Search for the associated production of a Higgs boson and a top quark pair in multilepton final states with the ATLAS detector

The Yukawa coupling of the Higgs boson to the top quark is a key parameter of the Standard Model, and can be constrained using the associated production process $pp\rightarrow t\bar{t}H+X$.\\ A search for this process using final states with multiple leptons, primarily targeting the decays $H\rightarrow WW^*$ and $H\rightarrow \tau \tau$, has been performed using 13.2 $fb^{-1}$ of data recorded by the ATLAS detector in 2015 and 2016 at a center of mass energy $\sqrt{s}$= 13 TeV.\\ The analysis includes four final states distinguished by the number and flavor of leptons: two same-charge light leptons ( e or $\mu$ ) and no hadronically-decaying $\tau$ lepton candidates ($2l0\tau_{had}$); two same-charge light leptons and one hadronically-decaying t lepton candidate ($2l1\tau_{had}$), three light leptons ($3l$) and four light leptons ($4l$). The different background sources are also presented for each channel. \\ The latest best-fit value of the ratio of observed and Standard Model cross sections as well as an upper limit on this parameter is given.