Measurement of the top quark decay branching fraction ratio $R_b = {\cal B}(t \to Wb)/{\cal B}(t \to Wq)$ and of the inclusive production cross section $pp$ $\to$ $t\bar{t}$ in the dilepton channel at ATLAS

The top quark branching fraction ratio $R_b$ and the inclusive production cross section $\sigma_{t\bar{t}}$ are measured in the dilepton channel using 4.6 fb$^{−1}$ of proton-proton collisions at the centre-of-mass energy $\sqrt{s}$ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Template fits to the distribution of the number of $b$-tagged jets find $R_b$ = 1.02 ± 0.01 (stat.) ± 0.07 (syst.) and $\sigma_{t\bar{t}}$ = 178 ± 3 (stat.) ± 19 (syst.) pb. Lower limits on $R_b$ and on the CKM matrix element |$V_{tb}$| are set with the measured value of $R_b$ to $R_b > 0.88$ and |$V_{tb}$| > 0.94 at 95% confidence level.