| Abstract |
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We report the results of a study of the exclusive charmless semileptonic
decays, B0 --> pi- ell+ nu, B+ --> pi0 ell+ nu, B+ --> omega ell+ nu,
B+ --> eta ell+ nu and B+ --> eta' ell+ nu, (ell = e or mu) undertaken with
approximately 462 x 10^6 BBbar pairs collected at the Upsilon(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 B0 --> pi- ell+ nu,
B+ --> pi0 ell+ nu, B+ --> omega ell+ nu and B+ --> eta ell+ nu. From these
distributions, we extract the form-factor shapes f+(q^2) and the
total branching fractions BF(B0 --> pi- ell+ nu = (1.45 +/- 0.04_{stat}
+/- 0.06_{syst}) x 10^-4 (combined pi- and pi0 decay channels assuming isospin
symmetry), BF(B+ --> omega ell+ nu = (1.19 +/- 0.16_{stat} +/- 0.09_{syst})
x 10^-4 and BF(B+ --> eta ell+ nu = (0.38 +/- 0.05_{stat} +/- 0.05_{syst})
x 10^-4. We also measure BF(B+ --> eta' ell+ nu = (0.24 +/- 0.08_{stat}
+/- 0.03_{syst}) x 10^-4. We obtain values for the magnitude of the CKM matrix
element |V_{ub}| by direct comparison with three different QCD calculations in
restricted q^2 ranges of B --> pi ell+ nu 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) x 10^-3,
where the error is the combined experimental and theoretical uncertainty.
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