Performance of the GE1/1 detectors for the upgrade of the CMS muon forward system

"The CERN Large Hadron Collider (LHC) will undergo major upgrades over the course of the decade that will lead to the High Luminosity LHC. The proton-proton collisions will reach a center-of-mass energy of 14 TeV, with an ultimate instantaneous luminosity of $7.5 \times 10^{34}$ cm$^{-2}$s$^{-1}$ delivered to each of the large experiments. The foreseen total integrated luminosity after ten years of operations is $4000$ fb$^{-1}$. While this allows for high-precision studies and potential new discoveries, it also imposes upgrades to the experiments in order to sustain the higher particle fluxes and the larger radiation doses. This thesis focuses on the Gas Electron Multipliers (GEM) detectors that will contribute to the performance enhancement of the forward regions of the Compact Muon Solenoid (CMS) experiment. Triple-GEM chambers will be installed in three stations of the CMS endcaps with the aim of ensuring redundancy and increased coverage of the muon system. The pilot project of the CMS muon system upgrade is the first GEM station (GE1/1), which was installed between July 2019 and September 2020 and will begin operation in the LHC Run 3 in 2022. The GE1/1 is meant to complement the present Cathode Strip Chambers (ME1/1) in order to improve the muon track reconstruction and to reduce the trigger rate, thanks to the suppression of fake candidates. The main subject of the thesis is the GE1/1 detector validation, which ensured their conformity to the established requirements prior to the installation. An analysis of the chamber signal amplification has been conducted with a focus on the uniformity across their $\mathcal{O}(0.5$ m$^2)$ surface area. This revealed mild gas gain non-uniformities, mostly concentrated around the central part. The high voltage stability has been verified and improved thanks to a dedicated training. The frontend electronics has also undergone an in-depth study that established its optimal configuration. This led to a reduction of the applied frontend thresholds of about $30\%$. Subsequently, the detection performance has been thoroughly investigated with cosmic ray muons. The chambers were characterized in terms of detection efficiency, signal spatial extension and spatial resolution. The attained results confirmed an average efficiency plateau of $96-97\%$, an average signal width of $600-700$ µm and a spatial resolution of $150-200$ µrad at the efficiency plateau. Each of the qualification tests will be detailed in the thesis, as well as the encountered issues and the developed solutions. Finally, an outlook on the GE1/1 commissioning activities will give an insight on the steps for a full integration of the GEM subsystem within the CMS environment."