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

Using a data sample of $(1.0087\pm 0.0044)\times {10}^{10}$ $J\mathrm{/}\psi$ events collected with the BESIII detector, we study the rare double Dalitz decay of $\eta \to e^{+}{e}^{-}{e}^{+}{e}^{-}$ through the processes $J\mathrm{/}\psi \to \gamma \eta$ and $J\mathrm{/}\psi \to \gamma {\eta }^{\mathrm{\prime }},{\eta }^{\mathrm{\prime }}\to {\pi }^{+}{\pi }^{-}\eta$. Clear $\eta$ signals are observed in the $e^{+}{e}^{-}{e}^{+}{e}^{-}$ invariant mass spectrum, with statistical significances of 5.9$\sigma$ and 7.8$\sigma$ for the two channels, respectively. By combining both modes, we determine the branching fraction of $\eta \to e^{+}{e}^{-}{e}^{+}{e}^{-}$ to be $(2.63\pm {0.34}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.16}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}})\times {10}^{-5}$. The result is consistent with the previous measurements within uncertainties and further constrains physics beyond the standard model.