We have proposed to use an effective theory to describe interactions of an $Nbar N$-system. The effective theory can be constructed in analogy to the existing effective theory for an $NN$-system. In this work we study the next-to-leading order correc
tion to $Nbar N$ scattering near the threshold in the effective theory. We find that the experimental data can be well described with the effective theory.
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.
The recently discovered $X$(3872) has many possible interpretations. We study the production of $X$(3872) with PANDA at GSI for the antiproton-proton collision with two possible interpretations of X(3872). One is as a loosely-bound molecule of $D$-me
sons, while another is a 2P charmonium state $chi_{c1}$ (2P). Using effective couplings we are able to give numerical predictions for the production near the threshold and the production associated with $pi^0$. The produced $X$(3872) can be identified with its decay $J/psi pi^+pi^-$. We also study the possible background near the threshold production for $X(3872) to J/psi pi^+pi^-$. With the designed luminosity $1.5{rm fb}^{-1}$ per year of PANDA we find that the event number of $pbar p to J/psi pi^+pi^-$ near the threshold is at the order of $10^6 sim 10^8$, where the large uncertainty comes from the total decay width of X(3872). Our study shows that at the threshold more than about 60% events come from the decay of X(3872) and two interpretations are distinguishable from the line-shape of the production. With our results we except that the PANDA experiments will shed light on the property of X(3872).
