Study of the operation and trigger performance of GEM detectors in the CMS experiment

At the end of 2018, the CERN Large Hadron Collider (LHC) accelerator started, in concert with its experiments, an upgrade campaign to reach the goal of the High-Luminosity LHC project: proton-proton collisions at a center-of-mass energy √s = 14TeV with an instantaneous luminosity around 5 − 7 · 10^34 cm−2s−1, aiming to deliver a 3000 fb−1 integrated luminosity. To cope with these new experimental conditions, the CMS experiment started, among others, an upgrading campaign of its muon system, programming the installation of three new stations using the Gas Electron Multiplier (GEM) technology: GE1/1, GE2/1 and ME0. The motivations of the installation of these stations are to increase the redundancy in the CMS muon system, to keep the trigger rate under control, to have radiation hard detectors in the CMS forward region, and to improve the detection of physics channels at high pseudorapidity (η), extending the angular coverage of the muon system to |η| < 2.8. This thesis was developed in the framework of the GEM upgrade, focusing, in particular, on the production and commissioning of the GE1/1 station and on the development of trigger algorithms for the Phase 2 upgrade, exploiting the possibilities offered by the foreseen GEM detectors. The first chapter presents the LHC accelerator and the CMS experiment, describing its foreseen upgrades. The second chapter introduces the GEM upgrade, then it moves to a detailed description of the GE1/1 station, and finally presents the GE2/1 and ME0 stations, whose installation is foreseen between 2023 and 2026. The third chapter describes the activities I have performed in the production and the validation process of GE1/1 chambers, before their installation in the CMS experiment. The second part of this chapter describes the activities I carried out during the commissioning in the experimental site, focusing on the monitoring of the power systems and on the study of HV trips in different experimental conditions, for example during the commissioning of the CMS magnet and in the early LHC collisions performed at the end of the Long Shutdown 2 period. The fourth chapter illustrates a trigger study I have carried out, dedicated to the τ → 3μ decay channel, a Lepton Flavour Violating decay with a branching ratio heavily suppressed in the Standard Model and brought in a statistically significant region by some Beyond the Standard Model models. This decay is characterised by a multi-muon final state, with low transverse momentum muons and collimated in the forward region. Due to these features, the research carried out on the τ → 3μ channel would turn out to be useful also for other channels with similar characteristics. In the discussion some trigger paths of interest, that exploit the possibilities introduced by the installation of the GEM stations and by other CMS Phase 2 upgrades, are presented.