Search for top-quark-pair associated Higgs production with tau leptons using the ATLAS detector at the LHC

The search for Higgs boson production in association with a pair of top quarks in multilepton final states is presented. Seven final states defined by the number and flavour of charged leptons are analysed in the data set corresponding to an integrated luminosity of $36.1~\text{fb}^{-1}$ recorded at $13~\text{TeV}$ with the ATLAS detector. The result is an observed excess of events over the expected backgrounds from Standard Model processes with a significance of 4.1 standard deviations (expected 2.8). The measured cross section of the $t\bar{t}H$ process $\sigma_{t\bar{t}H}=790\,\pm150\text{(stat.)}\,^{+170}_{-150}\text{(syst.)}\,\text{fb}=790\,^{+230}_{-210}\,\text{fb}$ is compatible with the expected one of $\sigma_{t\bar{t}H}^\text{SM}=507^{+35}_{-50}\,\text{fb}$. Particular focus is put on the two final states with two electrons and/or muons (of same or opposite electric charges) and one hadronically decaying tau lepton which contribute significantly to the result.