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 $\sqrt{s} = 8$ TeV with the ATLAS experiment at the LHC

This thesis focuses on the search for the Standard Model Higgs boson produced in association with a pair of top quarks, $t \bar{t} H$, using $20.3 fb^{-1}$ of pp collision data at $\sqrt{s}=8$ TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the $ H \to b\bar{b}$ decay mode and is performed in the single lepton channel, characterised by an isolated electron or muon, missing transverse energy and at least four jets. In order to improve the sensitivity of the search, events are categorised according to their jet and $b$-tagged jet multiplicities into nine analysis regions. The discrimination between signal and background, the latter being dominated by $t \bar{t}$+jets production, is obtained by employing neural networks in signal-enriched regions. No significant excess of events above the background expectation is found and an observed (expected) upper limit of 3.6 (2.6) times the Standard Model cross section is obtained at $95\%$ confidence level. The search in the single lepton channel significantly contributes to the combination of various $t \bar{t} H$ searches carried out by ATLAS Collaboration at $\sqrt{s} = 7$ and $8$ TeV, yielding a measured (expected) significance for the observation of the $t \bar{t} H$ production process of $2.3 \sigma$ ($1.5 \sigma$).