A partial wave analysis of the $p \bar{ p}$ mass-threshold enhancement in the reaction $J/\psi\rightarrow\gamma p\bar{ p}$ is used to determine: its $J^{ PC}$ quantum numbers to be $0^{ -+}$; its peak mass to be below threshold at $M=1832^{ +19}_{ -5} { \rm(stat.)}^{ +18}_{ -17} { \rm (syst.)}\pm19 { \rm (model)} { \rm MeV}/c^2$; and its total width to be $\Gamma<76 { \rm MeV}/c^2$ at the 90% C.L. The product branching ratio is measured to be $B(J/\psi\rightarrow\gamma X(p\bar{ p}))B(X(p\bar{ p})\rightarrow p\bar{ p})=(9.0^{ +0.4}_{ -1.1} { \rm (stat.)}^{ +1.5}_{ -5.0} { \rm (syst.)}\pm2.3 { \rm (model)})\times10^{ -5}$. A similar analysis performed on $\psi^\prime\rightarrow\gamma p\bar{ p}$ decays shows, for the first time, the presence of a corresponding enhancement with a production rate relative to that for $J/\psi$ decays of $R=(5.08^{ +0.71}_{ -0.45} { \rm (stat.)}^{ +0.67}_{ -3.58} { \rm (syst.)}\pm0.12 { \rm (model)})%$.
Categories: hep-ex