Limit of infinite target mass in Q.E.D. and non-abelian gauge theories

Abstract

General rules of computing amplitudes in the limit of infinite target mass are derived. While reproducing the usual rules of external potential in the case of q.e.d., the rules for gauge theories are more complicated in that the limiting vertices can either depend on the spin variables of the targets, or they can transfer energy. Simple applications of these rules are made to compute vector-boson-fermion scattering amplitude in the limit the masses of fermions go to infinity.