Measurement of the B→πlν Branching Fraction and Determination of |Vub| with Tagged B Mesons

We report a measurement of the B→πℓν 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 B(B0→π-ℓ+ν)=(1.33±0.17stat±0.11syst)×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 |Vub|=(4.5±0.5stat±0.3syst+0.7/-0.5FF)×10-3, where the last error is due to the normalization of the form factor.