A search for a Higgs boson produced in association with a pair of top quarks ($t\bar{t}H$) and decaying into a pair of bottom quarks ($H\to b\bar{b}$) is presented. The analysis uses 4.7 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=7$ 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 $t\bar{t}$ system which, together with the $H\to b\bar{b}$ 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, $\sigma(t\bar{t}H)\times BR(H\to b\bar{b})$, 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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