  We study the threshold enhancement effects of baryon-anti-baryon systems in
$J/\psi$ decays at BES using parity $P$, charge conjugation $C$, and flavor
SU(3) symmetries. The $P$ and $C$ symmetries restrict the $p\bar p$ in $J/\psi
\to \gamma p\bar p$ to be in a state with $C=+1$, while the $p\bar p$ in
$J/\psi \to \pi^0 p\bar p$ to be with $C=-1$. Combining the $C$ and $P$
symmetries with flavor SU(3) symmetry, i.e. the CPS symmetry, we find that the
$\Lambda \bar p$ system cannot be in $0^+$ and $0^-$ states in $J/\psi \to K^+
\Lambda \bar p$. We provide a consistent explanation of observation and
non-observation of near threshold enhancements in $J/\psi \to K^+ \Lambda \bar
p$ and $J/\psi \to \pi^0 p \bar p$, respectively. We also find that near baryon
pair threshold enhancement can happen in several channels for $J/\psi$ decays
and can be several times larger than that observed in $J/\psi \to K^+ \Lambda
\bar p$ in some channels.

High Energy Physics - Phenomenology;High Energy Physics - Experiment