Measurement of the decays B→ηℓνℓ and B→η′ℓνℓ in fully reconstructed events at Belle

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Abstract

We report branching fraction measurements of the decays ${B}^{+}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ and ${B}^{+}\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ based on $711\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data collected near the $\mathrm{\ensuremath{\Upsilon}}(4S)$ resonance with the Belle experiment at the KEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider. This data sample contains 772 million $B\overline{B}$ 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 $\ensuremath{\eta}$ meson in two decay channels, $\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ and $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$, and reconstruct the ${\ensuremath{\eta}}^{\ensuremath{'}}$ meson in ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ with subsequent decay of the $\ensuremath{\eta}$ into $\ensuremath{\gamma}\ensuremath{\gamma}$. Combining the two $\ensuremath{\eta}$ modes and using an extended maximum likelihood, the ${B}^{+}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ branching fraction is measured to be $(4.2\ifmmode\pm\else\textpm\fi{}1.1(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.3(\mathrm{syst}.))\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. For ${B}^{+}\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$, we observe no significant signal and set an upper limit of $0.72\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 90% confidence level.

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