Observation of the double Dalitz decay ${\eta }^{\prime }\to {e}^{+}{e}^{-}{e}^{+}{e}^{-}$

Based on $\left(10087±44\right)×{10}^{6}\text{}\text{}J/\psi $ events collected with the BESIII detector at BEPCII, the double Dalitz decay ${\eta }^{\prime }\to {e}^{+}{e}^{-}{e}^{+}{e}^{-}$ is observed for the first time via the $J/\psi \to \gamma {\eta }^{\prime }$ decay process. The significance is found to be $5.7\sigma $ with systematic uncertainties taken into consideration. Its branching fraction is determined to be $B\left({\eta }^{\prime }\to {e}^{+}{e}^{-}{e}^{+}{e}^{-}\right)=\left(4.5±1.0\left(\mathrm{stat}\right)±0.5\left(\mathrm{sys}\right)\right)×{10}^{-6}$.