Search for Standard Model $H\rightarrow \tau^+\tau^-$ Decays in the Lepton-Hadron Final State in Proton-Proton Collisions with the ATLAS Detector at the LHC

This thesis presents a search for Standard Model (SM) Higgs boson decays to a pair of tau leptons in the lepton-hadron final state with the ATLAS detector at the Large Hadron Collider (LHC). The analysis is based on proton-proton collision data recorded during Run 1 of the LHC, corresponding to integrated luminosities of 4.5 fb$^{-1}$ and 20.3 fb$^{-1}$ at centre-of-mass energies of 7 TeV and 8 TeV, respectively. Background events from various SM processes contribute to the selected event sample at a high rate. Their contribution is efficiently separated from the expected Higgs boson signal by using boosted decision trees (BDT) in two analysis categories, which are enriched in events emerging from vector boson fusion and gluon fusion processes. The expected number of events from background processes is modelled using data-driven estimation techniques. The signal contribution is measured using a maximum likelihood fit of the BDT output distributions. An excess of events over the expected level of background events is found and corresponds to an observed (expected) significance of 2.3 (2.4) standard deviations at a Higgs boson mass hypothesis of 125 GeV. The signal strength normalised to the Standard Model expectation is measured to be $0.98^{+0.5}_{-0.5}$. A combined analysis of all tau-tau final states rejects the background-only hypothesis at a level of 4.5 standard deviations at $m_H=125$ GeV, while a significance of 3.5 standard deviations is expected. This provides evidence for the direct coupling of the recently discovered Higgs boson to tau leptons. The measured normalised signal strength of $1.4^{+0.43}_{-0.37}$ is consistent with the predicted Yukawa coupling strength in the Standard Model.