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

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