Evidence for Standard Model Higgs boson decays to tau lepton pairs in the ATLAS detector supported by a search in the fully hadronic final state

This thesis presents the results of a search for Standard Model Higgs bosons decaying to tau lepton pairs, where both tau leptons decay hadronically. The search is performed using 20.3 fb$^{−1}$ of proton-proton collision data with a center-of-mass energy of 8 TeV collected in 2012 by the ATLAS detector at the Large Hadron Collider at CERN. The search is complicated by a significant source of background events from non- resonant multijet processes and by Z → ττ events that share similar features with the Higgs signal. The multijet background is estimated from a control region in data, and the Z → ττ background is estimated from Z → µµ events in data where the muons are re- placed by simulated tau leptons. Other minor backgrounds are estimated from simulation. Signal-sensitive regions in the data are only observed after presenting a satisfactory model of the background processes. A multivariate analysis using boosted decision trees is optimized to search for Higgs events produced via gluon-gluon fusion and vector boson fusion at m$_{H}$ = 125 GeV. A cut-based analysis is also developed and presented in support of the multivariate analysis. The compatibility of the multivariate and cut-based analyses at mH = 125 GeV is evaluated using the bootstrap method. An excess of events over the expected background is found, with an observed (expected) significance of 2.9 (1.8) standard deviations. The measured signal strength, normalized to the Standard Model expectation is µ = 1.77$\frac{+0.93}{-0.71}$. The combined excess of events from searches in the fully leptonic and semi-leptonic final states corresponds to an observed (expected) significance of 4.5 (3.5) standard deviations and a measured signal strength of µ = 1.42$\frac{+0.44}{-0.38}$. The combined excess provides direct evidence for the coupling of the recently discovered Higgs boson with m$_{H}$ = 125 GeV to fermions.