Study of B -> pi l nu and B -> rho l nu decays and determination of |Vub|

We present an analysis of exclusive charmless semileptonic B-meson decays based on 377×106 BB̅ pairs recorded with the BABAR detector at the Υ(4S) resonance. We select four event samples corresponding to the decay modes B0→π-l+ν, B+→π0l+ν, B0→ρ-l+ν, and B+→ρ0l+ν and find the measured branching fractions to be consistent with isospin symmetry. Assuming isospin symmetry, we combine the two B→πlν samples, and similarly the two B→ρlν samples, and measure the branching fractions B(B0→π-l+ν)=(1.41±0.05±0.07)×10-4 and B(B0→ρ-l+ν)=(1.75±0.15±0.27)×10-4, where the errors are statistical and systematic. We compare the measured distribution in q2, the momentum transfer squared, with predictions for the form factors from QCD calculations and determine the Cabibbo-Kobayashi-Maskawa matrix element |Vub|. Based on the measured partial branching fraction for B→πlν in the range q2<12 GeV2 and the most recent QCD light-cone sum-rule calculations, we obtain |Vub|=(3.78±0.13-0.40+0.55)×10-3, where the errors refer to the experimental and theoretical uncertainties. From a simultaneous fit to the data over the full q2 range and the FNAL/MILC lattice QCD results, we obtain |Vub|=(2.95±0.31)×10-3 from B→πlν, where the error is the combined experimental and theoretical uncertainty.