Cancellation of Sudakov logarithms in radiative decays of quarkonia

We study infrared QCD effects in radiative quarkonia decays. We examine the endpoint region z --> 1 of the photon spectrum. We point out a cancellatio n mechanism for the corrections in alpha (n)(s) ln(m) (1 - z), m less than or equal to 2n, in the short-distance coefficient for the color-singlet Foc k state in the quarkonium. The cancellation is due to the coherence of the color radiation, and applies even though logarithmic contributions are pres ent in the jet distributions associated with the decay. We comment on the i mplications of our results for the modeling of hadronization in the endpoin t region and for the role of color-octet states in the quarkonium. (C) 2001 Published by Elsevier Science B.V.