Study of \(B→πlν\) and \(B→ρlν\) Decays and Determination of \(|V_{ub}|\)

Abstract

We present an analysis of exclusive charmless semileptonic B-meson decays based on \(377×10^{6} B\overline{B}\) pairs recorded with the BABAR detector at the \(Υ(4S)\) 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 \(B→πℓν\) samples, and similarly the two \(B→ρℓν\) 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 \(|V_{ub}|\). Based on the measured partial branching fraction for \(B→πℓν\) in the range \(q^{2}<12 GeV^{2}\) and the most recent QCD light-cone sum-rule calculations, we obtain \(|V_{ub}|=(3.78±0.13^{+0.55}_{−0.40})×10^{−3}\), 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 \(|V_{ub}|=(2.95±0.31)×10^{−3}\) from \(B→πℓν\), where the error is the combined experimental and theoretical uncertainty.