# Measurement of the Decays $\boldsymbol{B\to\eta\ell\nu_\ell}$ and $\boldsymbol{B\to\eta^\prime\ell\nu_\ell}$ in Fully Reconstructed Events at Belle

## TL;DR

This paper measures the branching fractions of B meson decays to eta and eta' mesons with leptons, using a large data sample from the Belle experiment, providing new experimental results on these rare decays.

## Contribution

First measurement of B+ to eta and eta' semileptonic decays using fully reconstructed events at Belle.

## Key findings

- Measured B+ to eta l nu branching fraction as (4.2 ± 1.1 ± 0.3) × 10^{-5}
- Set an upper limit of 0.72 × 10^{-4} for B+ to eta' l nu at 90% CL
- Utilized 711 fb^{-1} of data with full reconstruction technique.

## Abstract

We report branching fraction measurements of the decays $B^+\to\eta\ell^+\nu_\ell$ and $B^+\to\eta^\prime\ell^+\nu_\ell$ based on 711~fb$^{-1}$ of data collected near the $\Upsilon(4S)$ resonance with the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider. This data sample contains 772 million $B\bar B$~events. One of the two $B$~mesons is fully reconstructed in a hadronic decay mode. Among the remaining ("signal-$B$") daughters, we search for the $\eta$~meson in two decay channels, $\eta\to\gamma\gamma$ and $\eta\to\pi^+\pi^-\pi^0$, and reconstruct the $\eta^{\prime}$~meson in $\eta^\prime\to\eta\pi^+\pi^-$ with subsequent decay of the $\eta$ into $\gamma\gamma$. Combining the two $\eta$ modes and using an extended maximum likelihood, the $B^+\to\eta\ell^+\nu_\ell$ branching fraction is measured to be $(4.2\pm 1.1 (\rm stat.)\pm 0.3 (\rm syst.))\times 10^{-5}$. For $B^+\to\eta^\prime\ell^+\nu_\ell$, we observe no significant signal and set an upper limit of $0.72\times 10^{-4}$ at 90\% confidence level.