Measurement of Exclusive $B\to X_u\ell \hat{I}½$ Decays with $D^{(\ast )}\ell \hat{I}½$ Decay Tagging

Abstract

We report a measurement of the charmless semileptonic decays $B^0\to \ddot{I}{€}^{-}/\ddot{I}{}^{-}{\ell }^{+}\hat{I}½$ and $B^{+}\to \ddot{I}{€}^0/\ddot{I}{}^0{\ell }^{+}\hat{I}½$, based on 253 fb${}^{-1}$ of data collected with the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric collider. In this analysis, the accompanying $B$ meson is reconstructed in the semileptonic mode $B\to D^{(\ast )}\ell \hat{I}½$, enabling detection of the signal modes with high purity. We measure the branching fractions $\mathcal{B}(B^0\to \ddot{I}{€}^{-}{\ell }^{+}\hat{I}½)=(1.48\pm 0.20\pm 0.16\pm 0.04)\times 10^{-4}$, $\mathcal{B}(B^0\to \ddot{I}{}^{-}{\ell }^{+}\hat{I}½)=(2.07\pm 0.47\pm 0.25\pm 0.14)\times 10^{-4}$, $\mathcal{B}(B^{+}\to \ddot{I}{€}^0{\ell }^{+}\hat{I}½)=(0.76\pm 0.13\pm 0.08\pm 0.04)\times 10^{-4}$ and $\mathcal{B}(B^{+}\to \ddot{I}{}^0{\ell }^{+}\hat{I}½)=(1.39\pm 0.23\pm 0.17\pm 0.03)\times 10^{-4}$, where the errors are statistical, experimental systematic, and systematic due to form-factor uncertainties, respectively. For each mode we also present the partial branching fractions in three $q^2$ intervals; $q^2<8$, $8\le q^2<16$, and $q^2\ge 16$ GeV${}^2/c^2$. Based on these results, the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$ is extracted. All of the presented results are preliminary.

16 pages, 19 figures, submitted to the HEP2005 Europhysics Conference in Lisbon, Portgal