Measurement of the B ->pi l nu branching fraction and determination of vertical bar V-ub vertical bar with tagged B mesons

We report a measurement of the B ->pi center dot nu branching fraction based on 211 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 B(B-0 ->pi(-)center dot(+)nu)=(1.33 +/- 0.17(stat)+/- 0.11(syst))x10(-4). We determine the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element vertical bar V-ub vertical bar 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 vertical bar V-ub vertical bar=(4.5 +/- 0.5(stat)+/- 0.3(syst)0.7/0.5(FF))x10(-3), where the last error is due to the normalization of the form factor.