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

We report the results of a study of the exclusive charmless semileptonic decays, B0→π ^-+ν, B ⁺→π0 ⁺ν, B ⁺→ω ⁺ν, B ⁺→η ⁺ν, and B ⁺→η ^′+ν (=e or μ) undertaken with approximately 462×106 BB̄ pairs collected at the Υ(4S) 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 q2, the square of the momentum transferred to the lepton-neutrino pair, for B0→π ^{- +}ν, B ⁺→π0 ⁺ν, B ⁺→ω ⁺ν, and B ⁺→η ⁺ν. From these distributions, we extract the form-factor shapes f ₊(q2) and the total branching fractions B(B0→π ^-+ν)=(1.45±0.04 _stat±0.06 _syst)×10 ^-4 (combined π ^- and π0 decay channels assuming isospin symmetry), B(B ⁺→ω ⁺ν)=(1.19±0.16 _stat±0.09 _syst)×10 ^-4 and B(B ⁺→η ⁺ν)=(0.38±0.05 _stat±0.05 _syst)×10 ^-4. We also measure B(B ⁺→η ^′+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 _ub| by direct comparison with three different QCD calculations in restricted q2 ranges of B→π ⁺ν decays. From a simultaneous fit to the experimental data over the full q2 range and the FNAL/MILC lattice QCD predictions, we obtain |V _ub|=(3.25±0.31)×10 ^-3, where the error is the combined experimental and theoretical uncertainty.