Search for the standard model Higgs boson produced in association with top quarks and decaying into a b ¯ b pair in p p 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, t ¯ t H , is presented. The analysis uses 36.1     fb − 1 of p p 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 → 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 + jets production. For a Higgs boson mass of 125 GeV, the ratio of the measured t ¯ t H signal cross-section to the standard model expectation is found to be μ = 0.8 4 + 0.64 − 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 ¯ t H signal.