Measurements of Higgs boson properties in the four-lepton final state at $\sqrt{s}$ = 13 TeV with the CMS experiment at the LHC

This thesis reports a study of Higgs boson production in proton-proton (pp) collisions at $\sqrt{s}=13~{\rm TeV}$ recorded with the CMS detector at the CERN Large Hadron Collider (LHC), exploiting the decay channel into a pair of Z bosons that in turn decay into pairs of electrons or muons $(H\rightarrow{\rm Z}{\rm Z}\rightarrow4\ell$, $\ell={\rm e},\mu$). This work is carried out in the context of the beginning of Run II of the LHC, a new data-taking period that started in 2015, following a two-year shutdown. This restart is marked by an increase of the centre-of-mass energy from 8 TeV to 13 TeV and an upgrade of collider parameters that increases the luminosity, setting new constraints on the triggering, reconstruction and analysis of pp collision events. Therefore, considerable effort is devoted to the improvement and reoptimization of the CMS trigger system for Run II, focusing on the reconstruction and selection of electrons and on the preparation of multilepton trigger paths that preserve a maximal efficiency for the $H\rightarrow{\rm Z}{\rm Z}\rightarrow4\ell$ channel. Secondly, the offline algorithms for electron and muon selection are optimized and their efficiencies are measured in data, while the selection logic of four-lepton candidates is improved. In order to extract rare production modes of the Higgs boson such as vector boson fusion, VH associated production, and $t\bar{t}H$ associated production, a new classification of selected events into exclusive categories is introduced, using discriminants based on matrix-element calculations and jet flavour tagging. Results of the analysis of first 13 TeV data are presented for two data sets recorded in 2015 and early 2016, corresponding to integrated luminosities of $2.8~{\rm fb}^{-1}$ and $12.9~{\rm fb}^{-1}$, respectively. A standalone rediscovery of the Higgs boson in the four-lepton channel is achieved at the new energy. The signal strength relative to the standard model prediction, the mass and decay width of the boson, and a set of parameters describing the contributions of its main predicted production modes are measured. All results are in good agreement with standard model expectations for a 125 GeV Higgs boson within the uncertainties, which are dominated by their statistical component with the current data set. Finally, a search for an additional high-mass resonance decaying to four leptons is performed, and no significant excess is observed.