Measurement of $CP$ asymmetries in $B^0\to {\eta }^{\prime }{K}_s^0$ decays at Belle II

Abstract

We describe a measurement of charge-parity ($CP$) violation asymmetries in $B^0\to {\eta }^{\prime }{K}_S^0$ decays using Belle II data. We consider ${\eta }^{\prime }\to \eta (\to \gamma \gamma ){\pi }^{+}{\pi }^{-}$ and ${\eta }^{\prime }\to \rho (\to {\pi }^{+}{\pi }^{-})\gamma$ decays. The data were collected at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider between the years 2019 and 2022, and contain $(387\pm 6)\times 10^6$ bottom-antibottom meson pairs. We reconstruct $829\pm 35$ signal decays and extract the $CP$ violating parameters from a fit to the distribution of the proper-decay-time difference between the two $B$ mesons. The measured direct and mixing-induced $CP$ asymmetries are ${\text{C}}_{{\eta }^{\prime }{K}_S^0}=-0.19\pm 0.08\pm 0.03$ and ${\text{S}}_{{\eta }^{\prime }{K}_S^0}=+0.67\pm 0.10\pm 0.04$, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with current world averages and standard model predictions.

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