Measurements of branching fractions and q2 distributions for B → π ℓ ν and B → ρ ℓ ν decays with B → D(*) ℓ ν decay tagging

We report measurements of the charmless semileptonic decays B0 → π- / ρ- ℓ+ ν and B+ → π0 / ρ0 ℓ+ ν, based on a sample of 2.75 × 108B over(B, ̄) events collected at the Υ{hooked} (4 S) 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 ν, enabling detection of the signal modes with high purity. We measure the branching fractions B (B0 → π- ℓ+ ν) = (1.38 ± 0.19 ± 0.14 ± 0.03) × 10-4, B (B0 → ρ- ℓ+ ν) = (2.17 ± 0.54 ± 0.31 ± 0.08) × 10-4, B (B+ → π0 ℓ+ ν) = (0.77 ± 0.14 ± 0.08 ± 0.00) × 10-4 and B (B+ → ρ0 ℓ+ ν) = (1.33 ± 0.23 ± 0.17 ± 0.05) × 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 q2 intervals: q2 < 8, 8 ≤ q2 < 16, and q2 ≥ 16   GeV2 / c2. From our partial branching fractions for B → π ℓ ν and recent results for the form factor from unquenched Lattice QCD calculations, we obtain values of the CKM matrix element | Vu b |. © 2007 Elsevier B.V. All rights reserved.