First measurement of B(t -\> Wb)/B(t -\> Wq) in the dilepton channel in pp collisions at sqrt(s)=7 TeV

The flavor content of $t \bar{t} \rightarrow (l^{+}\nu q)(l^{-}\bar{\nu} \bar{q})$ events is studied using a sample corresponding to an integrated luminosity of 2.2~fb$^{-1}$ of proton-proton collisions produced at $\sqrt{s}=$7~TeV and acquired by the CMS detector. Data-driven strategies are used to constrain the background contributions. % which contaminate the sample. %and which may mimic true top quark decays. A measurement of the ratio of branching fractions $R=B(t\rightarrow Wb)/B(t\rightarrow Wq)$ is performed by counting the jets identified as $b$-jets.%, using a dedicated tagging algorithm. We measure $R=$~0.98~$\pm$~0.04 which is consistent with the SM prediction. We also set a lower limit $R>$0.85 at 95\% C.L. which is obtained after requiring $R\leq$1.