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, $B^{+}\to {\eta }^{(\mathrm{\prime })}{\mathrm{\ell }}^{+}\nu$ and $B^{0} \rightarrow \pi^{-} \ell^{+} \nu$,undertakenwithapproximately464million$\ensuremath{B\kern 0.18em\bar{\kern -0.18em B}}$pairscollectedatthe$\Upsilon(4S)$resonancewiththe${{\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}}}$ 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 $B^{+}\to \eta {\mathrm{\ell }}^{+}\nu$ and $B^{0} \rightarrow \pi^{-} \ell^{+} \nu$decaysinthreeandtwelvebinsof$q^2$, respectively, from which we extract the $f_{+}(q^2)$ form-factor shapes and the total branching fractions $\text{ensuremath}\mathcal{B}(B^{+}\to \eta {\mathrm{\ell }}^{+}\nu )$ $ = (0.36 \pm 0.05_{stat} \pm 0.04_{syst}) \times 10^{-4}$and${\ensuremath{\cal B}}(B^{0} \rightarrow \pi^- \ell^{+} \nu)$ $ = (1.42 \pm 0.05_{stat} \pm 0.07_{syst}) \times 10^{-4}$\mathrm{.}Wealsomeasure${\ensuremath{\cal B}}(B^{+} \rightarrow \eta^{\prime} \ell^{+} \nu)$ $ = (0.24 \pm 0.08_{stat} \pm 0.03_{syst}) \times 10^{-4}$\mathrm{.}WeobtainvaluesforthemagnitudeoftheCKMmatrixelement$\ensuremath{|V_{ub}|}$ using three different QCD calculations.