Search for the Standard Model Higgs boson produced in association with top quarks and decaying to $\bar{b}$ at $\sqrt{s}$ = 7 TeV

A search for a Higgs boson produced in association with a pair of top quarks ($tar{t}H$) and decaying into a pair of bottom quarks ($H o bar{b}$) is presented. The analysis uses 4.7~fb$^{-1}$ of $pp$ collision data at $sqrt{s}=7 ev$ collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The search is focused on the semileptonic decay mode of the $tar{t}$ system which, together with the $H o bar{b}$ decay, typically results in a final state signature characterised by one high transverse momentum isolated electron or muon, high transverse missing momentum and six jets, of which four jets originate from $b$ quarks. Events are classified in nine different topologies depending on their jet and $b$-tagged jet multiplicities in order to improve the sensitivity of the search. No significant excess of events above the background expectation is observed and 95% confidence-level upper limits on the production cross section times branching ratio, $sigma(tar{t}H) imes BR(H o bar{b})$, are derived for a Higgs boson with a mass between 110 and 140~gev. For a Higgs boson with a mass of 125~gev, an observed (expected) 95% confidence-level upper limit of 13.1 (10.5) times the Standard Model cross section is obtained.