Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector

The observation of Higgs boson production in association with a top quark pair ($t\bar{t}H$), based on the analysis of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb${}^{-1}$, and considering Higgs boson decays into $b\bar{b}$, $W{W}^{\ast }$, $\tau \tau$, $\gamma \gamma$, and $Z{Z}^{\ast }$, the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the $t\bar{t}H$ searches using a dataset corresponding to integrated luminosities of 4.5 fb${}^{-1}$ at 7 TeV and 20.3 fb${}^{-1}$ at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total $t\bar{t}H$ production cross section at 13 TeV is measured to be 670 $\pm$ 90 (stat.) ${}_{-100}^{+110}$ (syst.) fb, in agreement with the Standard Model prediction.