Measurement of the branching fractions of $B\to {\eta }^{\prime }K$ decays using 2019/2020 Belle II data

Abstract

This note describes the rediscovery of $B\to {\eta }^{\prime }K$ decays in Belle II data, both in the charged and neutral final state: $B_0\to {\eta }^{\prime }{K}_S$ and $B^{\pm }\to {\eta }^{\prime }{K}^{\pm }$. The ${\eta }^{\prime }$ is searched for in two decay modes: ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ with $\eta \to \gamma \gamma$, and ${\eta }^{\prime }\to \rho \gamma$. The analysis uses data collected in 2019 and 2020 at the SuperKEKB asymmetric $e^{+}{e}^{-}$ collider, with an integrated luminosity of $62.8f{b}^{-1}$, corresponding to $68.2$ million of $B\bar{B}$ pairs produced. The signal yield is obtained via an unbinned maximum likelihood fit to signal sensitive variables, obtaining branching ratios: $$\mathcal{B}\left(B^{\pm }\to {\eta }^{\prime }{K}^{\pm }\right)=\left(63.4{}_{-3.3}^{+3.4}(stat)\pm 3.2(syst)\right)\times 10^{-6}$$ $$\mathcal{B}\left(B_0\to {\eta }^{\prime }{K}_S\right)=\left(59.9{}_{-5.5}^{+5.8}(stat)\pm 2.9(syst)\right)\times 10^{-6}$$ which are consistent with world average.

17 pages, 6 figures