Measurement of the branching fraction of $\eta \to \mu^+ \mu^-$ and search for $\eta \to e^+ e^-$

Abstract

We analyze the decay of $\eta\rightarrow \ell^+\ell^-(\ell=e, \mu)$ via $J/\psi\rightarrow\gamma\eta'$ and $\eta'\rightarrow\pi^+\pi^-\eta$, based on (10087 $\pm$ 44) $\times$ 10$^{6}$ $J/\psi$ events collected with the BESIII detector at the BEPCII storage rings. The branching fraction of $\eta\rightarrow\mu^+ \mu^-$ is measured to be $(5.8 \pm 1.0_{\rm stat} \pm 0.2_{\rm syst}) \times 10^{-6}$, which is consistent with the previous measurements and theoretical expectations. In addition, no significant $\eta\to e^+e^-$ signal is observed in the $e^+ e^-$ invariant mass spectrum, and an improved upper limit of ${\cal B}(\eta \to e^+ e^-) < 2.2 \times 10^{-7}$ is set at 90\% confidence level.