Using a sample of dilepton top-quark pair ($t\overline{t}$) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of $t\overline{t}+b+X$ and $t\overline{t}+c+X$, collectively referred to as $t\overline{t}+\mathrm{HF}$. The data set used corresponds to an integrated luminosity of $4.7{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF ($b$ or $c$) quarks in the $t\overline{t}$ sample is inferred by looking for events with at least three $b$-tagged jets, where two are attributed to the $b$ quarks from the $t\overline{t}$ decays and the third to additional HF production. The dominant background to $t\overline{t}+\mathrm{HF}$ in this sample is $t\overline{t}+\text{jet}$ events in which a light-flavor jet is misidentified as a heavy-flavor jet. To determine the heavy- and light-flavor content of the additional $b$-tagged jets, a fit to the vertex mass distribution of $b$-tagged jets in the sample is performed. The result of the fit shows that $79\pm 14(\text{stat})\pm 22(\text{syst})$ of the 105 selected extra $b$-tagged jets originate from HF quarks, 3 standard deviations away from the hypothesis of zero $t\overline{t}+\mathrm{HF}$ production. The result for extra HF production is quoted as a ratio ($R_{\mathrm{HF}}$) of the cross section for $t\overline{t}+\mathrm{HF}$ production to the cross section for $t\overline{t}$ production with at least one additional jet. Both cross sections are measured in a fiducial kinematic region within the ATLAS acceptance. $R_{\mathrm{HF}}$ is measured to be $[6.2\pm 1.1(\text{stat})\pm 1.8(\text{syst})]\%$ for jets with $p_T>25\mathrm{GeV}$ and $|\eta |<2.5$, in agreement with the expectations from Monte Carlo generators.

• Received 23 April 2013

DOI:https://doi.org/10.1103/PhysRevD.89.072012

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© 2014 CERN, for the ATLAS Collaboration