Using $(2.712\pm0.014)\times10^{ 9}$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, we report an evidence of $\Xi^{ 0}$ transverse polarization with a significance of 4.4$\sigma$, and a precise measurement of the branching fraction of $\psi(3686)\to\Xi^{ 0}\bar\Xi^{ 0}$. The weak decay parameters ($\phi_{ \Xi^0/\bar{ \Xi}^{ 0}}$, $\alpha_{ \Xi^0/\bar{ \Xi}^{ 0}}$) and the angular distribution ($\alpha_{ \psi}$) are also measured with higher precision compared to the previous measurements. Furthermore, two the $C\!P$ observables are also determined to be $A^{ \Xi^0}_{ C\!P} = -0.014 \pm 0.030 \pm 0.010$ and $\Delta\phi^{ \Xi^0}_{ C\!P} = 0.000 \pm 0.028 \pm 0.003$ rad, which are still consistent with $C\!P$ conservation at 1$\sigma$ level under the current statistics.
Categories: hep-ex