Observation of a near-threshold enhancement in the pp-bar mass spectrum from radiative J/ψ→γpp-bar decays

We observe a narrow enhancement near 2mp in the invariant mass spectrum of pp(bar sign) pairs from radiative J/ψ→γpp-bar decays. No similar structure is seen in J/ψ→π0pp-bar decays. The results are based on an analysis of a 58x106 event sample of J/ψ decays accumulated with the BESII detector at the Beijing electron-positron collider. The enhancement can be fit with either an S- or P-wave Breit-Wigner resonance function. In the case of the S-wave fit, the peak mass is below 2mp at M=1859-10+3 (stat)-25+5(syst) MeV/c2 and the total width is Γ<30 MeV/c2 at the 90% confidence level. These mass and width values are not consistent with the properties of any known particle