The effect of Yukawa couplings on unification predictions and the non-perturbative limit

We investigate the effects of Yukawa couplings on the phenomenological predictions for a class of supersymmetric models which allows for the presence of complete SU(5) multiplets in addition to the minimal supersymmetric standard model spectrum. We develop a two-loop analytical approach to quantify the predictions for gauge unification including Yukawa couplings. The effects of the heavy thresholds of the model are also included. In some cases accurate predictions can be made for the unification scale, irrespective of the initial (unknown) Yukawa couplings, so long as perturbation theory remains valid. We also consider the limit of a large number of extra states and compute the predictions in a resummed perturbation series approach to show that the results are stable in this limit. Finally we consider the possibility of making predictions for the case the gauge and Yukawa couplings enter the non-perturbative domain below the unification scale and estimate the errors which affect these predictions. (orig.)