First Measurement of theB→πℓνandB→ρ(ω)ℓνBranching Fractions

CLEO has studied $B$ decays to $\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\nu}$, $\ensuremath{\rho}\ensuremath{\ell}\ensuremath{\nu}$, and $\ensuremath{\omega}\ensuremath{\ell}\ensuremath{\nu}$, where $\ensuremath{\ell}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}eor\ensuremath{\mu}$, by incorporating the missing momentum into full $B$ reconstruction. With the ${B}^{0}$ and ${B}^{+}$ modes combined according to isospin predictions for the relative partial widths, we obtain $B({B}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\ell}}^{+}\ensuremath{\nu})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(1.8\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.3\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and $B({B}^{0}\ensuremath{\rightarrow}{\ensuremath{\rho}}^{\ensuremath{-}}{\ensuremath{\ell}}^{+}\ensuremath{\nu})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(2.5\ifmmode\pm\else\textpm\fi{}{0.4}_{\ensuremath{-}0.7}^{+0.5}\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, where the errors are statistical, systematic, and the estimated model dependence. We also estimate $|{V}_{\mathrm{ub}}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(3.3\ifmmode\pm\else\textpm\fi{}{0.2}_{\ensuremath{-}0.4}^{+0.3}\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.