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

The decay channel psi' -> pi(+)pi(-)J/psi(J/psi -> gamma p (p) over bar) is studied using a sample of 1.06 x 10(8) psi' events collected by the BESIII experiment at BEPCII. A strong enhancement at threshold is observed in the pp 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 Gamma < 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.