Origin of cancellation of infrared divergences in coherent state approach

The idea of Kulish and Faddeev for treating asymptotic dynamics in QED by a deductive construction of asymptotic states corresponding to the asymptotic behavior of the hamiltonian operator for vertical stroke t vertical stroke → infinite, in the interaction representation, is further extended in QCD to provide a more complete analysis of the combinatorial structure of the asymptotic states obtained by the analogous construction in QCD. In the process so far considered in low orders in perturbative QCD, we find that with these asymptotic states, IR-divergent contributions are systematically generated in the matrix element such as to cancel the leading-order IR divergences arising from the unitarity cuts of the standard covariant graph of perturbative QCD. This occurs separately for each topological type of covariant graph. The cancellation of the forward scattering, leading-order IR divergences in the matrix element in qq → qq + gluon to lowest order in perturbation theory is discussed in detail. (orig.)