Search for the associated production of a single top quark and a Z or Higgs boson with the CMS experiment

This thesis presents 4 different studies based on the data collected by the CMS experiment during the Run 2 of the LHC. Firstly, a search for the associated production of a single top quark and a Z boson (tZq) is conducted. The measurement of the cross section of this rare process probes Standard Model (SM) couplings, and is sensitive to Beyond the Standard Model (BSM) processes. This search uses data collected in 2016 and targets the trilepton final state. As part of this thesis, the analysis is carried out from the event selection up to the final results. A particular emphasis is put on the signal extraction, using advanced techniques, and on the data-driven estimation of reducible backgrounds. The first evidence for the tZq process is presented. The second analysis targets the associated production of a single top quark and a Higgs boson (tH) in multilepton final states. It is the only process sensitive at leading order both to the magnitude and sign of the top quark Yukawa coupling, which may provide insights on both SM and BSM physics. The entire analysis is performed, using 2017 data. Notably, constraints are placed on the value of the top Yukawa, and on the tH cross section. In addition, the analysis is re-adapted to search for top-Higgs FCNC processes with similar final states. The most stringent upper limits to date are set for these processes. The last study aims at monitoring the ageing of the silicon strip tracker of CMS with irradiation. This detector is subjected to high levels of radiation, which is expected to change the properties of its sensors over time. Dedicated bias scans were regularly analysed to monitor the evolution of the important full depletion voltage parameter. The algorithm extracting the relevant information from the data was improved, and the analysis was extended to the entire subdetector.