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

Abstract

Using a data sample of $(1.0087\pm 0.0044)\times 10^{10}$ $J/\psi$ events collected with the BESIII detector, we study the rare double Dalitz decay of $\eta \to e^{+}{e}^{-}{e}^{+}{e}^{-}$ through the processes $J/\psi \to \gamma \eta$ and $J/\psi \to \gamma {\eta }^{\prime },{\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$. 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 $(2.63\pm 0.34_{\mathrm{stat}}\pm 0.16_{\mathrm{syst}})\times 10^{-5}$. The result is consistent with the previous measurements within uncertainties and further constrains physics beyond the standard model.

11 pages, 5 figures, consistent with the Publication in PRD