  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 tbar to ell nu q
q' b bbar) or dileptons (t tbar to ell(+) nu ell(-) nu b bbar), ell being an
electron or a muon. The search is optimized for the decay mode H to b bbar. The
largest background to the t tbar 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.
