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

Abstract

We report a measurement of the B → π ℓ ν 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 → π − ℓ + ν ) = ( 1.33 ± 0.17 stat ± 0.11 syst ) × 10 − 4 . We determine the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element | V u b | 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 u b | = ( 4.5 ± 0.5 stat ± 0.3 syst + 0.7 − 0.5 FF ) × 10 − 3 , where the last error is due to the normalization of the form factor.