Study of Exclusive Charmless Semileptonic $B$ Decays and $|V_{ub}|$

We study semileptonic $B$ decay to the exclusive charmless states $\pi$, $\rho /\omega$, $\eta$, and ${\eta }^{\prime }$ using the $16{\mathrm{fb}}^{-1}$ CLEO ${\rm Y}(4S)$ data sample. We find $\mathcal{B}(B^0\to {\pi }^{-}{\ell }^{+}\nu )=(1.37\pm {0.15}_{\mathrm{stat}}\pm {0.11}_{\mathrm{sys}})\times {10}^{-4}$ and $\mathcal{B}(B^0\to {\rho }^{-}{\ell }^{+}\nu )=(2.93\pm {0.37}_{\mathrm{stat}}\pm {0.37}_{\mathrm{sys}})\times {10}^{-4}$ and find evidence for $B^{+}\to {\eta }^{\prime }{\ell }^{+}\nu$, with $\mathcal{B}(B^{+}\to {\eta }^{\prime }{\ell }^{+}\nu )=(2.66\pm {0.80}_{\mathrm{stat}}\pm {0.56}_{\mathrm{sys}})\times {10}^{-4}$. From our $B\to \pi \ell \nu$ rate for $q^2>16{\mathrm{GeV}}^2$ and lattice QCD, we find $|V_{ub}|=(3.6\pm {0.4}_{\mathrm{stat}}\pm {0.2}_{\mathrm{syst}-0.4\mathrm{thy}}^{+0.6})\times {10}^{-3}$.

Figure 1

$M_{h\ell \nu }$ for candidates in the $\Delta E$ signal region [$-0.15$, 0.25 GeV] for the $B\to \pi \ell \nu$ modes combined and summed over $q^2$. Dotted lines indicate the binning used by the fitter. Fit components, from bottom to top, are $b\to c$, continuum, fake signal leptons, other $B\to X_u\ell \nu$, $B\to \eta \ell \nu$ cross feed, $B\to \rho \ell \nu$ cross feed, $B\to \pi \ell \nu$ cross feed, and signal.

Figure 2

$M_{\pi \pi }$ in the restricted signal region for the combined $B\to \rho \ell \nu$ modes, with components as described in Fig. 1. Arrows indicate the range included in the fit.