General criteria for radiation amplitude zeros

We consider the general phenomenon of radiation amplitude zeros. It has been shown recently by Brodsky and Brown and by Samuel that such zeros occur, at the tree level, in a variety of radiative processes provided the $\frac{k·p_i}{Q_i}$ are the same for all external particles. These zero conditions define the so-called null zone. In this paper we show, in general, how to determine in the case of massive charged particles whether or not a null zone exists in the physical region for a given set of particle energies. The answer is given in the form of simple inequalities. We also show how to characterize the null zone if the zero conditions are satisfied.