Spin-parity analysis of $p\bar{p}$ mass threshold structure in $J/\psi$ and $\psi (3686)$ radiative decays

Abstract

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 $J^\text{PC}$ quantum numbers to be $0^{-+}$, its peak mass to be below threshold at $M=1832_{-5}^{+19}(\text{stat})_{-17}^{+18}(\text{syst})±19(\text{model})$ $\text{MeV}/c^2$, and its total width to be $\Gamma <76$ $\text{MeV}/c^2$ at the 90% C.L. The product of branching ratios is measured to be $\text{BR}[J/\psi\rightarrow\gamma X(p\bar{p})]$ $\text{BR}[X(p\bar{p})\rightarrow p\bar{p}]=[9.0_{-1.1}^{+0.4}(\text{stat})_{-5.0}^{+1.5}(\text{syst})±2.3(\text{model})]×10^{-5}$. A similar analysis performed on $\psi (3686)\rightarrow\gamma p\bar{p}$ decays shows, for the first time, the presence of a corresponding enhancement with a production rate relative to that for $J/\psi$ decays of $R=[5.08_{-0.45}^{+0.71}(\text{stat})_{-3.58}^{+0.67}(\text{syst}) ±0.12(\text{model})]%$.