Wing Sheung Chan

32 The Large Hadron Collider and the ATLAS detector 2.2. The ATLAS detector The ATLAS detector [67] is one of the two general-purpose detectors at the LHC. It was designed to measure various parameters of the SM and search for BSM phenomena by probing pp or heavy-ion collisions at the LHC. The idea is to detect, identify and reconstruct particles produced in pp collisions, and use those particles to reconstruct the collision events. To achieve that, the detector must be able to (1) track particles to determine where they came from and in what directions they were travelling, (2) measure the energies or momenta of the particles and (3) produce differential responses to different types of particles in order to identify them. Equipped with a number of subsystems, each specialised in a certain type of measurement, the ATLAS detector is able to accomplish all of the above. On top of these, the detector also has to be efficient, meaning that it should collect as much useful data as possible for a given integrated luminosity delivered by the LHC. Therefore, the ATLAS detector is designed to cover a solid angle of almost 4 π to capture as many particles as possible, and have a fast and efficient trigger and read-out system to record as many useful events as possible. 2.2.1. Coordinate system A coordinate system is defined and used consistently in the ATLAS experiment. The origin of the coordinate system is defined to be the nominal collision point. The Cartesian positive x -axis points toward the centre of the LHC ring while the positive y -axis points vertically upward. The z -axis is the beam axis and its direction is defined such that the coordinate system is right-handed. The x – y plane is referred to as the transverse plane. Components of a spatial vector in the transverse plane are denoted by a subscript T as in p T of vector p . Because of the symmetry of the detector as well as the expected symmetry of the physical events, cylindrical and spherical coordinates are also often used. As in common mathematical conventions, the azimuthal angle is denoted by φ and is measured from the positive x -axis on the transverse plane, while the polar angle is denoted by θ and is measured from the positive z -axis. The radius in cylindrical coordinates is denoted by R . The pseudorapidity η is conventionally used in place of θ and is a monotonic transformation of θ : η = − ln tan θ 2 . (2.6) The magnitude of a spatial vector p and its components can be related by the pseudorapidity with the formulae tanh η = p z | p | (2.7)