First observation of the ${\eta }_c\to {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0$ decay

Using $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the decay ${\eta }_c\to {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0$. The interference between $J\mathrm{/}\psi \to \gamma {\eta }_c\to \gamma {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0$ and $J\mathrm{/}\psi \to \gamma {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0{\mathrm{\mid }}_{\mathrm{n}\mathrm{o}\mathrm{n}-\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}}$ is considered in the ${\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0$ mass spectrum fits. The branching fractions are measured to be $B({\eta }_c\to {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0)=(1.33\pm 0.03\pm 0.18)\times {10}^{-3}$ for the constructive interference and $B({\eta }_c\to {\mathrm{\Xi }}^0{\bar{\mathrm{\Xi }}}^0)=(1.63\pm 0.04\pm 0.21)\times {10}^{-3}$ for the destructive interference, where the first uncertainties are statistical and the second are systematic.