A search for the standard model Higgs boson produced in association with a top-quark pair is presented using data samples corresponding to an integrated luminosity of 5.0 inverse femtobarns (5.1 inverse femtobarns) collected in pp collisions at the center-of-mass energy of 7 TeV (8 TeV). Events are considered where the top-quark pair decays to either one lepton+jets ($t\bar{t} \to \ell\nu q\bar{q}' b\bar{b}$) or dileptons ($t\bar{t} \to \ell^+\nu \ell^-\bar{\nu}b \bar{b}$), $\ell$ being an electron or a muon. The search is optimized for the decay mode $H \to b \bar{b}$. The largest background to the $t \bar{t} H$ signal is top-quark pair production with additional jets. Artificial neural networks are used to discriminate between signal and background events. Combining the results from the 7 TeV and 8 TeV samples, the observed (expected) limit on the cross section for Higgs boson production in association with top-quark pairs for a Higgs boson mass of 125 GeV is 5.8 (5.2) times the standard model expectation.

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