Observation of a near-threshold enhancement in the p(p)over-bar mass spectrum from radiative J/ψ→γp(p)over-bar deecays -: art. no. 022001

We observe a narrow enhancement near 2m(p) in the invariant mass spectrum of p (p) over bar pairs from radiative J/psi-->gammap (p) over bar decays. No similar structure is seen in J/psi-->pi(0)p (p) over bar decays. The results are based on an analysis of a 58x10(6) event sample of J/psi 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 2m(p) at M=1859(-10)(+3) (stat)(-25)(+5)(syst) MeV/c(2) and the total width is Gamma<30 MeV/c(2) at the 90% confidence level. These mass and width values are not consistent with the properties of any known particle.