Étude des propriétés CP du boson de Higgs dans le canal tau tau dans l’expérience CMS auprès du LHC

The subject of this thesis is the search for a CP violation in tau tau decays of the Higgs boson ($\mathrm{H\rightarrow\tau\tau\rightarrow a_1\nu a_1\nu\rightarrow3\pi\nu3\pi\nu}$) in the CMS experiment at LHC. The analysis focused on a data sample corresponding to the entire LHC Run 2 data, with an integrated luminosity of 137 $\mathrm{fb^{−1}}$. The polarimetric vector method was used to define the observable characterizing the CP state of the Higgs boson. This analysis is accompanied by two other studies. The first one is the improvement of the identification of taus leptons for the high luminosity phase of the LHC (HL-LHC), for the rejection of electrons, by the use of boosted decision trees. The electrons emitting photons by bremsstrahlung radiation, they constitute a consequent background to the identification of taus leptons in their hadronic decay modes. The second study is the hit efficiency in the silicon tracker. The origin of the observed inefficiencies was evaluated in the context of the presence of highly ionizing particles.