Riken-bnl Preprint Long–range Interactions of Small Color Dipoles

We study the scattering of small color dipoles (e.g., heavy quarkonium states) at low energies. We find that even though the couplings of color dipoles to the gluon field can be described in perturbation theory, at large distances the interaction becomes totally non–perturbative. The structure of the scattering amplitude, however, is fixed by the (broken) chiral and scale symmetries of QCD; the leading long–distance contribution arises from the correlated two–pion exchange. We use the spectral representation technique to evaluate both perturbative and nonperturbative contributions to the scattering amplitude. Our main result is the sum rule which relates the overall strength of the non–perturbative interaction between color dipoles to the energy density of QCD vacuum.