Measurement of the $pp\to H\to ZZ^* \to 4 \ell$ Production and $HZZ$ Tensor Coupling with the ATLAS Detector at 13 TeV Centre-of-Mass Energy

In this master thesis the measurement of the Higgs boson production in the $H\to~ZZ^*~\to 4~\ell$ decay channel ($\ell=e,\mu$) is performed together with the measurement of the tensor structure of the Higgs boson couplings to $Z$ bosons. The results are based on the Run~II dataset of LHC's proton-proton collisions at a centre-of-mass energy of 13~TeV, with the ATLAS detector and corresponding to a total integrated luminosity of $14.78$~fb$^{-1}$. Special emphasis is given to the estimation of the reducible background contribution. Based on the signal and background estimations, there are $32.0\pm3.2$ Higgs boson candidates expected after the final event selection, while $44$ candidates are observed. The difference is compatible at the level of about $2$ standard derivations with the Standard Model predictions. All selected candidates are used in the study of the tensor structure of the $HZZ$ coupling between the Higgs boson and the two $Z$ bosons. For this study a dedicated signal model is introduced to describe the signal dependence on the anomalous contribution relative to the Standard Model $HZZ$ coupling. The allowed range of anomalous CP-even and CP-odd coupling parameters has been reduced by a factor of $4$ to $10$ compared to the constraints from Run~I data.