Observation of a p mass threshold enhancement in ψ′→π+π-J/ψ(J/ψ→γp) decay

The decay channel ψ′→ π 〉＋π^-J/ψ（J/ψ 〉 γp^-p） is studied using a sample of 1.06 × 10^8 ψ′ events collected by the BESIII experiment at BEPCII. A strong enhancement at threshold is observed in the p^-p invariant mass spectrum. The enhancement can be fitted with an S-wave Breit-Wigner resonance function with a resulting peak mass of M = 1861-13^＋6 （stat）-26^＋7 （syst） MeV/c^2 and a narrow width that is Г 〈 38 MeV/c^2 at the 90% confidence level. These results are consistent with published BESII results. These mass and width values do not match with well established mesons.