Measurement of the ratio B(t to Wb)/B(t to Wq)

The ratio of the top quark branching fractions $\mathcal{R}=B(t\rightarrow Wb)/B(t\rightarrow Wq)$, where $q$ can be a $d$, $s$, or a $b$ quark, is measured in the $t\bar{t}$ dilepton final state. The measurement is performed with 16.7~\fbinv of proton-proton collision data at $\sqrt{s}$=8\TeV collected with the CMS detector. Data-driven strategies are used to constrain the main background contributions to the data sample. By counting the number of $b$-jets identified per event, the ratio of branching fractions is determined to be $\mathcal{R}=~1.023^{+0.036}_{-0.034}$~(stat+syst), in good agreement with the SM prediction. A lower limit $\mathcal{R}>$0.945 at 95\% CL is obtained after requiring that $R\leq$1. Assuming a unitary, three-generation CKM matrix, $\lvert V_{\rm tb}\rvert=1.011^{+0.018}_{-0.017}$~(stat.+syst.) is measured and $|V_{\rm tb}|>$0.972 is obtained at 95\% CL.