Search for the Standard Model Higgs Boson Produced in Association with Top Quarks and Decaying into a bb̅ Pair in pp Collisions at √s=13 TeV with the ATLAS Detector

A search for the standard model Higgs boson produced in association with a top-quark pair, tt̅ H, is presented. The analysis uses 36.1 fb⁻¹ of pp collision data at √s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. The search targets the H → bb̅ decay mode. The selected events contain either one or two electrons or muons from the top-quark decays, and are then categorized according to the number of jets and how likely these are to contain b-hadrons. Multivariate techniques are used to discriminate between signal and background events, the latter being dominated by tt̅ + jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured tt̅ H signal cross-section to the standard model expectation is found to be μ = 0.84{{_(-0.61)}^{(+0.64)}}. A value of μ greater than 2.0 is excluded at 95% confidence level (C.L.) while the expected upper limit is μ < 1.2 in the absence of a tt̅ H signal.