Measurement of the Higgs boson coupling with tau leptons and search for an additional neutral MSSM Higgs boson with the ATLAS detector

The aim of this thesis is the search for the Standard Model Higgs boson and for additional heavy MSSM Higgs bosons in the $\tau\tau$ channel with the ATLAS detector of the LHC. The search for the Higgs boson predicted by the Standard Model is performed using the full integrated luminosity of the Run 1 of the LHC: 4.5 fb$^{-1}$ of data collected at an energy in the center of mass of 7 TeV and 20.3 fb$^{-1}$ of data collected at an energy in the center of mass of 8 TeV. The analysis enhances the sensitivity to signal events by implementing a Boosted Decision Tree trained on the signal and background models. The BDT score distribution shows an excess of data over the background expectation, with an observed (expected) statistical significance of 4.5 $\sigma$ (3.4 $\sigma$), which provides evidence for the direct coupling of the Higgs boson to fermions. The measured signal strength normalized to the Standard Model expectation of $\mu = 1.43^{+0.43}_{-0.37}$ is consistent with the predicted Yukawa coupling strength in the Standard Model. The search for additional neutral Higgs bosons predicted by the Minimal Super-symmetric extension of the Standard Model is performed using 3.21 fb$^{-1}$ of data collected during the first year of the Run 2 of the LHC, at an energy in the center of mass of 13 TeV. The data is in good agreement with the background predicted by the Standard Model. Therefore, exclusion limits on the cross-section times branching ratio of the different MSSM production modes and on the $\tan \beta -m_A$ space for different MSSM benchmark scenarios are extracted. The most stringent constraints exclude at 95% confidence level $\tan \beta > 7.6 $ for $m_A = 200 $ GeV in the MSSM $m_h^{\textrm{mod+}}$ scenario.