Measurements of B→{π,η,η′}lνl Branching Fractions and Determination of |Vub| with Semileptonically Tagged B Mesons

We report measurements of branching fractions for the decays B→Pℓνℓ, where P are the pseudoscalar charmless mesons π-, π0, η and η′, based on 348  fb-1 of data collected with the BABAR detector, using B0 and B+ mesons found in the recoil of a second B meson decaying as B→D(*)ℓνℓ. Assuming isospin symmetry, we combine pionic branching fractions to obtain B(B0→π-ℓ+νℓ)=(1.54±0.17(stat)±0.09(syst))×10-4; we find 3.2σ evidence of the decay B+→ηℓ+νℓ and measure its branching fraction to be (0.64±0.20(stat)±0.03(syst))×10-4, and determine B(B+→η′ℓ+νℓ)<0.47×10-4 to 90% confidence level. Using partial branching fractions for the pionic decays in ranges of the momentum transfer and a variety of form factor calculation, we obtain values of the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element |Vub| in ranging from 3.6×10-3 to 4.1×10-3.