We make a complete analysis of radiative symmetry breaking in the MSSM and its SU(5) extensions including low- and high-energy threshold effects in the framework of the two-loop renormalization group. In particular, we consider minimal SU(5), the missing-doublet SU(5), a Peccei-Quinn-invariant version of SU(5), as well as a version with light adjoint remnants. We derive permitted ranges for the parameters of these models in relation to predicted ${\alpha }_s$ and $M_G$ values within the present experimental accuracy. The parameter regions allowed under the constraints of radiative symmetry breaking, perturbativity, and proton stability, include the experimentally designated domain for ${\alpha }_s.$ In the case of the minimal SU(5), the values of ${\alpha }_s$ obtained are somewhat 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 the minimal model, leading to small values. In the case of the Peccei-Quinn version, however, the presence of an extra intermediate scale allows us to achieve an excellent agreement with the experimental ${\alpha }_s$ values. Finally, the last considered version, with light remnants, exhibits unification of couplings at string scale at the expense, however, of rather large ${\alpha }_s$ values.

• Received 7 October 1996

DOI:https://doi.org/10.1103/PhysRevD.55.2955