Study of the couplings between the Higgs boson and the electroweak gauge bosons in off-shell processes with the CMS experiment

Abstract: The focus of this doctoral thesis is the experimental investigation of the couplings between the Brout-Englert-Higgs boson and the massive electroweak bosons (V). The analysis is performed on proton-proton collision data at 13 TeV collected by the CMS experiment during run 2 of the CERN LHC collider, which corresponds to about 140 fb-1 of integrated luminosity. The thesis mainly concentrates on the study of the production of highly virtual H bosons, known as "off-shell" (mH > 200 GeV), through gluon fusion and vector boson fusion production modes, followed by decay in the H->ZZ->l+l-nunu channel. The verification of an important prediction of the Standard Model of particle physics, namely the destructive interference of this process and the ZZ background, is achieved through the observation of events with an "off-shell" H boson. This interference ensures the unitarity of cross-section calculations. The H->ZZ->l+l-nunu channel offers a higher branching fraction rate (about 6 times higher) than the H->ZZ->l+l-l’+l’- channel and good sensitivity to the off-shell production process. The thesis provides evidence for the sought interference for the first time, with a significance level of 3.6 sigma, by analyzing the data from the H->ZZ->l+l-nunu channel and combining it with previously published results by the CMS collaboration in the H->ZZ->l+l-l’+l’- channel. Moreover, The high energy scale of the off-shell processes can constrain the presence of physics beyond the Standard Model in the H-V couplings, and an improvement of the constraints on anomalous H-V couplings of about 10% was achieved. Furthermore, the measurement of the off-shell production cross-section in the H->ZZ channel allows for the extraction of the total decay width of the H boson, which is proportional to the ratio between the off-shell cross-section and the on-shell cross-section measured in the H->ZZ->l+l-l’+l’- channel. This ratio is the only known method that can test the expected total width value within the framework of the Standard Model, 4.1 MeV. The total width was measured for the first time with an accuracy of about 50%, following the analysis and combination with measurements in the H->ZZ->l+l-l’+l’- channel, and the result is 3.2+2.4-1.7 MeV. The author of this thesis contributed to reducing two dominant sources of uncertainty in the analysis of the H->ZZ->l+l-nunu channel. The first is a theoretical uncertainty arising from the description of off-shell production processes, which was addressed by enabling the modeling of the signal and background that interfere in the off-shell region using event reweighting techniques. The second is experimental uncertainty arising from measurement errors in the tails of the missing transverse momentum distribution. This was reduced by optimizing event selection and rejection of the background constituted by the production of a Z boson accompanied by jets, and by testing the algorithms developed by the CMS collaboration to reduce the contribution of measurement errors in the tails of the missing transverse momentum distribution.