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 (tt 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¯ b, W W *, t +t -, ?? , and Z Z*, the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the tt 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 tt H¯ production cross section at 13 TeV is measured to be 670 ± 90 (stat.) +110 -100 (syst.) fb, in agreement with the Standard Model prediction.