Response of the high granularity calorimeter HGCAL and characterisation of the Higgs boson with the CMS experiment at the LHC

This thesis presents a comprehensive characterisation of the properties of the Higgs boson in the four-lepton decay channel exploiting proton-proton collisions collected at a centre-of-mass energy of 13 TeV with the CMS experiment at the CERN LHC. The dataset analyzed corresponds to the 137 fb$^{-1}$ statistics collected during the Run-II (2015-2016). The analysis results in the first CMS measurement of the Stage 1.2 Simplified Template Cross Sections (STXS), for a total of nineteen cross sections defined in mutually exclusive phase space regions. The inclusive signal strength modifier, defined as the ratio of the Higgs boson production cross section to the corresponding SM prediction, is found to be $\mu$=0.94$\pm$0.07(stat)$\pm$0.09(syst) at $m_\mathrm{H}$=125.38 GeV. The signal strength modifers of each production mechanism are also measured. Fiducial and differential cross sections are also measured, at generator level, after unfolding the detector effects, in a volume defined to closely match the experimental acceptance. Limits on the trilinear self-coupling of the Higgs boson are extracted for the first time in a CMS single-Higgs analysis using differential information. Additional studies of CP-violation and anomalous couplings of the Higgs boson to vector bosons and fermions are also presented. All the results are found to be in agreement with the SM predictions within their uncertainties. Given the remarkable predictive power of the SM when including the H boson, possible new physics will require even more extensive studies at higher statistics. A massive upgrade of the detectors is necessary to maintain the current physics performance in the harsh environment of the High-Luminosity LHC (HL-LHC) project, expected to start by the end of 2027. The CMS Collaboration will replace the current endcap calorimeters with a High Granularity Calorimeter (HGCAL). With its 6 millions individual readout channels, the HGCAL will be the very first large-scale silicon-based imaging calorimeter ever employed in a high energy physics experiment. This thesis presents the results of the analysis of the test beam data collected with the first large-scale prototype of the HGCAL when exposed to positron beams with energies ranging from 20 to 300~GeV. All the results are compared with a dedicated Monte Carlo simulation and an excellent agreement are found for all the observables studied, thus corroborating the final design of the HGCAL and the nominal physics performance expected for the HL-LHC operations.