Observation of an anomalous line shape of the $eta^{prime}pi^{+}pi^{-}$ mass spectrum near the $pbar{p}$ mass threshold in $J/psirightarrowgammaeta^{prime}pi^{+}pi^{-}$

Using $1.09times10^{9}$ $J/psi$ events collected by the BESIII experiment in 2012, we study the $J/psirightarrowgammaeta^{prime}pi^{+}pi^{-}$ process and observe a significant abrupt change in the slope of the $eta^{prime}pi^{+}pi^{-}$ invariant mass distribution at the proton-antiproton ($pbar{p}$) 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 $pbar{p}$ final states or a narrow state just below the $pbar{p}$ mass threshold. Although we cannot distinguish between the fits, either one supports the existence of a $pbar{p}$ molecule-like state or bound state with greater than $7sigma$ significance.