Cross section measurements of the Higgs boson exploiting the $H\rightarrow\tau\tau$ decay mode and their combination with other decay modes using the ATLAS detector

This thesis presents cross section measurements of the Higgs boson exploiting the $H \rightarrow \tau \tau$ decay mode and their combination with other decay modes using the ATLAS detector. Proton-proton collision data corresponding to an integrated luminosity of up to 139 fb$^{-1}$ taken between 2015 and 2017 is studied in several individual analyses. Two analyses are performed in the $H \rightarrow \tau \tau$ decay mode, where one is based on the data recorded in 2015 and 2016 and one on the data recorded from 2015 until 2018. The first analysis measured the inclusive cross section of the $H \rightarrow \tau \tau$ decay mode to be (3.77$\pm$1.00) pb and the cross sections of the two main production modes of the Higgs boson gluon-gluon fusion (ggF) and vector-boson fusion (VBF) to be (3.1$\pm$1.8) pb and (0.28$\pm$0.13) pb. In addition, the Simplified Template Cross Section (STXS) framework is used to measure the cross sections of three individual regions of phase space. None of these measurements show a significant deviation from the Standard Model (SM) predictions. A combination of this analysis with the analysis based on the data recorded in 2011 and 2012 achieved an observed significance of 6.4 standard deviations corresponding to the first observation of the $H \rightarrow \tau \tau$ decay mode with the ATLAS experiment. The analysis based on the data recorded from 2015 until 2018 measured the inclusive cross section of the $H \rightarrow \tau \tau$ decay mode to be (2.94$\pm$0.40) pb. The cross sections of the four dominant production modes ggF, VBF, Higgs strahlung (VH) and the associated production with top quarks (ttH) are measured to be (2.65$\pm$0.85) pb, (0.20$\pm$0.04) pb, (0.12$\pm$0.07) pb and (0.031$\pm$0.003) pb. This measurement is the first observation of the VBF production mode in the $H \rightarrow \tau \tau$ decay mode with an observed significance of 5.3 standard deviations. Furthermore, the STXS framework is used to measure the cross sections of nine independent regions of phase space. None of these measurements show a significant deviation from the SM predictions. In addition, this analysis is used to estimate the expected sensitivity of the ATLAS experiment in the $H \rightarrow \tau \tau$ decay mode with the full high-luminosity LHC data set, which will be taken approximately until 2038. Both $H \rightarrow \tau \tau$ analyses are combined with analyses of other Higgs boson decay modes. In both combinations the inclusive production mode cross sections of dominant Higgs boson production modes and cross sections within the STXS framework are measured. Both combined measurements are interpreted within an SM effective field theory. No significant deviations from the SM predictions have been observed in both combinations.