Wing Sheung Chan

The Large Hadron Collider and the ATLAS detector 35 Figure 2.4.: A schematic illustration showing a sector of a cross section of the ATLAS detector and how different particles are typically detected [69] . In Run 2, the LHC delivered pp collisions at a rate of 40 million bunch crossings per second. With such a rate, it is practically impossible to process and store all of the data collected. Because of this, the ATLAS detector has a trigger system that decides what data are necessary for analysis and should therefore be kept for further processing, while discarding the rest to save the data processors and storage from being flooded by the overwhelming amount of data. 2.2.3. The inner detector The inner detector (ID) is the innermost subsystem that records trajectories of charged particles as “hits”, which are then reconstructed as “tracks”. The ID is placed closest to the collisions, so that it can precisely determine the interaction vertices. Since the main purpose of the ID is not to measure the energies of particles, it is designed to work with a minimal amount of material that particles can interact with, so that the energies of the particles are mostly unaltered when they leave the ID and enter the calorimeters. The ID consists of four subdetectors: the insertable b -layer (IBL), the pixel detector (Pixels), the semiconductor tracker (SCT) and the transition radiation tracker (TRT). Cutaway views of the ID are illustrated in Figure 2.5.