Measurements of branching fractions and q2 distributions for B→πℓν and B→ρℓν decays with B→D(∗)ℓν decay tagging

We report measurements of the charmless semileptonic decays B0→π−/ρ−ℓ+ν and B+→π0/ρ0ℓ+ν, based on a sample of 2.75×108BB¯ events collected at the ϒ(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(∗)ℓν, 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+→π0ℓ+ν)=(0.77±0.14±0.08±0.00)×10−4 and B(B+→ρ0ℓ+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⩾16GeV2/c2. From our partial branching fractions for B→πℓν and recent results for the form factor from unquenched Lattice QCD calculations, we obtain values of the CKM matrix element |Vub|.