Wing Sheung Chan

The Large Hadron Collider and the ATLAS detector 33 and cosh η = p T | p | . (2.8) In the ultrarelativistic limit, the pseudorapidity of a particle converges to the rapidity y † : y = 1 2 ln E + p z E − p z , (2.9) where E is the energy of the particle. Differences in rapidity are Lorentz invariant under boosts in the z -direction, and thus transform additively. The distance in the rapidity– azimuth space ∆ R ‡ : ∆ R = q (∆ y ) 2 + (∆ φ ) 2 (2.10) is often used to quantify the directional proximity of two measured particle tracks. 2.2.2. Overview of the detector The ATLAS detector is a massive machine made up of layers of detector components that surround the collision point (Figure 2.3) . The entire detector is 25m high and 44m long and weighs approximately 7000 t . Components of the detector are arranged in a cylindrical manner, with the axis of symmetry being the beam axis. The detector has a forward-backward reflectional symmetry and a nominal eight-fold azimuthal symmetry. The ATLAS detector consists of three main detector subsystems: the inner detector, the calorimeters and the muon spectrometer. The inner detector is the innermost layer of the detector and its mission is to trace charged particles that traverse it. Outside of the inner detector are the calorimeters. The calorimeters measure the energies of particles when the particles interact with them. The outermost part of the detector is the muon spectrometer, which is designed to detect muons and measure their momenta. Besides the three detector subsystems, the ATLAS detector is also equipped with a magnet system that bends charged particle trajectories, which helps the identification and measurement of these particles. The subsystems, which will be discussed in greater detail in the following subsections, are designed to work together to reconstruct and identify particles in the following way: Particles produced by the collisions are first met by the inner detector. If the particles are charged, they interact with the inner detector and leave “hits” while traversing it. These hits can then be used to reconstruct the trajectories, or “tracks”, of the particles. Alone, a track is useful in determining the travelling direction of an individual charged particle. Together, tracks can be used to reconstruct the “primary vertex”, the precise position where † Not to be confused with the Cartesian y -coordinate, even though they conventionally, and inconve- niently, share the same symbol. ‡ Not to be confused with changes or differences in R , the radius in cylindrical coordinates.