Quantum-chromodynamic multipole expansion and soft-gluon effects in quarkonium decays

General techniques of the QCD multipole expansion are developed and applied to the study of soft-gluon effects in quarkonium decays. In contrast to short-distance corrections, nonperturbative QCD corrections turn out to reduce the ratio GAMMA(eta/sub c/→2#betta#)/GAMMA (J/psi→e+e-); the ratio GAMMA(eta/sub c/→2#betta#)/GAMMA(eta/sub c/→2g) thereby remains unaffected. An application to the QCD sum rules is also presented; multipole-expansion techniques provide a systematic and powerful means of calculating nonperturbative terms in the sum-rule approach