Measurement of the Higgs boson production in the $H\rightarrow\tau\tau\rightarrow \tau_{lep}\tau_{had}$ decay channel at $\sqrt{s} = 13$ TeV with the ATLAS detector at the LHC

This thesis presents a measurement of the Standard Model Higgs boson production in the $H\rightarrow\tau\tau$ decay with particular emphasis on the final state in which one tau decays hadronically and the other one decays leptonically, referred to as the $H\rightarrow\tau\tau\rightarrow \tau_{lep}\tau_{had}$ channel. The analysis uses a dataset corresponding to 36.1 fb$^{-1}$ collected by the ATLAS detector in the 2015 and 2016 Run 2 data taking period at a center of mass energy $\sqrt{s} = 13$ TeV. \\ The decay of the Higgs boson into a tau lepton pair is currently the only accessible channel to establish the Higgs-Yukawa coupling to leptons. In this context, the $H\rightarrow\tau\tau\rightarrow \tau_{lep}\tau_{had}$ channel plays an important role due to the high branching ratio and the moderate background. \\ A cut-based analysis has been performed to enhance the sensitivity to the signal considering the kinematic features of the two dominant Higgs boson production modes: gluon fusion and vector-boson fusion. \\ The results of a profile likelihood fit to the di-tau mass distribution will be presented considering the $\tau_{lep}\tau_{had}$ standalone fit, as well as the combination of all possible tau pair decay channels. From the combined fit, a signal strength $\mu = 1.08^{+0.175}_{-0.171} \textrm {(stat.)} ^{+0.265}_{-0.222} \textrm{(syst.)}$ and the cross section times branching ratio $\sigma_{H\tau\tau} (VBF) = 0.28^{+0.09}_{-0.09} \textrm{(stat.)} ^{+0.10}_{-0.09} \textrm{(syst.)}$ pb, $\sigma_{H\tau\tau} (ggH) = 2.97^{+1.03}_{-1.01} \textrm{(stat.)} ^{+1.67}_{-1.23} \textrm{(syst.)}$ pb values have been measured. The results are in good agreement with the Standard Model prediction within the uncertainties. The observed (expected) significance, considering only the Run 2 data, is 4.36 (4.13) standard deviations from the background-only hypothesis. The combination of Run 1 and Run 2 data gives a significance of 6.37 (5.43), which is above the 5 standard deviations observation threshold.