Observation of $\eta^{\prime}\to\pi^{+}\pi^{-}\pi^{+}\pi^{-}$ and   $\eta^{\prime}\to\pi^{+}\pi^{-}\pi^{0}\pi^{0}$
  Using a sample of $1.3\times 10^9$ $J/\psi$ events collected with the BESIII detector, we report the first observation of $\eta^{\prime}\to\pi^{+}\pi^{-}\pi^{+}\pi^{-}$ and $\eta^{\prime}\to\pi^{+}\pi^{-}\pi^{0}\pi^{0}$. The measured branching fractions are $\mathcal{B}$($\eta^{\prime}\to\pi^{+}\pi^{-}\pi^{+}\pi^{-}$) = $(8.53\pm0.69({\rm stat.})\pm0.64({\rm syst.}))\times10^{-5}$ and $\mathcal{B}$($\eta^{\prime}\to\pi^{+}\pi^{-}\pi^{0}\pi^{0}$) = $(1.82\pm0.35({\rm stat.})\pm0.18({\rm syst.}))\times10^{-4}$, which are consistent with theoretical predictions based on a combination of chiral perturbation theory and vector-meson dominance. 
BESIII Collaboration: M. Ablikim (1), M. N. Achasov (8, a), X. C. Ai   (1), O. Albayrak (4), M. Albrecht (3), D. J. Ambrose ...
hep-ex