First observation of the 𝜂𝑐 →Ξ0⁢Ξ0bar decay

Using \left(10087\pm 44\right)\times {10}^{6}J/\psi events collected with the BESIII detector at the BEPCII collider, we report the first observation of the decay {\eta }_{c}\rightarrow {\Xi }^{0}{\bar{\Xi }}^{0}. The interference between J/\psi \rightarrow \gamma {\eta }_{c}\rightarrow \gamma {\Xi }^{0}{\bar{\Xi }}^{0}and the nonresonant process J/\psi \rightarrow \gamma {\Xi }^{0}{\bar{\Xi }}^{0}is taken into account in the fits to the {\Xi }^{0}{\bar{\Xi }}^{0}invariant mass spectrum. The branching fractions are measured to be \mathcal{B}\left({\eta }_{c}\rightarrow {\Xi }^{0}{\bar{\Xi }}^{0}\right)=\left(1.33\pm 0.03\pm 0.18\right)\times {10}^{-3} for the constructive interference solution, and\mathcal{B}\left({\eta }_{c}\rightarrow {\Xi }^{0}{\bar{\Xi }}^{0}\right)=\left(1.63\pm 0.04\pm 0.21\right)\times {10}^{-3} for the destructive interference solution, where the first uncertainties are statistical and the second are systematic.