Richard Wall

In this thesis, we show evidence for the production of ttbar + b + X and ttbar + c + X, together refered to as ttbar + HF, at the Large Hadron Collider. A sample of dilepton ttbar candidate events with three or more b-tagged jets is used to isolate a $\ttbar$ 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 ttbar + HF production using a correction factor from Monte Carlo simulation. The cross-section for ttbar events with at least one additional jet is also measured. 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(ttbar + HF) / sigma(ttbar + jet) to be 7.9 +/- 1.4 (stat.) +5.5,-2.1 (syst.)%, compared to the leading-order Standard Model expectation of 4.1 +/- 1.3%.