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, B^0→π^-ℓ^+ν, B^+→π^0ℓ^+ν, B^+→ωℓ^+ν, B^+→ηℓ^+ν, and B^+→η′ℓ^+ν (ℓ=e or μ) undertaken with approximately 462×10^6 BB 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 q^2, the square of the momentum transferred to the lepton-neutrino pair, for B^0→π^-ℓ^+ν, B^+→π^0ℓ^+ν, B^+→ωℓ^+ν, and B^+→ηℓ^+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+→ωℓ+ν)=(1.19±0.16_(stat)±0.09_(syst))×10^(-4) and B(B^+→ηℓ^+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_(ub)| by direct comparison with three different QCD calculations in restricted q^2 ranges of B→πℓ^+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_(ub)|=(3.25±0.31)×10^(-3), where the error is the combined experimental and theoretical uncertainty