Analysis of $t\bar{t}H$ and $t\bar{t}W$ production in multilepton final states with the ATLAS detector

A search for the associated production of a top-quark pair with the Higgs boson ($t\bar{t}H$) in multilepton final states is presented. The search is based on a dataset of proton--proton collisions at $\sqrt{s}=13~$TeV recorded with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 80 fb$^{-1}$. Six final states, defined by the number and flavour of charged-lepton candidates, and 25 event categories are defined to simultaneously search for the \ttH signal and constrain several leading backgrounds. The $t\bar{t}W$ background normalisation is left unconstrained in the statistical analysis and the resulting $t\bar{t}W$ normalisation is found to be higher than the theoretical prediction. An excess of events consistent with $t\bar{t}H$ production, over the expected background from Standard Model processes, is found with an observed significance of 1.8 standard deviations, compared to an expectation of 3.1 standard deviations. Assuming Standard Model branching fractions, the best-fit value of the $t\bar{t}H$ production cross section is $\sigma_{t\bar{t}H} = 294^{+182}_{-162}$~fb, which is consistent with the Standard Model prediction. The impact on the $t\bar{t}H$ cross section measurement of the assumptions made on the $t\bar{t}W$ background modelling is discussed.