Search for the standard model Higgs boson produced in association with top quarks and decaying into a b(b)overbar pair in pp collisions at root s=13 TeV with the ATLAS detector

A search for the standard model Higgs boson produced in association with a top-quark pair, t¯ tH, is presented. The analysis uses 36.1 fb−1 of pp collision data atffiffi p ¼ 13 TeV collected with the ATLAS s detector at the Large Hadron Collider in 2015 and 2016. The search targets the H → b¯ b 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 t¯ t þ jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured t¯ tH signal cross-section to the standard model expectation is found to be μ¼ 0.84þ0.64 −0.61 . 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 t¯ tH signal.