Measurement of $CP$ asymmetries in $B^0\to \phi K^0_S$ decays with Belle II

TL;DR

This paper reports a measurement of time-dependent CP asymmetries in $B^0 o \,phi K^0_S$ decays using Belle II data, testing for physics beyond the Standard Model in $b\to q\overline{q}s$ transitions.

Contribution

First measurement of CP asymmetries in $B^0\to \phi K^0_S$ decays with Belle II, providing data to compare with Standard Model predictions.

Findings

Measured direct CP asymmetry $A=0.31\pm0.20\pm0.05$.

Measured mixing-induced CP asymmetry $S=0.54\pm0.26^{+0.06}_{-0.08}$.

Results are consistent with CP asymmetries in $b\to c\overline{c} s$ transitions.

Abstract

We present a measurement of time-dependent rate asymmetries in $B^0\to \phi K^0_S$ 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 $\Upsilon(4S)$ resonance decays. We reconstruct $162\pm17$ 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 $A=0.31\pm0.20\pm0.05$ and $S=0.54\pm0.26^{+0.06}_{-0.08}$, 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.