Infra-red fixed points revisited

We reconsider how Yukawa couplings may be determined in terms of a gauge coupling through the infra-red fixed point structure paying particular regard to the rate of approach to the fixed point. Using this we determine whether the fixed point structure of an underlying unified theory may play a significant rôle in fixing the couplings at the gauge unification scale. We argue that, particularly in the case of compactified theories, this is likely to be the case and illustrate this by a consideration of phenomenologically interesting theories. We discuss in particular what the infra-red fixed point structure implies for the top quark mass.