Measurement of the Decays $\mathbf{B}\to \mathbf{\eta }\mathbf{\ell }{\mathbf{\nu }}_{\mathbf{\ell }}$ and $\mathbf{B}\to {\mathbf{\eta }}^{\prime }\mathbf{\ell }{\mathbf{\nu }}_{\mathbf{\ell }}$ in Fully Reconstructed Events at Belle

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 ${\rm Y}(4S)$ 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 $(4.2\pm 1.1(stat.)\pm 0.3(syst.))\times 10^{-5}$. 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.

arXiv comment8 pages, 4 figures

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• Semileptonic $B$ decays and $|V_{xb}|$ update