THRESHOLD EFFECTS AND RADIATIVE ELECTROWEAK SYMMETRY-BREAKING IN SU(5) EXTENSIONS OF THE MSSM

We make a complete analysis of radiative symmetry breaking in the MSSM and its SU(5) extensions including low- and high-energy threshold eff ects in the framework of the two-loop renormalization group. In partic ular, we consider minimal SU(5), the missing-doublet SU(5), a Peccei-Q uinn-invariant version of SU(5), as well as a version with light adjoi nt remnants. We derive permitted ranges for the parameters of these mo dels in relation to predicted alpha(s) and M(G) values within the pres ent experimental accuracy. The parameter regions allowed under the con straints of radiative symmetry breaking, perturbativity, and proton st ability, include the experimentally designated domain for alpha(s). In the case of the minimal SU(5), the values of alpha(s) obtained are so mewhat large in comparison with the experimental average. The missing- doublet SU(5), generally, predicts smaller values of alpha(s). In both versions of the missing doublet, the high-energy threshold effects on alpha(s) operate in the opposite direction than that in the case of t he minimal model,leading to small values. In the case of the Peccei-Qu inn version, however, the presence of an extra intermediate scale allo ws us to achieve an excellent agreement with the experimental alpha(s) values. Finally, the last considered version, with light remnants, ex hibits unification of couplings at string scale at the expense, howeve r, of rather large alpha(s) values.