Spin-Parity Analysis of p(p)over-bar Mass Threshold Structure in J/psi and psi(3686) Radiative Decays

A partial wave analysis of the p (p) over bar mass-threshold enhancement in the reaction J/psi -> gamma p (p) over bar is used to determine its J(PC) quantum numbers to be 0(-+), its peak mass to be below threshold at M 1832(-5)(+19)(stat)(-17)(+18)(syst) +/- 19(model) MeV/c(2), and its total width to be Gamma <76 MeV/c(2) at the 90% C. L. The product of branching ratios is measured to be BR[J/psi -> gamma X(p)]BR[X(p (p) over bar) -> p (p) over bar] = [9:0(-1.1)(+0.4)(stat)(-5.0)(+1:5)(syst) +/- 2: 3(model)] x 10(-5). A similar analysis performed on psi(3686) -> gamma p (p) over bar decays shows, for the first time, the presence of a corresponding enhancement with a production rate relative to that for J/psi decays of R = [5.08(-0.45)(+0: 71)(stat)(-058)(+0: 67) (syst) +/- 0: 12(model)%.