Search for the Standard Model Higgs boson produced in association with top quarks and decaying into

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$${{\,\mathrm GeV\,}}$$