Measurement of $CP$ asymmetries in $B^0\to \varphi K_S^0$ decays with Belle II

Abstract

We present a measurement of time-dependent rate asymmetries in $B^0\to \varphi K_S^0$ decays to search for non-standard-model physics in $b\to q\overline{q}s$ transitions. The data sample is collected with the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider in 2019-2022 and contains $(387\pm 6)\times 10^6$ bottom-antibottom mesons from ${\rm Y}(4S)$ resonance decays. We reconstruct $162\pm 17$ signal events and extract the charge-parity ($CP$) violating parameters from a fit to the distribution of the proper-decay-time difference of the two $B$ mesons. The measured direct and mixing-induced $CP$ asymmetries are $A=0.31\pm 0.20\pm 0.05$ and $S=0.54\pm 0.26_{-0.08}^{+0.06}$, respectively, where the first uncertainties are statistical and the second are systematic. The results are compatible with the $CP$ asymmetries observed in $b\to c\overline{c}s$ transitions.

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• Phenomenological analysis of charmless $B\to PV$ decays in the modified perturbative QCD approach