First Measurement of the $\mathit{B}\to \pi \ell \nu$ and $\mathit{B}\to \rho \left(\omega \right)\ell \nu$ Branching Fractions

CLEO has studied $B$ decays to $\pi \ell \nu$, $\rho \ell \nu$, and $\omega \ell \nu$, where $\ell =eor\mu$, 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 $B(B^0\to {\pi }^{-}{\ell }^{+}\nu )=(1.8\pm 0.4\pm 0.3\pm 0.2)\times {10}^{-4\mathrm{}}$ and $B(B^0\to {\rho }^{-}{\ell }^{+}\nu )=(2.5\pm {0.4}_{-0.7\mathrm{}}^{+0.5\mathrm{}}\pm 0.5)\times {10}^{-4\mathrm{}}$, where the errors are statistical, systematic, and the estimated model dependence. We also estimate $|V_{\mathrm{ub}}|=(3.3\pm {0.2}_{-0.4\mathrm{}}^{+0.3\mathrm{}}\pm 0.7)\times {10}^{-3\mathrm{}}$.