Momentum-dependent effects in the decays of charmonium and upsilon

Decay widths of quarkonia are determined in a calculation in which the momentum distribution resulting from the localization of the quarks is taken into account. The e+e- decay modes of charmonium and upsilon are considered with the momentum-dependent corrections to the well-known van Royen-Weisskopf formula for leptonic widths. The momentum-dependent effects are found to be significant, and amount to approximately a 20% correction. The γη(1440) decay mode is also examined to investigate a decay involving a propagating quark in the bound state. Here the momentum-dependent corrections to the static approximation are very large and suppress the decay widths by well over a factor of 3. Therefore one must exercise caution in using a static approximation. (orig.)