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Measurement of the B^0 --> pi^-ell^+nu and B^+ --> eta^(')ell^+nu Branching Fractions, the B^0 --> pi^-ell^+nu and B^+ --> eta ell^+nu Form-Factor Shapes, and Determination of |V_{ub}|

We report the results of a study of the exclusive charmless semileptonic decays, $B^{+} \rightarrow η^{(\prime)} \ell^{+} ν$ and $B^{0} \rightarrow π^{-} \ell^{+} ν$, undertaken with approximately 464 million $\ensuremath{B\kern 0.18em\bar{\kern -0.18em B}}$ pairs collected at the $Υ(4S)$ resonance with the ${{\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}}}$ detector. The analysis uses events in which the signal B decays are reconstructed with a loose neutrino reconstruction technique. We obtain partial branching fractions for $B^{+} \rightarrow η\ell^{+} ν$ and $B^{0} \rightarrow π^{-} \ell^{+} ν$ decays in three and twelve bins of $q^2$, respectively, from which we extract the $f_+(q^2)$ form-factor shapes and the total branching fractions ${\ensuremath{\cal B}}(B^{+} \rightarrow η\ell^{+} ν)$ $ = (0.36 \pm 0.05_{stat} \pm 0.04_{syst}) \times 10^{-4}$ and ${\ensuremath{\cal B}}(B^{0} \rightarrow π^- \ell^{+} ν)$ $ = (1.42 \pm 0.05_{stat} \pm 0.07_{syst}) \times 10^{-4}$. We also measure ${\ensuremath{\cal B}}(B^{+} \rightarrow η^{\prime} \ell^{+} ν)$ $ = (0.24 \pm 0.08_{stat} \pm 0.03_{syst}) \times 10^{-4}$. We obtain values for the magnitude of the CKM matrix element $\ensuremath{|V_{ub}|}$ using three different QCD calculations.