Observation of a pp mass threshold enhancement in Ψ′ → π + π − J/Ψ(J/Ψ → γ pp) decay

The decay channel Ψ' → π + π − J/Ψ(J/Ψ → γp ) 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 invariant mass spectrum. The enhancement can be fitted with an S -wave Breit–Wigner resonance function with a resulting peak mass of M = 1861 +6 −13 (stat) +7 −26 ( s y s t) MeV/c 2 and a narrow width that is Γ < 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.