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Observation of an anomalous line shape of the $η^{\prime}π^{+}π^{-}$ mass spectrum near the $p\bar{p}$ mass threshold in $J/ψ\rightarrowγη^{\prime}π^{+}π^{-}$

Using $1.09\times10^{9}$ $J/ψ$ events collected by the BESIII experiment in 2012, we study the $J/ψ\rightarrowγη^{\prime}π^{+}π^{-}$ process and observe a significant abrupt change in the slope of the $η^{\prime}π^{+}π^{-}$ invariant mass distribution at the proton-antiproton ($p\bar{p}$) mass threshold. We use two models to characterize the $η^{\prime}π^{+}π^{-}$ line shape around $1.85~\text{GeV}/c^{2}$: one which explicitly incorporates the opening of a decay threshold in the mass spectrum (Flatté 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 $7σ$ significance.