Wing Sheung Chan

Chapter 3. Object reconstruction and identification In the fields of observation chance favours only the prepared mind. — Louis Pasteur The ATLAS detector is an incredibly convoluted machine with many components and literally millions of data readout channels. When particles traverse the detector, the traces they leave are recorded as nothing but a bunch of disjointed digital signals. Without proper processing, these raw data will not be of much use. Therefore, the first step of any physics analysis in the ATLAS experiment is to convert these raw data into meaningful “physics objects”. In view that these are common efforts shared by all physics analyses, the reconstruction of SM particles are harmonised within the collaboration. The algorithms that are used to reconstruct and identify physics objects are products of collaborative work, and are developed, maintained and constantly improved by various dedicated working groups. In this chapter, we will introduce the algorithms for reconstructing and identifying objects that are relevant to the search for LFV Z → `τ decays. Particularly, attention will be given to the reconstruction and identification of hadronic tau decays, which are crucial for the search and are work that the author has actively and proudly contributed to. 3.1. Jets and flavour tagging Given that the LHC is a hadron collider, gluons and quarks can be found in almost every collision event. The hadronisations of these gluons and quarks produce showers of particles that travel in directions close to that of the original particles, also known as jets. By reconstructing these jets, we can learn about the kinematic properties of the particles that initiated them, and sometimes even their flavour. 45