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

We analyze the decay of $\eta \to {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}(\mathrm{\ell }=e,\mu )$ via $J\mathrm{/}\psi \to \gamma {\eta }^{\mathrm{\prime }}$ and ${\eta }^{\mathrm{\prime }}\to {\pi }^{+}{\pi }^{-}\eta$, based on (10087 $\pm$ 44) $\times$ 10${}^6$ $J\mathrm{/}\psi$ events collected with the BESIII detector at the BEPCII storage rings. The branching fraction of $\eta \to {\mu }^{+}{\mu }^{-}$ is measured to be $(5.8\pm {1.0}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.2}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}})\times {10}^{-6}$, 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 $\mathcal{B}(\eta \to e^{+}{e}^{-})<2.2\times {10}^{-7}$ is set at 90% confidence level.