Search for the Standard Model Higgs boson produced in association with a pair of top quarks and decaying into a b anti b-pair in the single lepton channel at √(s) = 13 TeV with the ATLAS experiment at the LHC

This thesis presents a search for Standard Model Higgs boson produced in association with top quark pairs, t anti tH. The analysis uses 13.2 fb^-1 of pp collision data at √(s)=13 TeV collected in 2015 and 2016 with the ATLAS detector at the LHC. The considered decay mode for the Higgs boson is H→ b anti b and the single lepton decay channel (electron or muon) for the t anti t pair. The sensitivity of this channel is improved by an event categorisation according to the jet multiplicity and the number of jets containing a b-hadron decay. Multivariate techniques are used to distinguish the signal events from the background events, which are dominated by t anti t+jets production, in particular by the t anti t+b anti b component. The data are found to be consistent with both the background-only hypothesis and with the Standard Model t anti tH prediction. The ratio of the measured t anti tH signal cross-section to the Standard Model expectation is found to be μ=1.6±1.1, assuming a Higgs boson mass of 125 GeV. No significant excess of events above the background expectation is found and an observed (expected) upper limit on μ of 3.6(2.2) is set at a 95% confidence level.