Cagnotta, Antimo (2024) Search for Vector-Like quark T single production in the tZ channel with a new Machine Learning approach for Top quark tagging with the CMS detector at LHC. [Tesi di dottorato]

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Abstract

My thesis project is centred on a signature-based study involving top quarks produced in association with invisible particles using proton-proton collisions with CMS data. The final state involves one top quark decaying hadronically and invisible particles that can be indirectly detected trough the missing transverse momentum of the event. The signature under study is predicted in many BSM theories, where the invisible particles could be neutrinos originated from Z in the tZ channel of a Vector-Like Quark T’. A crucial part of the work is the reconstruction of top quarks through hadronic jets. The reconstruction can be performed through three different topologies, making use of jets reconstructed by Anti-kt algorithms with different radii: several topologies in which the three quarks in the final state can be collimated generating different combinations of narrow/fat jets. A new approach to this problem was developed making use of Machine Learning algorithms, trained on the different kinds of reconstructed top quarks to improve signal efficiency and background rejection over all the possible range of transverse momentum of the top quark. A part of my research has been dedicated to the examination of the triple-GEM detectors utilized by the CMS experiment, in three different stations: GE1/1, GE2/1, and ME0. I am contributing to a comprehensive investigation of the current and voltage characteristics of CMS triple-GEM detectors under diverse operational conditions. This study is particularly focused on the analysis of electric discharges, which has been conducted in different experimental setups. These experiments have been conducted in both laboratory settings at CERN and directly within the CMS detector itself. The GE1/1 Triple-GEM station was subjected to magnetic field testing in order to establish a safety protocol for the station during the ramps up and down of the CMS magnet. Furthermore, I conducted a dedicated analysis on discharges and short circuits during the initial two years of commissioning in CMS. With regard to the GE2/1 demonstrator module installed in CMS, I conducted monitoring during 2022 and 2023 from the standpoint of high-voltage considerations. This entailed the observation of potential issues that could arise during operation, including the possibility of inefficiency in the detector. The tests on ME0 were conducted to ascertain the readiness of the high-voltage system designed for the new station, with a particular focus on the new CAEN board. Furthermore, a dedicated study of the high-voltage filters is currently underway, employing test beams that simulate the anticipated high background rate for this station.

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