Back to the doublet-triplet splitting problem

Examples of supersymmetric grand unified models are explicitly illustrated, in which the Higgs doublets are identified, at the tree level, with the massless degrees of freedom describing an "accidental" degeneracy of the vacuum at the first stage of gauge symmetry breaking. This degeneracy occurs as a consequence either of the gauge symmetry or of a discrete exact symmetry. The effect of radiative corrections, which lift the vacuum degeneracy and break the SU (2) ⊗ U (1) group, is discussed, showing that the consistency of the theory, at low energy, only sets a rather weak and m_t-dependent lower bound on the lightest Higgs mass, with no constraint on the top-quark itself.