Search for the Standard Model Higgs Boson in Hadronic $\tau^{+}\tau^{-}$ Decays with the ATLAS Detector

The discovery of a Higgs boson in di-boson decays, the evidence of its decays into fermion pairs and the compatibility of its measured properties with the Standard Model predictions support the electroweak symmetry breaking mechanism of the Standard Model. The topic of this thesis is the search for the Higgs boson decays into a pair of $\tau$ leptons, important for probing the coupling of the Higgs boson to fermions. The search is performed in final states where both $\tau$ leptons decay hadronically using $4.6\,\rm{fb}^{-1}$ and $20.3\,\rm{fb}^{-1}$ of data collected by the ATLAS detector in proton-proton collisions at the Large Hadron Collider at center-of-mass energies of 7 and 8 TeV, respectively. The signal selection is optimised for events with highly boosted Higgs bosons produced via gluon fusion with additional jet or via vector boson fusion. In order to reduce systematic uncertainties, the major background contributions from $Z\to\tau\tau$ and multi-jet production processes have been measured using signal-free control data samples. The trigger and identification efficiencies of hadronically decaying $\tau$ leptons have been measured using dedicated calibration data samples. An excess of events above the predicted background is found with observed (expected) significance of $2.3\,\sigma\,(2.1\,\sigma)$. The observation is compatible with the production of a Higgs boson with mass of 125 GeV and corresponds to a Higgs boson production cross section times branching ratio of $\rm \mu=1.2\pm0.4(stat)^{+0.5}_{-0.4}(syst)$ times the Standard Model prediction. The Higgs boson mass determined from the observed invariant $\tau\tau$ mass distribution is $125^{+16}_{\,\,\,-7}$ GeV. The sensitivity of this analysis and its results are compatible with the ones obtained using a multivariate approach. The combination of searches for $H\to\tau^{+}\tau^{-}$ decays in fully leptonic and semi-leptonic and fully hadronic $\tau^{+}\tau^{-}$ final states in $\sqrt{s}=8$ TeV data with multivariate analyses leads to a $4.1\,\sigma$ evidence for Higgs boson decays into $\tau$ lepton pairs.