Physics of the Higgs boson in the ditau decay channel with the ATLAS detector

This thesis presents two measurements performed in the $H$$\tau$$\tau$ channel and based on the proton-proton collisions data collected by the ATLAS experiment during 2015 and 2016 and corresponding to an integrated luminosity of 36.1~fb$^{-1}$. The $H$$\tau$$\tau$ is a considerably important decay channel because it allows to directly measure the Higgs Yukawa coupling and to probe the Higgs boson properties. Measurements of the Higgs production cross section in the $H$$\tau$$\tau$ decay channel are presented. The observed (expected) significance of the $H \rightarrow \tau \tau$ signal excess over the expected background amounts to 4.4 (4.1) standard deviations. This result, combined with the data taken at $\sqrt{s}=7$ and 8 TeV, leads to an observed (expected) significance of 6.4 (5.4) and constitutes the first ATLAS observation of $H \rightarrow \tau \tau$. The measured total cross section of $H \rightarrow \tau \tau$, using the data collected at $\sqrt{s}=13$~TeV, is $3.77^{+0.60}_{-0.59}~(\text{stat.})~^{+0.87}_{-0.74}~(\text{syst.})$~pb which is consistent with the Standard Model expectation. In addition, total cross sections for the vector boson fusion and gluon-gluon fusion processes have been measured separately and similar results are reported based on the simplified template cross section framework. All of these measurements are in agreement with the Standard Model predictions.\\ The second measurement presented in this manuscript concerns a CP violation study in the same decay channel. The coupling structure of the vertex where the Higgs couples to two electroweak gauge bosons in its vector boson fusion production is investigated. The Optimal Observable method is used to directly test CP invariance in this vertex. No sign of CP violation is observed and constraints on the $\tilde{d}$ parameter, which governs the strength of CP violation, have been set. A 68\% CL interval is observed for $\tilde{d}$ of [-0.090, 0.035], compared to an expected interval of $\tilde{d} \in [-0.035, 0.033]$. No constraints can be set at 95\% CL despite an expected interval $\tilde{d} \in [-0.21, 0.15]$ at 95\% CL was determined.