Study of B{yields}{pi}l{nu} and B{yields}{rho}l{nu} decays and determination of |V{sub ub}|

We present an analysis of exclusive charmless semileptonic B-meson decays based on 377x10{sup 6} BB pairs recorded with the BABAR detector at the {Upsilon}(4S) resonance. We select four event samples corresponding to the decay modes B{sup 0}{yields}{pi}{sup -}l{sup +}{nu}, B{sup +}{yields}{pi}{sup 0}l{sup +}{nu}, B{sup 0}{yields}{rho}{sup -}l{sup +}{nu}, and B{sup +}{yields}{rho}{sup 0}l{sup +}{nu} and find the measured branching fractions to be consistent with isospin symmetry. Assuming isospin symmetry, we combine the two B{yields}{pi}l{nu} samples, and similarly the two B{yields}{rho}l{nu} samples, and measure the branching fractions B(B{sup 0}{yields}{pi}{sup -}l{sup +}{nu})=(1.41{+-}0.05{+-}0.07)x10{sup -4} and B(B{sup 0}{yields}{rho}{sup -}l{sup +}{nu})=(1.75{+-}0.15{+-}0.27)x10{sup -4}, where the errors are statistical and systematic. We compare the measured distribution in q{sup 2}, the momentum transfer squared, with predictions for the form factors from QCD calculations and determine the Cabibbo-Kobayashi-Maskawa matrix element |V{sub ub}|. Based on the measured partial branching fraction for B{yields}{pi}l{nu} in the range q{sup 2}<12 GeV{sup 2} and the most recent QCD light-cone sum-rule calculations, we obtain |V{sub ub}|=(3.78{+-}0.13{sub -0.40}{sup +0.55})x10{sup -3}, where the errors refer to the experimental and theoretical uncertainties. From a simultaneous fit to the data over the full q{sup 2} range and the FNAL/MILC lattice QCD results, we obtain |V{sub ub}|=(2.95{+-}0.31)x10{sup -3} from B{yields}{pi}l{nu}, where themore » error is the combined experimental and theoretical uncertainty.« less