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 $pp \to t\bar t H + X$. A search for this process using final states with multiple leptons, primarily targeting the decays $H\to WW^*$ and $H\to \tau\tau$, 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 best-fit value of the ratio of observed and Standard Model cross sections is 2.5 $\pm$ 0.7 (stat) $^{+1.1}_{-0.9}$ (syst), and an upper limit on this ratio of 4.9 (2.3 expected) is found at 95% confidence level.