Search for the Standard Model Higgs boson produced in association with a pair of top quarks and decaying into a $b\bar{b}$-pair in the single lepton channel at $\sqrt{s}$ = 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, $t\bar{t}H$. The analysis uses 13.2 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}$ = 13 TeV collected in 2015 and 2016 with the ATLAS detector at the LHC. The considered decay mode for the Higgs boson is $H\rightarrow b\bar{b}$ and the single lepton decay channel (electron or muon) for the $t\bar{t}$ 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 $t\bar{t}$+jets production, in particular by the $t\bar{t}b\bar{b}$ component. The data are found to be consistent with both the background-only hypothesis and with the Standard Model $t\bar{t}H$ prediction. The ratio of the measured $t\bar{t}H$ signal cross-section to the Standard Model expectation is found to be $\mu$ = 1.6 $\pm$ 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.