Measurement of the B^0 --> pi^-ell^+nu and B^+ --> eta^(')ell^+nu Branching Fractions, the B^0 --> pi^-ell^+nu and B^+ --> eta ell^+nu Form-Factor Shapes, and Determination of |V_{ub}|

TL;DR

This paper measures branching fractions and form-factor shapes for specific B meson decays, providing insights into the CKM matrix element |V_{ub}| using a large dataset from the Belle experiment.

Contribution

It presents the first detailed measurement of branching fractions and form-factor shapes for B→η(')ℓν and B→πℓν decays, and extracts |V_{ub}| with multiple QCD calculations.

Findings

Branching fraction for B+→ηℓν: (0.36 ± 0.05_{stat} ± 0.04_{syst})×10^{-4}

Branching fraction for B0→π−ℓ+ν: (1.42 ± 0.05_{stat} ± 0.07_{syst})×10^{-4}

Branching fraction for B+→η′ℓν: (0.24 ± 0.08_{stat} ± 0.03_{syst})×10^{-4}

Abstract

We report the results of a study of the exclusive charmless semileptonic decays, $B^{+} \rightarrow \eta^{(\prime)} \ell^{+} \nu$ and $B^{0} \rightarrow \pi^{-} \ell^{+} \nu$, undertaken with approximately 464 million $\ensuremath{B\kern 0.18em\bar{\kern -0.18em B}}$ pairs collected at the $\Upsilon(4S)$ resonance with the ${{\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}}}$ detector. The analysis uses events in which the signal B decays are reconstructed with a loose neutrino reconstruction technique. We obtain partial branching fractions for $B^{+} \rightarrow \eta \ell^{+} \nu$ and $B^{0} \rightarrow \pi^{-} \ell^{+} \nu$ decays in three and twelve bins of $q^2$, respectively, from which we extract the $f_+(q^2)$ form-factor shapes and the total branching fractions ${\ensuremath{\cal B}}(B^{+} \rightarrow \eta \ell^{+} \nu)$ $ = (0.36 \pm 0.05_{stat} \pm 0.04_{syst}) \times 10^{-4}$ and ${\ensuremath{\cal B}}(B^{0} \rightarrow \pi^- \ell^{+} \nu)$ $ = (1.42 \pm 0.05_{stat} \pm 0.07_{syst}) \times 10^{-4}$. We also measure ${\ensuremath{\cal B}}(B^{+} \rightarrow \eta^{\prime} \ell^{+} \nu)$ $ = (0.24 \pm 0.08_{stat} \pm 0.03_{syst}) \times 10^{-4}$. We obtain values for the magnitude of the CKM matrix element $\ensuremath{|V_{ub}|}$ using three different QCD calculations.