Measurement of the branching fractions of $B\to\eta' K$ decays using 2019/2020 Belle II data

TL;DR

This paper reports the first measurement of $B o ext{eta'} K$ decay branching fractions using Belle II data from 2019/2020, confirming previous world averages with improved precision.

Contribution

It provides the first measurement of $B o ext{eta'} K$ branching fractions using Belle II data, demonstrating the detector's capability to study rare B decays.

Findings

Branching ratio for $B^ o ext{eta'} K^ ext{+}$ is approximately 63.4×10^{-6}.

Branching ratio for $B_0 o ext{eta'} K_S$ is approximately 59.9×10^{-6}.

Results are consistent with previous world averages.

Abstract

This note describes the rediscovery of $B\to\eta' K$ decays in Belle II data, both in the charged and neutral final state: $B_0\to\eta' K_S$ and $B^\pm\to\eta' K^\pm$. The $\eta'$ is searched for in two decay modes: $\eta'\to\eta\pi^+\pi^-$ with $\eta\to\gamma\gamma$, and $\eta'\to\rho\gamma$. The analysis uses data collected in 2019 and 2020 at the SuperKEKB asymmetric $e^+e^-$ collider, with an integrated luminosity of $62.8~fb^{-1}$, 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: $$\mathcal{B}\left(B^\pm\to\eta'K^\pm\right) = \left(63.4~^{+3.4}_{-3.3}\,(stat)\,\pm3.2\,(syst)\,\right) \times10^{-6} $$ $$\mathcal{B}\left(B_0\to\eta'K_S\right) = \left(59.9~^{+5.8}_{-5.5}\,(stat)\,\pm2.9\,(syst)\,\right) \times10^{-6} $$ which are consistent with world average.