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

TL;DR

This study measures the rare double Dalitz decay of the eta meson using a large J/psi dataset, providing a precise branching fraction that aligns with previous results and constrains new physics.

Contribution

First measurement of the eta to four-electron decay branching fraction using BESIII data, improving precision and testing standard model predictions.

Findings

Branching fraction determined as (2.63 ± 0.34_stat ± 0.16_syst) × 10^{-5}

Significant eta signals observed with 5.9σ and 7.8σ significance

Results consistent with previous measurements and constrain physics beyond the standard model

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 $\eta\rightarrow e^+e^-e^+e^-$ through the processes $J/\psi\rightarrow \gamma \eta$ and $J/\psi\rightarrow \gamma \eta' ,\eta' \to \pi^+\pi^-\eta$. 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 $\eta\rightarrow e^+ e^- e^+ e^-$ to be $(2.63~\pm~0.34_{\rm stat}~\pm~0.16_{\rm syst}) \times10^{-5}$. The result is consistent with the previous measurements within uncertainties and further constrains physics beyond the standard model.