Observation of an anomalous line shape of the $\eta^{\prime}\pi^{+}\pi^{-}$ mass spectrum near the $p\bar{p}$ mass threshold in $J/\psi\rightarrow\gamma\eta^{\prime}\pi^{+}\pi^{-}$

TL;DR

This study reports an anomalous line shape near the proton-antiproton threshold in the $ ext{J}/ ext{psi} ightarrow ext{gamma} ext{eta}^{ ext{'}} ext{pi}^+ ext{pi}^-$ decay, suggesting possible par{p} bound or molecular states with high significance.

Contribution

First observation of an anomalous line shape near the par{p} threshold in this decay, indicating potential par{p} bound or molecular states.

Findings

Significant slope change at par{p} threshold in mass spectrum

Data compatible with broad or narrow par{p} states

Evidence supports par{p} molecule-like state with >7σ significance

Abstract

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.