Cancellation of Infrared Divergence in Inclusive Production of Lepton   Pair Near the Threshold of Heavy Quarkonia
  The detailed proof of cancellation of topologically unfactorized infrared divergences in the inclusive production of lepton pair close to the threshold of heavy quarkonia is presented. To make the effects of transition between states containing heavy quark pairs, which is important in such cancellation, the final detected states are constrained to be lepton pair near the threshold of the heavy quarkonia instead of heavy quarkonia themselves. Such cancellation is crucial for the NRQCD factorization of these processes. 
Gao-Liang Zhou
hep-ph