Search for the Standard Model Higgs boson produced in association with top quarks and decaying into $b\bar{b}$ in pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

A search for the Standard Model Higgs boson produced in association with a top-quark pair, $t\bar{t}H$ , is presented. The analysis uses 20.3 fb$^{−1}$ of pp collision data at $\sqrt{s}=8{{\,\mathrm TeV}}$ , collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the $H \rightarrow b\bar{b}$ decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by $t\bar{t}$ +jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible $t\bar{t}{+}b\bar{b}$ background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured ${t\bar{t}H}$ signal cross section to the Standard Model expectation is found to be $\mu = 1.5 \pm 1.1$ assuming a Higgs boson mass of 125 ${{\,\mathrm GeV\,}}$ .