Measurement of the Higgs Boson Decay to τ-Pairs in the Lepton plus Hadrons Final State in Proton-Proton Collisions with the ATLAS Detector

In this thesis, the decay of the Standard Model Higgs boson into one hadronically and one leptonically decaying τ lepton is investigated using multivariate methods. Proton-proton collisions recorded by the ATLAS experiment at the LHC in 2011 and 2012 at a center-of-mass energy of √ s =7 TeV and √ s =8 TeV are analysed. The data correspond to an integrated luminosity of 4.5 fb−1 and 20.3 fb−1 , respectively. Within the analysis two event categories are defined, which are sensitive to the two main Higgs production mechanisms, gluon-gluon fusion and vector-boson fusion. Further improvements of the data-driven technique used to model the irreducible Z → ττ background in ττ final states were performed. Under the assumption of a Higgs boson with mass mH=125 GeV, an excess in data over the expected background is found with an observed (expected) significance of 2.5 (2.4) σ. The measured signal strength of µ = 1.1 +0.5 −0.5 is in agreement with the Standard Model prediction. The presented analysis is an important part of the combined analysis of the H → ττ decay channel, which includes all possible τ decay modes. The combined analysis yields an excess of events over the predicted background with an observed (expected) significance of 4.5 (3.4) σ and a measured signal strength of µ = 1.4 +0.4 −0.4 . The combined result is evidence of the Higgs boson Yukawa coupling to τ leptons. Furthermore, studies are presented which address potential advancements of the analysis strategy towards future operation times of the LHC.