Measurements of the Top Quark Pair Production Cross Section and Branching Ratio to a W-boson and bottom quark Using the Semi-Leptonic and Dilepton Final States with the ATLAS Detector at the LHC

In the Standard Model, the top quark plays a unique role as the heaviest known fundamental particle and as a quark that decays before it is able to hadronize. Top quarks are expected to decay to a W-boson and a b-quark nearly 100% of the time. If the branching ratio of t to Wb is lower than one, the distribution of the number of b-tagged jets will shift to lower multiplicities. A simultaneous likelihood t to the number of b-tagged jets distributions in the lepton+jets and dilepton channels is performed on 4.7 fb^-1 of data collected by the ATLAS detector to extract both the branching ratio and the tt cross section. The branching ratio of t to Wb, R, is measured to be 1.06 +-0.11, which is consistent with the Standard Model value. This is the first measurement of the t to Wb branching ratio performed with the ATLAS detector using both the lepton+jets and dilepton channels at the LHC. The tt cross section is measured to be 167 +27-26 pb, which agrees with NNLO predictions.