The observed enhancement of $pbar p$-production near the threshold in radiative decays of $J/psi$ and $e^+e^-$-annihilations can be explained with final state interactions among the produced $Nbar N$ system, where the enhancement is essentially deter
mined by $Nbar N$ elastic scattering amplitudes. We propose to use an effective theory for interactions in a $Nbar N$ system near its threshold. The effective theory is similar to the well-known one for interactions in a $NN$ system but with distinctions. It is interesting to note that in the effective theory some corrections to scattering amplitudes at tree-level can systematically be summed into a simple form. These corrections are from rescattering processes. With these corrected amplitudes we are able to describe the enhancement near the threshold in radiative decays of $J/psi$ and $e^+e^-$-annihilations, and the $pbar p$ elastic scattering near the threshold.
We study the observed enhancement of a $pbar p$ system near the threshold in the process $J/psi to gamma pbar p$ and $e^+ e^- to pbar p$. 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 $Nbar N$ 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 $J/psi to gamma pbar p$ and $e^+ e^- to pbar p$ can be well described with the rescattering effects.
