The ${\mathrm{\Xi }}^0$ asymmetry parameters are measured using entangled quantum ${\mathrm{\Xi }}^0-{\overline{\mathrm{\Xi }}}^0$ pairs from a sample of $(448.1\pm 2.9)\times {10}^6\psi (3686)$ events collected with the BESIII detector at BEPCII. The relative phase between the transition amplitudes of the ${\mathrm{\Xi }}^0{\overline{\mathrm{\Xi }}}^0$ helicity states is measured to be $\mathrm{\Delta }\mathrm{\Phi }=-0.050\pm 0.150\pm 0.020\mathrm{rad}$, which implies that there is no obvious polarization at the current level of statistics. The decay parameters of the ${\mathrm{\Xi }}^0$ hyperon $({\alpha }_{{\mathrm{\Xi }}^0},{\alpha }_{{\overline{\mathrm{\Xi }}}^0},{\varphi }_{{\mathrm{\Xi }}^0},{\varphi }_{{\overline{\mathrm{\Xi }}}^0})$ and the angular distribution parameter $[{\alpha }_{\psi (3686)}]$ and $\mathrm{\Delta }\mathrm{\Phi }$ are measured simultaneously for the first time. In addition, the $CP$ asymmetry observables are determined to be $A_{CP}^{{\mathrm{\Xi }}^0}=({\alpha }_{{\mathrm{\Xi }}^0}+{\alpha }_{{\overline{\mathrm{\Xi }}}^0})/({\alpha }_{{\mathrm{\Xi }}^0}-{\alpha }_{{\overline{\mathrm{\Xi }}}^0})=-0.007\pm 0.082\pm 0.025$ and $\mathrm{\Delta }{\varphi }_{CP}^{{\mathrm{\Xi }}^0}=({\varphi }_{{\mathrm{\Xi }}^0}+{\varphi }_{{\overline{\mathrm{\Xi }}}^0})/2=-0.079\pm 0.082\pm 0.010\mathrm{rad}$, which are consistent with $CP$ conservation.

• Received 20 February 2023
• Accepted 6 June 2023

DOI:https://doi.org/10.1103/PhysRevD.108.L011101

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Published by the American Physical Society