Study of beyond the Standard Model processes in leptonic final states with the CMS detector at the Run II of the LHC

The work presented in this thesis explores high energy physics in final states with two or more leptons (multileptonic). Data taken by the CMS experiment, corresponding to the Run 2 of the LHC, is used to explore both the physics of the Standard Model of Particle Physics (SM), and possible models of new physics beyond the Standard Model (BSM). The electroweak sector of the SM is first probed through the study of processes involving the production of two massive gauge bosons (WW, WZ, and ZZ). The first measurements of the cross sections for all three processes at a center-of-mass energy of 5.02 TeV are presented, leading to the first observation of WW production at this energy regime. A detailed set of studies on WZ production at a center of mass energy of 13 TeV is also discussed. The results are lead by the most precise measurement of the WZ production cross section, with an overall relative uncertainty of 4%. The charge asymmetry in WZ production, including an interpretation in terms of parton distribution function constrainings, is measured at the 2.5% precision level. Estimations of the boson polarization fractions for both the W and Z bosons lead to the first observation of longitudinally polarized W bosons in diboson production. A search for BSM physics modelled through effective field theory (EFT) operators lead to constraints in five different dimension six components. All measurements are confirmed through the production of detector-free differential cross section measurements with respect to all relevant high level observables. The Yukawa coupling of the top quark is studied through the ttH production mechanism. A dedicated single variable-based signal extraction criteria has been developed to optimize the sensitivity of the measurement while keeping the result easily reinterpretable. The results correspond to evidence for the ttH production mode with a significance of 4 statistical deviations. A final set of measurements corresponding to searches for electroweak supersymmetry (SUSY) in multileptonic final states is also performed. An exhaustive strategy is followed in which twelve final states consisting on different lepton flavours, charges and multiplicities are explored in detail. Both a simple signal region (SR), approach based on single variable selection requirements, and a multivariate approach based on parametric neural networks (PNN) are designed to look for BSM physics. While no significant presence is found, limits are set in SUSY particle masses and production cross sections through the usage of simplified models (SMS). Depending on the model assumptions, these limits are up to the most stringent in chargino and neutralino masses up to date. In order to prepare for the next operating years of the LHC, several improvements developed for the Phase II upgrade of the CMS detector are introduced. The different algorithms presented correspond to different upgrades to the current L1 trigger strategy so it can cope with the harsher running conditions of the High Luminosity LHC with equal or better performance that the current ones did for the Run 2.