Search for the Standard Model Higgs boson produced in association with a pair of top quarks and decaying into a bb-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, ttH. 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->bb and the single lepton decay channel (electron or muon) for the tt 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 tt+jets production, in particular by the ttbb component. The data are found to be consistent with both the background-only hypothesis and with the Standard Model ttH prediction. The ratio of the measured ttH signal cross-section to the Standard Model expectation is found to be mu = 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 mu of 3.6 (2.2) is set at a 95% confidence level