Observation of an anomalous line shape of the   $\eta^{\prime}\pi^{+}\pi^{-}$ mass spectrum near the $p\bar{p}$ mass   threshold in $J/\psi\rightarrow\gamma\eta^{\prime}\pi^{+}\pi^{-}$
  Using $1.09\times10^{9}$ $J/\psi$ events collected by the BESIII experiment in 2012, we study the $J/\psi\rightarrow\gamma\eta^{\prime}\pi^{+}\pi^{-}$ process and observe a significant abrupt change in the slope of the $\eta^{\prime}\pi^{+}\pi^{-}$ invariant mass distribution at the proton-antiproton ($p\bar{p}$) mass threshold. We use two models to characterize the $\eta^{\prime}\pi^{+}\pi^{-}$ line shape around $1.85~\text{GeV}/c^{2}$: one which explicitly incorporates the opening of a decay threshold in the mass spectrum (Flatt\'{e} formula), and another which is the coherent sum of two resonant amplitudes. Both fits show almost equally good agreement with data, and suggest the existence of either a broad state around $1.85~\text{GeV}/c^{2}$ with strong couplings to $p\bar{p}$ final states or a narrow state just below the $p\bar{p}$ mass threshold. Although we cannot distinguish between the fits, either one supports the existence of a $p\bar{p}$ molecule-like state or bound state with greater than $7\sigma$ significance. 
M. Ablikim, M. N. Achasov, S. Ahmed, X. C. Ai, O. Albayrak, M.   Albrecht, D. J. Ambrose, A. Amoroso, F. F. An, Q. An, J. Z....
hep-ex