Search for the associated production of a Higgs boson and a top quark pair in multilepton (2 leptons, no hadronically-decaying $\tau$ lepton candidates and 4 leptons) 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. It 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 the data set recorded by the ATLAS detector in 2015 and 2016 at a center of mass energy $\sqrt{s}$= 13 TeV. The analysis presented here includes two of the 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 ($2l0\tau_{had}$) and four light leptons ($4l$), the remaining channels not covered being two same-charge light leptons and one hadronically-decaying $\tau$ lepton candidate ($2l1\tau_{had}$) and three light leptons ($3l$). The different background sources are also presented for each channel considered. The latest best-fit value for the ratio of observed versus Standard Model cross sections is then given as well as an upper limit on this parameter.