Resummation of relativistic corrections to eta(c)->gamma gamma

Abstract

By using the threshold-expansion approach, we compute the relativistic corrections to the two-photon decay width of a spin-singlet S-wave heavy quarkonium. The corrections are resummed to all orders in the heavy-quark velocity upsilon in the quarkonium-rest frame, where we keep all the contributions of the color-singlet Q (Q) over bar potential. Our result agrees with a previous derivation based on the exact spin-projector approach. We also discuss the phenomenological implications of the resummed formula.