Search for high mass resonances with multi-lepton channels and study of timing performance of the CMS High Granular Calorimeter

This thesis takes place at a critical moment when all the data collected by the CMS experiment during the LHC Run-2 are available and when the new generation of detectors aiming to probe even further our knowledge of physics are being developed. In this context, this thesis is structured around two problematics, the first one concerns the study of the performance of a prototype of the CMS High Granularity Calorimeter (HGCAL). It is part of a vast campaign of upgrades for the future life phase of the LHC, also known as the High-Luminosity LHC, which should be in service by 2029. It aims to generate ten times more data than during the entire first phase of the LHC, in order to measure even more accurately the predictions of the Standard Model (SM) of particle physics, as well as to study rare phenomena that are not observable currently. The higher luminosity will also be associated with a significant increase in the number of simultaneous events and the radiation dose that the detectors will receive. The current detectors are not designed to operate in such an environment, so the endcap calorimeters of CMS will be replaced by HGCAL. It will be the first imaging calorimeter based on silicon sensors to be used. To determine its feasibility, a first prototype has been built and used in beam tests using positrons and pions with energies ranging from 20 to 300 GeV. In this thesis, the focus will be on the study of the temporal performance of the prototype, through the various stages of calibration and up to the calculation of the temporal resolution for a single HGCAL cell and for the whole electromagnetic or hadronic showers. The results obtained in this thesis are compared with those obtained from Monte Carlo simulations and the very good agreement between both confirms the viability of the HGCAL concept. The second problematic addressed in this thesis is about the search for high mass resonances using the multi-leptonic decay channel. Such a search is explained by the fact that the SM does not describe some phenomena, but other theories propose an explanation with the addition of new bosons. The aim of such an analysis is to see if there is a resonance with a mass between 200 GeV and 3 TeV that could correspond to a new type of scalar boson. For this purpose, the 138 fb^-1 of data collected by the CMS experiment during Run-2 are used in the H → ZZ → 4l decay channel. The interest in using this particular channel is that it corresponds to one of the dominant high-mass production modes and has a fully reconstructed final state with excellent resolution and good signal-to-noise ratio. Firstly, this thesis presents how the different objects constituting the events of interest are reconstructed and selected. Secondly, the construction of a signal model is done in such a way that any combination of the resonance parameters can be compared to the data. For this purpose, this modelling consists of a part describing the theoretical signal and another part that takes into account all the effects of the detector. In order to find out whether the experimental data can be described by the signal constructed in this thesis, a statistical test is performed by maximizing a likelihood function taking into account the signal, as well as the background and interference between both. As a result, limits on the expected signal cross-section are calculated and allow excluding or not a region of mass in which a high mass resonance can be found. (author)