Study of the CP properties of the top- quark Yukawa interaction in $ttH$ and $tH$ events with $H\rightarrow\gamma\gamma$.

This poster presents the first search for $\cal{CP}$-violation in the top Yukawa coupling using $t\bar{t}H$ and $tH$ production modes in the diphoton decay channel ($H\to\gamma\gamma$). The analysis is based on the 139~fb$^{-1}$ of proton-proton collision data at center-of-mass energy $\sqrt{s}$~=~13~\TeV\ recorded with the ATLAS detector at the Large Hadron Collider. Two separate set of selections are introduced to capture the full hadronic and leptonic decay of the $t\bar{t}$\ system each. This search is performed using a simultaneous fit to the $m_{\gamma\gamma}$ distribution in analysis categories defined to enhance signal over continuum background and also to separate the CP even and CP odd signal hypotheses. The expected significance for $t\bar{t}H$ signal is 4.4$\sigma$. The observed significance is 5.2$\sigma$. The expected and observed limits for $tH$ production are $11.7\times$ and $11.6\times$ the Standard Model prediction, respectively. Two-dimensional contours of $\kappa_t\cos(\alpha)$ and $\kappa_t\sin(\alpha)$ are derived. A $\cal{CP}$-mixing angle above $43^\circ$ ($63^\circ$ expected) is excluded at 95\% confidence level, and $\cal{CP}$-odd hypothesis is rejected at the 3.9$\sigma$ level.