The vacuum of the minimal nonsupersymmetric SO(10) unification

We study a class of nonsupersymmetric SO(10) grand-unified scenarios where the first stage of the symmetry breaking is driven by the vacuum expectation values of the 45-dimensional adjoint representation. Three-decade-old results claim that such a Higgs setting may lead exclusively to the flipped SU(5) - U(1) intermediate stage. We show that this conclusion is actually an artifact of the tree-level potential. The study of the accidental global symmetries emerging in various limits of the scalar potential offers a simple understanding of the tree-level result and a rationale for the drastic impact of quantum corrections. We scrutinize in detail the simplest and paradigmatic case of the 45H 16H Higgs sector triggering the breaking of SO(10) to the standard electroweak model. We show that the minimization of the one-loop effective potential allows for intermediate SU(4)C - SU(2)L - U(1)R and SU(3)c - SU(2)L - SU(2)R - U(1)B-L symmetric stages as well. These are the options favored by gauge unification. Our results, that apply whenever the SO(10) breaking is triggered by 45H, open the path for hunting the simplest realistic scenario of nonsupersymmetric SO(10) grand unification. © 2010 The American Physical Society.