Search for the double Dalitz decays $\eta/\eta' \to e^+e^-\mu^+\mu^-$ and $\eta' \to \mu^+\mu^-\mu^+\mu^-$

TL;DR

This study searches for rare double Dalitz decays of eta and eta' mesons using BESIII data, setting new upper limits on their branching fractions due to no observed excess events.

Contribution

First to set experimental upper limits on specific eta and eta' double Dalitz decay modes with large data sample and improved sensitivity.

Findings

No excess events observed for the decay modes.

Established new upper limits on branching fractions.

Improved previous limits by about two orders of magnitude.

Abstract

Using a data sample of $(10087 \pm 44) \times {10^{6}}$ $J/{\psi}$ events collected with the BESIII detector, we search for the decays $\eta/\eta'\to e^+e^-\mu^+\mu^-$ and $\eta' \to \mu^+\mu^-\mu^+\mu^-$ via the radiative decays $J/{\psi}\to\gamma\eta$/$\gamma\eta'$. No excess of events over expected background is observed for any of the decays of interest. At 90% confidence level, we report the first upper limits on the branching fractions of $\eta' \to e^{+}e^{-}\mu^{+}\mu^{-}$ and $\eta' \to \mu^{+}\mu^{-}\mu^{+}\mu^{-}$ to be $ 1.75 \times {10^{-6}}$ and $5.28 \times {10^{-7}}$, respectively. In addition, we set an upper limit on the branching fraction of $\eta \to e^{+}e^{-}\mu^{+}\mu^{-}$ to be $6.88 \times {10^{-6}}$, which improves the previous result by about two orders of magnitude.