Measurement of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math> asymmetries in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>B</mml:mi><mml:mn>0</mml:mn></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:mi>ϕ</mml:mi><mml:msubsup><mml:mi>K</mml:mi><mml:mi>s</mml:mi><mml:mn>0</mml:mn></mml:msubsup></mml:math> decays with Belle II

We present a measurement of time-dependent rate asymmetries in ${B}^{0}\ensuremath{\rightarrow}\ensuremath{\phi}{K}_{S}^{0}$ decays to search for non-standard-model physics in $b\ensuremath{\rightarrow}q\overline{q}s$ transitions. The data sample is collected with the Belle II detector at the SuperKEKB asymmetric-energy ${e}^{+}{e}^{\ensuremath{-}}$ collider in 2019--2022 and contains $(387\ifmmode\pm\else\textpm\fi{}6)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ bottom-antibottom mesons from $\mathrm{\ensuremath{\Upsilon}}(4S)$ resonance decays. We reconstruct $162\ifmmode\pm\else\textpm\fi{}17$ 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 $C=\ensuremath{-}0.31\ifmmode\pm\else\textpm\fi{}0.20\ifmmode\pm\else\textpm\fi{}0.05$ and $S=0.54\ifmmode\pm\else\textpm\fi{}0.2{6}_{\ensuremath{-}0.08}^{+0.06}$, respectively, where the first uncertainties are statistical and the second are systematic. The results are compatible with the $CP$ asymmetries observed in $b\ensuremath{\rightarrow}c\overline{c}s$ transitions.

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