Wing Sheung Chan

38 The Large Hadron Collider and the ATLAS detector straws in the barrel region are 144 cm long and are parallel to the beam direction, while the straws in the end-cap region are 37 cm long and are arranged radially in wheels. Although the TRT does not provide as much spatial information as the precision tracking detectors, it provides a valuable particle identification capability. Since the probability for a particle to emit transition radiation depends on the relativistic Lorentz factor, ultrarelativistic particles such as electrons leave drastically more hits in the TRT than heavier particles such as pions. The information provided by the TRT is important in, for example, distinguishing electrons from hadronically decaying τ leptons. 2.2.4. The calorimeters The ATLAS detector is built with a set of calorimeters surrounding the ID. The main mission of the calorimeters is to trigger particles to produce showers and subsequently measure their energy and structure. The calorimeters in ATLAS can be divided into the electromagnetic calorimeter (EMCal) and the hadronic calorimeters (HCal), which are designed to measure and contain EM showers and hadronic showers respectively. Figure 2.6 shows a cutaway view of the calorimeters. Figure 2.6.: A computer-generated cutaway view of the ATLAS calorimeters [67] . The electromagnetic calorimeter Electromagnetic (EM) showers are produced by electrons or photons via bremsstrahlung and e + e − pair production. EM showers typically develop faster than hadronic showers and