Measurement ofB(t→Wb)/B(t→Wq)at the Collider Detector at Fermilab

We present a measurement of the ratio of top-quark branching fractions $R=\mathcal{B}(t\to Wb)/\mathcal{B}(t\to Wq)$, where $q$ can be a $b$, $s$, or a $d$ quark, using lepton-plus-jets and dilepton data sets with an integrated luminosity of $\sim 162{\mathrm{pb}}^{-1}$ collected with the Collider Detector at Fermilab during Run II of the Tevatron. The measurement is derived from the relative numbers of $t\overline{t}$ events with different multiplicity of identified secondary vertices. We set a lower limit of $R>0.61$ at 95% confidence level.