider_Detector_at_Fermilab/HcqPvu9s">CDF</a> or at the <a href="/facts/Large_Hadron_Collider/XWJCFSqT">LHC</a>) and to fully reconstruct events (as in many current particle physics experiments, such as <a href="/facts/Atlas_experiment/hqWY5WQD">ATLAS</a>, <a href="/facts/BaBar_experiment/qh6z0s5M">BaBar</a>, <a href="/facts/Belle_experiment/hor4Nxnp">Belle</a> and <a href="/facts/Compact_Muon_Solenoid/zrdCWwVk">CMS</a>).
In particle physics there have been many devices used for tracking. These include <a href="/facts/Cloud_chamber/89YSNbpN">cloud chambers</a> (1920–1950), <a href="/facts/Nuclear_emulsion/yQGYX5hQ">nuclear emulsion</a> plates (1937–), <a href="/facts/Bubble_chamber/awVVhk4G">bubble chambers</a> (1952–), <a href="/facts/Spark_chamber/2n40vcHl">spark chambers</a> (1954-), <a href="/facts/Wire_chambers/BKstxtE9">multi wire proportional chambers</a> (1968–) and <a href="/facts/Wire_chamber/BKstxtE9">drift chambers</a> (1971–), including <a href="/facts/Time_projection_chamber/4ysP0jL1">time projection chambers</a> (1974–). With the advent of <a href="/facts/Semiconductor/zFf3mGTE">semiconductors</a> plus modern <a href="/facts/Photolithography/FTHsauvy">photolithography</a>, solid state trackers, also called <a href="/facts/Semiconductor_detector/HzvpIobR">silicon trackers</a> (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 <a href="/facts/LHC/XWJCFSqT">LHC</a>.

Tracking (particle physics) open-in-new

Tracking (particle physics)