The singular behaviour of QCD amplitudes at two-loop order

We discuss the structure of infrared singularities in on-shell QCD amplitudes at two-loop order. We present a general factorization formula that controls all the ϵ-poles of the dimensionally regularized amplitudes. The dependence on the regularization scheme is considered and the coefficients of the 1/ϵ4,1/ϵ3 and 1/ϵ2 poles are explicitly given in the most general case. The remaining single-pole contributions are also explicitly evaluated in the case of amplitudes with a qq̄ pair.