| Summary: | A search for a Higgs boson produced in association with a pair of top quarks (tt¯$tar t$H) and decaying into a pair of bottom quarks (H → bb¯$bar b$) is presented. The analysis uses 4.7 fb−1 of pp collision data at √s = 7 TeV collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The search is focused on the semileptonic decay mode of the tt¯$tar t$ system which, together with the H → bb¯$bar b$ 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, σ(tt¯H)×BR(H→bb¯)$sigma (tar tH) imes BR(H o bar b)$, 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.
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