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Using a sample of dilepton top-quark pair (ttbar) candidate events, a studyis performed of the production of top-quark pairs together with heavy-flavor(HF) quarks, ttbar+b+X or ttbar+c+X, collectively referred to as ttbar+HF. Thedataset used corresponds to an integrated luminosity of 4.7 fb-1 ofproton--proton collisions at a center-of-mass energy of 7 TeV recorded by theATLAS detector at the CERN Large Hadron Collider. The presence of additional HFquarks in the ttbar sample is inferred by looking for events with at leastthree b-tagged jets, where two are attributed to the b-quarks from the ttbardecays and the third to additional HF production. The dominant background tottbar+HF in this sample is ttbar+jet events in which a light-flavor jet ismisidentified as a heavy-flavor jet. To determine the heavy- and light-flavorcontent of the additional b-tagged jets, a fit to the vertex mass distributionof b-tagged jets in the sample is performed. The result of the fit shows that79 +/- 14 (stat.) +/- 22 (syst.) of the 105 selected extra b-tagged jetsoriginate from HF quarks, three standard deviations away from the hypothesis ofzero ttbar+HF production. The result for extra HF production is quoted as aratio (R_HF) of the cross section for ttbar+HF production to the cross sectionfor ttbar production with at least one additional jet. Both cross sections aremeasured in a fiducial kinematic region within the ATLAS acceptance. R_HF ismeasured to be [6.2 +/- 1.1 (stat.) +/- 1.8 (syst.)]% for jets with pT>25 GeVand |eta|<2.5, in agreement with the expectations from Monte Carlo generators.

A study of heavy flavor quarks produced in association with top quark pairs at sqrt(s) = 7 TeV using the ATLAS detector