Branching fraction and form-factor shape measurements of exclusive charmless semileptonic B decays, and determination of |Vub|

Abstract

We report the results of a study of the exclusive charmless semileptonic decays, B 0 → π − ℓ + ν , B + → π 0 ℓ + ν , B + → ω ℓ + ν , B + → η ℓ + ν , and B + → η ′ ℓ + ν ( ℓ = e or μ ) undertaken with approximately 462 × 10 6 B ¯¯¯ B pairs collected at the Υ ( 4 S ) resonance with the BABAR detector. The analysis uses events in which the signal B decays are reconstructed with a loose neutrino reconstruction technique. We obtain partial branching fractions in several bins of q 2 , the square of the momentum transferred to the lepton-neutrino pair, for B 0 → π − ℓ + ν , B + → π 0 ℓ + ν , B + → ω ℓ + ν , and B + → η ℓ + ν . From these distributions, we extract the form-factor shapes f + ( q 2 ) and the total branching fractions B ( B 0 → π − ℓ + ν ) = ( 1.45 ± 0.04 stat ± 0.06 syst ) × 10 − 4 (combined π − and π 0 decay channels assuming isospin symmetry), B ( B + → ω ℓ + ν ) = ( 1.19 ± 0.16 stat ± 0.09 syst ) × 10 − 4 and B ( B + → η ℓ + ν ) = ( 0.38 ± 0.05 stat ± 0.05 syst ) × 10 − 4 . We also measure B ( B + → η ′ ℓ + ν ) = ( 0.24 ± 0.08 stat ± 0.03 syst ) × 10 − 4 . We obtain values for the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element | V u b | by direct comparison with three different QCD calculations in restricted q 2 ranges of B → π ℓ + ν decays. From a simultaneous fit to the experimental data over the full q 2 range and the FNAL/MILC lattice QCD predictions, we obtain | V u b | = ( 3.25 ± 0.31 ) × 10 − 3 , where the error is the combined experimental and theoretical uncertainty.