Measurement of $t\bar t H$ production cross section times branching ratio in the $\gamma\gamma$ decay channel with the full Run2 pp collision dataset collected by the ATLAS experiment at $\sqrt {s}$= 13 TeV

In this study, the measurement of Higgs boson production in association with a top quark-antiquark pair in the two photons decay channel is presented. The analysis is based on 139 fb $^{−1}$ of proton-proton collision data recorded b√y the ATLAS detector at the Large Hadron Collider at center-of-mass energy of $\sqrt{s}$ = 13TeV. The analysis is performed using a simultaneous fit in seven signal-enriched event categories. The $t\bar t H$ process is observed in the diphoton decay channel with a significance of 4.9σ relative to the background-only hypothesis, while the expected significance is 4.2σ. The $t\bar t H$ cross-section times the $H$ → $\gamma\gamma$ branching ratio is measured to be σ$_{t\bar t H}$ × $B$$\gamma\gamma$ = 1.59$_{-0.39}^{+0.43}$, in agreement with the Standard Model prediction.