Measurement of the $\mathit{B}$ Semileptonic Branching Fraction with Lepton Tags

We have used the CLEO II detector and $2.06{\mathrm{fb}}^{-1\mathrm{}}$ of $\Upsilon \left(4S\right)$ data to measure the $B$-meson semileptonic branching fraction. The $B\to \mathrm{Xe}\nu$ momentum spectrum was obtained over nearly the full momentum range by using charge and kinematic correlations in events with a high-momentum lepton tag and an additional electron. We find $B(B\to \mathrm{Xe}\nu )=(10.49\pm 0.17\pm 0.43)\%$, with overall systematic uncertainties less than those of untagged single-lepton measurements. We use this result to calculate the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $V_{\mathrm{cb}}$ and to set an upper limit on the fraction of $\Upsilon \left(4S\right)$ decays to final states other than $B\overline{B}$.