Measurement of the top pair-production in association with a Z boson at LHC and WZ scattering in view of the High Luminosity LHC

This thesis describes two studies towards measurements of couplings of heavy particles of the standard model that will be useful in establishing the electroweak symmetry breaking. The first study describes a cross section measurement of top quark pair production in association with a Z boson ($\text t\bar{\text {t}Z}$), which is carried out using proton-proton collision data at a center-of-mass energy of 13 TeV, recorded with the CMS detector at the LHC. This is followed by a feasibility study of W and Z boson scattering in view of the High Luminosity LHC program. The first study is one of the most interesting analyses at the LHC as it allows to study the coupling between the top quark and Z boson. In addition, the $\text t\bar{\text {t}Z}$ is one of the main backgrounds for top quark pair production in association with a Higgs boson and for many beyond the standard model final states. The $\text t\bar{\text {t}Z}$ is measured to be $0.99^{+0.09}_{-0.08}(\text{stat.})^{+0.12}_{-0.10}(\text{sys.})$ pb, in agreement with the theoretical next-to-leading order calculation. It represents the most precise observation of this process to date attaining a 14$\%$ total relative uncertainty. The second analysis is the scattering of W and Z bosons, which has not yet been experimentally observed. The feasibility of this process and its properties are studied with the upgraded CMS detector at the High Luminosity LHC. The results are presented in terms of the precision of the cross section, the sensitivity to the longitudinal component of the WZ scattering, and the exclusion of partial unitarization scenarios. Additionally, the performance of the upgraded detector is compared with the performance of the current detector, which will be strongly degraded due to the high radiation damage.