Measurement of the B to pi l nu Branching Fraction and Determination of |Vub| with Tagged B Mesons

We report a measurement of the B → π(ell)ν branching fraction based on 211 fb-1 of data collected with the BABAR detector. We use samples of B0 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 Β(B0 → π-(ell)+ν) = (1.33 ± 0.17stat ± 0.11syst) x 10-4. We determine the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element |Vub| 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 |ub| = (4.5 ± 0.5stat ± 0.3syst-0.5+0.7FF) x 10-3, where the last error is due to the normalization of the form factor