SINGULAR TERMS OF HELICITY AMPLITUDES AT ONE-LOOP IN QCD AND THE SOFT LIMIT OF THE CROSS-SECTIONS OF MULTI-PARTON PROCESSES

We describe a general method that enables us to obtain all the singular terms of helicity amplitudes of n-parton processes at one loop. The algorithm uses helicity amplitudes at tree level and simple color algebra. We illustrate the method by calculating the singular part of the one-loop helicity amplitudes of all 2 --> 3 parton subprocesses. The results are used to derive the soft gluon limit of the cross sections of all 2 --> 4 parton scattering subprocesses which provide a useful initial condition for the angular ordering approximation to coherent multiple soft gluon emission, incorporated in existing Monte Carlo simulation programs.