Search for new particles decaying into vector boson pairs at LHC with the CMS detector and prospects for the vector boson scattering at the future High Luminosity LHC

This thesis presents a three year work inside the Compact Muon Solenoid (CMS) collaboration, mainly focused on data analysis from proton-proton collisions collected by the Large Hadron Collider (LHC) at a centre of mass energy of 8 TeV (LHC Run-I). My efforts were dedicated to searches for heavy particles decaying into di-boson pairs exploring boosted topologies, where the fragmentation products of hadronically decaying vector bosons overlap in the detector, preventing their identification as resolved jets. Several benchmark scenarios for new physics have been considered, from heavy Standard Model (SM) like Higgs boson to beyond the SM (BSM) extensions of the Higgs sector and massive gravitons predicted by Warped Extra Dimension (WED) models. In order to explore these boosted signatures, I developed specific algorithms to identify boosted vector bosons decaying hadronically (V-jets), which are also applied in other searches for new physics characterized high p_{T} jets originating from W/Z-boson, Higgs boson or top quark decays. These algorithms have been improved and new methods have been tested on detailed simulation, reproducing the conditions of the present data taking (LHC Run-II), and finally commissioned with first 13 TeV collisions. In particular, I performed a semi-leptonic WW search for heavy resonances with 8 TeV data, as well as the combination with other exclusive searches in di-boson final states. With no significant excess observed above the predicted background yield, upper limits have been produced in the context of massive gravitons predicted by Bulk and Randall-Sundrum WED models. Eventually, a model independent re-interpretation of the analysis is also performed. In addition, I developed also a search for SM-like Higgs boson at high mass, in the boosted semi-leptonic WW final state. The analysis is categorised in exclusive jet bins to disentangle the gluon fusion production from the vector boson fusion mode. The result is also interpreted in the context of a BSM heavy Higgs scenario, called electroweak singlet model, which predicts the existence of two Higgs scalars with the mass of the lighter one being around 125 GeV. Upper limits on these models are set through an unbinned fit to the di-boson invariant mass spectrum, where a deviation with a local significance of about 2.5 $\sigma$ has been observed in the VBF category. Eventually, I performed feasibility studies for the Vector Boson Scattering (VBS) in view of the CMS detector upgrade, foreseen for the high luminosity LHC (HL-LHC) data taking. If no evidence for new physics will be observed with Run-II collisions, this rare process would become crucial to search for BSM physics via precision electroweak measurements. Two scattering topologies have been considered: the scattering of same sign W-boson pairs and the fully leptonic WZ boson one. The ultimate goal is to assess the future CMS sensitivity to the longitudinal electroweak scattering, to partial unitarized scenarios and to new physics in the EWSB sector, which is parametrized through anomalies in the vector boson self couplings. In addition, the difference in the performance between the proposed upgraded CMS detector and the present one, which will undergo a strong degradation due to radiation damage, is investigated showing how the upgraded technology allows better performances in all the considered benchmark measurements.