Observation of ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$

Using $\left(1310.6±7.0\right)×{10}^{6}$ $J/\psi $ events acquired with the BESIII detector at the BEPCII storage rings, the decay ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ is observed for the first time with a significance of $8\sigma $ via the process $J/\psi \to \gamma {\eta }^{\prime }$. We measure the branching fraction of ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ to be $B\left({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}\right)=\phantom{\rule{0ex}{0ex}}\left(1.97±0.33\left(\mathrm{stat}\right)±0.19\left(\mathrm{syst}\right)\right)×{10}^{-5}$, where the first and second uncertainties are statistical and systematic, respectively.