Measurement of theB→DℓνBranching Fractions and Form Factor

Using a sample of $3.3\times {10}^6$ $B$-meson decays collected with the CLEO detector at the Cornell Electron Storage Ring, we have studied $B^{-}\to D^0{\ell }^{-}\overline{\nu }$ and ${\overline{B}}^0\to D^{+}{\ell }^{-}\overline{\nu }$ decays, where ${\ell }^{-}$ can be either $e^{-}$ or ${\mu }^{-}$. We distinguish $B\to D\ell \nu$ from other $B$ semileptonic decays by examining the net momentum and energy of the particles recoiling against $D-\ell$ pairs. We find $\Gamma (B\to D\ell \nu )=(14.1\pm 1.0\pm 1.2){\mathrm{ns}}^{-1\mathrm{}}$ and derive branching fractions for $B^{-}\to D^0{\ell }^{-}\overline{\nu }$ and ${\overline{B}}^0\to D^{+}{\ell }^{-}\overline{\nu }$ of $(2.32\pm 0.17\pm 0.20)\%$ and $(2.20\pm 0.16\pm 0.19)\%$, respectively, where the uncertainties are statistical and systematic. We also investigate the $B\to D\ell \nu$ form factor and the implication of the result for $|V_{\mathrm{cb}}|$.