Rescattering Effect and Near Threshold Enhancement of $p\bar p$ System

We study the observed enhancement of a $p\bar p$ system near the threshold in the process $J/\psi \to \gamma p\bar p$ and $e^+ e^- \to p\bar p$. From early studies the enhancement can be explained by final state interactions, which are in general taken into account with some potential models. In this work we offer a simple approach within quantum field theory to explain the observed enhancement. We point out that among different final state interactions the rescattering in a $N\bar N$ system though exchange of $\pi$ is the most important. The effects of the rescattering is completely fixed by the well-known coupling $g_{\pi NN}$. Our results show that the enhancement in $J/\psi \to \gamma p\bar p$ and $e^+ e^- \to p\bar p$ can be well described with the rescattering effects.