Measurements of branching fractions and q**2 distributions for B ---> pi l nu and B ---> rho l nu decays with B ---> D(*) l nu decay tagging

We report measurements of the charmless semileptonic decays B-0 -> pi(-)/rho(-)l(+)nu and B+ -> pi(0)/rho(0)l(+)nu, based on a sample of 2.75 x 10(8) BB events collected at the Y(4S) resonance with the Belle detector at the KEKB e(+)e(-) asymmetric collider. In this analysis, the accompanying B meson is reconstructed in the semileptonic mode B -> D((*))l nu, enabling detection of the signal modes with high purity. We measure the branching fractions B(B-0 -> pi(-)l(+)nu) = (1.38 +/- 0.19 +/- 0.14 +/- 0.03) x 10(-4), B(B-0 -> rho(-)l(+)nu) = (2.17 +/- 0.54 +/- 0.31 +/- 0.08) x 10(-4), B(B+ -> pi(0)l(+)nu) = (0.77 +/- 0.14 +/- 0.08 +/- 0.00) x 10(-4) and B(B+ -> rho(0)l(+)nu) = (1.33 +/- 0.23 +/- 0.17 +/- 0.05) x 10(-4), where the errors are statistical, experimental systematic, and systematic due to form factor uncertainties, respectively. For each mode we also present the partial branching fractions in three q(2) intervals: q(2) = 16 GeV2/c(2). From our partial branching fractions for B -> pi l nu and recent results for the form factor from unquenched Lattice QCD calculations, we obtain values of the CKM matrix element vertical bar V-ub vertical bar. (C) 2007 Elsevier B.V. All rights reserved.