A study of heavy-flavor quarks produced in association with top-quark pair events at a center-of-mass energy of 7 TeV using the ATLAS detector

In this thesis, we show evidence for the production of $t\bar{t} + b + X$ and $t\bar{t} + c + X$, together refered to as $t\bar{t} + \rm{HF}$, at the Large Hadron Collider. A sample of dilepton $t\bar{t}$ candidate events with three or more $b$-tagged jets is used to isolate a $t\bar{t}$ sample rich in extra heavy flavor jets. A fit to the vertex mass distribution for the $b$-tagged jets in this region is performed to extract the flavor composition of the additional $b$-tags. This measurement is converted to a cross section for $t\bar{t} + \rm{HF}$ production $(\sigma_{\rm fid}(t\bar{t} + \rm{HF}))$ using a correction factor from Monte Carlo simulation. The cross section for $t\bar{t}$ events with at least one additional jet is also measured $(\sigma_{\rm fid}(t\bar{t} + j))$. The final result is quoted as a ratio of cross sections within the visible ATLAS acceptance to reduce the overall systematic uncertainty. Using $4.7 fb^{−1}$ of data collected during the 2011 run, we find the ratio of $\sigma_{\rm fid}(t\bar{t} + \rm{HF})$ to $\sigma_{\rm fid}(t\bar{t} + j)$ to be $[7.9 \pm 1.4 {\rm (stat.)} ^{+5.5}_{−2.1} {\rm (syst.)}]$%, compared to the leading-order Standard Model expectation of $4.1 \pm 1.3$%.