The Negative-$Σ\barΣ$ Angular-Parameter Puzzle in $J/ψ$ and $ψ(2S)$ Decays

Experimental measurements of $J/\psi$ and $\psi(2S)$ decays to octet baryon pairs reveal a negative-$\Sigma\bar{\Sigma}$ puzzle in the angular distributions: $\alpha_\Sigma^{J/\psi}<0$, whereas the other measured $J/\psi$ octet channels and the corresponding $\psi(2S)$ channels have positive angular parameters. We show that an $SU(3)_F$-exact singlet description fails decisively in a $\chi^2$ fit, giving $\chi^2\simeq10^4$. By contrast, a fit that allows a common final-state interaction to mix the ${\rm S}$- and ${\rm D}$-wave amplitudes, together with a symmetric $SU(3)_F$-breaking octet, gives $\chi^2=4.43$ for two nominal degrees of freedom and strongly favors this breaking pattern. The negative $\Sigma\bar{\Sigma}$ angular parameter is traced to the suppression of the magnetic form factor by the common final-state rescattering.