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−l+ν, B+ →π0l+ν,B+ →ωl+ν,B+ →ηl+νandB+ →η′l+ν,(l=eorμ)undertakenwithap- proximately 462 × 106 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 recon- struction 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−l+ν, B+ → π0l+ν, B+ → ωl+ν and B+ → ηl+ν. From these distributions, we extract the form-factor shapes f+(q2) and the total branchingfractionsB(B0 →π−l+ν)=(1.45±0.04stat ±0.06syst)×10−4 (combinedπ− andπ0 de- cay channels assuming isospin symmetry), B(B+ → ωl+ν) = (1.19 ± 0.16stat ± 0.09syst) × 10−4 and B(B+ → ηl+ν) = (0.38 ± 0.05stat ± 0.05syst) × 10−4. We also measure B(B+ → η′l+ν) = (0.24 ± 0.08stat ± 0.03syst) × 10−4. We obtain values for the magnitude of the CKM matrix element |Vub| by direct comparison with three different QCD calculations in restricted q2 ranges of B → πl+ν decays. From a simultaneous fit to the experimental data over the full q2 range and the FNAL/MILC lattice QCD predictions, we obtain |Vub| = (3.25 ± 0.31) × 10−3, where the error is the combined experimental and theoretical uncertainty.