Study of B→πlν and B→ρlν decays and determination of |Vub|

Abstract

We present an analysis of exclusive charmless semileptonic B -meson decays based on 377 × 10 6 B ¯¯¯ B pairs recorded with the BABAR detector at the Υ ( 4 S ) resonance. We select four event samples corresponding to the decay modes B 0 → π − ℓ + ν , B + → π 0 ℓ + ν , B 0 → ρ − ℓ + ν , and B + → ρ 0 ℓ + ν and find the measured branching fractions to be consistent with isospin symmetry. Assuming isospin symmetry, we combine the two B → π ℓ ν samples, and similarly the two B → ρ ℓ ν samples, and measure the branching fractions B ( B 0 → π − ℓ + ν ) = ( 1.41 ± 0.05 ± 0.07 ) × 10 − 4 and B ( B 0 → ρ − ℓ + ν ) = ( 1.75 ± 0.15 ± 0.27 ) × 10 − 4 , where the errors are statistical and systematic. We compare the measured distribution in q 2 , the momentum transfer squared, with predictions for the form factors from QCD calculations and determine the Cabibbo-Kobayashi-Maskawa matrix element | V u b | . Based on the measured partial branching fraction for B → π ℓ ν in the range q 2 < 12 GeV 2 and the most recent QCD light-cone sum-rule calculations, we obtain | V u b | = ( 3.78 ± 0.13 + 0.55 − 0.40 ) × 10 − 3 , where the errors refer to the experimental and theoretical uncertainties. From a simultaneous fit to the data over the full q 2 range and the FNAL/MILC lattice QCD results, we obtain | V u b | = ( 2.95 ± 0.31 ) × 10 − 3 from B → π ℓ ν , where the error is the combined experimental and theoretical uncertainty.