Study of exclusive charmless semileptonicBdecays and extraction of|Vub|at CLEO

We have studied semileptonic $B$ decay to the exclusive charmless states $\pi$, $\rho /\omega$, $\eta$ and ${\eta }^{\prime }$ using the full $15.5{\mathrm{fb}}^{-1}$ CLEO ${\rm Y}(4S)$ sample, with measurements performed in subregions of phase space to minimize dependence on a priori knowledge of the form factors involved. We find total branching fractions $\mathcal{B}(B^0\to {\pi }^{-}{\ell }^{+}\nu )=(1.37\pm {0.15}_{\mathrm{stat}}\pm {0.11}_{\mathrm{sys}})\times {10}^{-4}$ and $\mathcal{B}(B^0\to {\rho }^{-}{\ell }^{+}\nu )=(2.93\pm {0.37}_{\mathrm{stat}}\pm {0.37}_{\mathrm{sys}})\times {10}^{-4}$. We find evidence for $B^{+}\to {\eta }^{\prime }{\ell }^{+}\nu$, with $\mathcal{B}(B^{+}\to {\eta }^{\prime }{\ell }^{+}\nu )=(2.66\pm \pm {0.80}_{\mathrm{stat}}\pm {0.56}_{\mathrm{sys}})\times {10}^{-4}$ and $1.20\times {10}^{-4}<\mathcal{B}(B^{+}\to {\eta }^{\prime }{\ell }^{+}\nu )<4.46\times {10}^{-4}$ (90% CL). We also limit $\mathcal{B}(B^{+}\to \eta {\ell }^{+}\nu )<1.01\times {10}^{-4}$ (90% CL). By combining our $B\to \pi \ell \nu$ information with unquenched lattice calculations, we find $|V_{ub}|=(3.6\pm {0.4}_{\mathrm{stat}}\pm 0.2_{\mathrm{sys}-0.4\mathrm{thy}}{}^{+0.6})\times {10}^{-3}$.