Evidence for the Higgs Boson Decay to t+t- in the Lepton + Hadrons Final State with ATLAS

The data recorded with the ATLAS experiment during the first run period of the Large Hadron Collider are analysed by selecting final states with two τ-leptons, which further decay to a lighter lepton and to hadrons, respectively. The modelling of these final states is first validated in the context of the Z boson production cross section in ττ final states. The event selection and the data-driven techniques for the estimation of important background processes are then adapted to a search for H→ττ decays of the Higgs boson. In this search, Z→ττ final states contribute the dominant background and are therefore modelled in a data-driven way by replacing the Z decay products in Z→μμ data events with simulated decays of τ-leptons. Including all different background estimates, the search strategy is developed, and its sensitivity is documented in two intermediate results. With the final multi-variate analysis employing boosted decision trees, an excess of data over the estimated background in lepton + hadrons final states is observed with a statistical significance of 3.2$sigma$. It is compatible with a mass of mHapprox125 GeV, which has been measured in other final states originating from bosonic decays. This result is evidence for the Yukawa coupling of the Higgs boson to μ-leptons.