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 ($t\bar{t}H$) is presented, based on the analysis of up to 79.7 $fb^{-1}$ of proton$-$proton collision data produced at 13 TeV center-of-mass energy by the Large Hadron Collider and recorded with the ATLAS detector. Considering Higgs boson decays to $b\bar{b}$, $WW^*$, $\tau^+\tau^-$, $\gamma\gamma$, and $ZZ^*$, 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 and 20.3 $fb^{-1}$ at 7 and 8 TeV, respectively, 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) $^{+110}_{-100}$ (sys) fb, while the Standard Model prediction is 507$^{+35}_{-50}$ fb.