Search for heavy neutrinos with the CMS experiment and studies for the upgrade of its electromagnetic calorimeter

In this thesis, the search for heavy right-handed neutrinos through the decay of a right-handed $W$ ($W_R$) boson into two leptons and two jets with the CMS experiment at the Large Hadron Collider is presented. The $W_R$ boson decays into a lepton and a massive neutrino ($N_R$), and the neutrino decays into another lepton of the same flavor and into a pair of jets. The data collected by the CMS experiment in proton-proton collisions at $13$ TeV during 2016 and corresponding to a total integrated luminosity of $35.9$ fb$^{-1}$ were used for this analysis. These heavy neutrinos are missing in the Standard Model (SM) of particle physics and thus they are important to shed light on physics beyond the SM. My main contributions were the simulation and validation of the signal samples, the estimation of one of the main backgrounds, the production of the $1$D and $2$D limits on the $W_R$ cross section (as a function of the $W_R$ mass and of the $W_R$ mass versus $N_R$ mass), and the systematics and statistical analysis. The results obtained with this analysis give the most stringent limits to date. Studies on the upgrade of the CMS electromagnetic calorimeter for the LHC Phase II, that foresees a partial replacement of the electronics in the barrel, are also presented. My main contributions were in the preparation of the setup and in the data taking of test beams performed at the H4 beam line in the SPS North Area of CERN, whose goal was to study the performance of the prototype for the new Very-Front-End boards (CATIA chips with 160 MHz ADC), and in studying the shapes and noise of the signals collected by a matrix of $5\times 5$ crystals calculating their timing and energy resolution. The document starts with a theoretical introduction about the Standard Model and the physics beyond the Standard Model, in particular on heavy neutrinos, followed by a description of the CMS detector and its electromagnetic calorimeter. The upgrade studies on the electromagnetic calorimeter are presented in a dedicated chapter, then the analysis on the search for heavy neutrinos and an overview on the overall neutrino physics at the LHC and other experiments are described.