Measurement of theB→πlνBranching Fraction and Determination of|Vub|with TaggedBMesons

We report a measurement of the $B\to \pi \ell \nu$ branching fraction based on $211{\mathrm{fb}}^{-1}$ of data collected with the BABAR detector. We use samples of $B^0$ and $B^{+}$ mesons tagged by a second $B$ meson reconstructed in a semileptonic or hadronic decay and combine the results assuming isospin symmetry to obtain $\mathcal{B}(B^0\to {\pi }^{-}{\ell }^{+}\nu )=(1.33\pm {0.17}_{\mathrm{stat}}\pm {0.11}_{\mathrm{syst}})\times {10}^{-4}$. We determine the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ by combining the partial branching fractions measured in ranges of the momentum transfer squared and theoretical calculations of the form factor. Using a recent lattice QCD calculation, we find $|V_{ub}|=(4.5\pm {0.5}_{\mathrm{stat}}\pm {0.3}_{\mathrm{syst}}{\genfrac{}{}{0ex}{}{+0.7}{-0.5}}_{\mathrm{FF}})\times {10}^{-3}$, where the last error is due to the normalization of the form factor.