Radiative Breaking of Gauge Symmetries in the MSSM and in its Extensions

The radiative electroweak symmetry breaking, the b-\tau Yukawa and gauge couplings unification in the MSSM and its SU(5) extensions are studied in detail. In the framework of the two-loop renormalization group equations both low- and high-energy threshold effects are included. In the case of the minimal SU(5), the values of \alpha_s obtained are somewhat larger than the experimental average. The Peccei-Quinn version of the missing-doublet SU(5) model generally predicts smaller values of \alpha_s and b-\tau unification, in excellent agreement with all low energy experimental data. We also study the generation of the GUT scale through radiative corrections in the context of an R-symmetric ``flipped" SU(5) x U(1)_X model and we find that this is possible in a certain range of values of the parameter space.