Abstract
Despite the successes of the Standard Model of particle physics, it is known to suffer from a number of deficiencies. Several of these can be addressed within non-supersymmetric theories of grand unification based on text {SO}(10). However, achieving gauge coupling unification in such theories is known to require additional physics below the unification scale, such as symmetry breaking in multiple steps. Many such models are disfavored due to bounds on the proton lifetime. Corrections arising from threshold effects can, however, modify these conclusions. We analyze all seven relevant breaking chains with one intermediate symmetry breaking scale, assuming the “survival hypothesis” for the scalar masses. Two are allowed by proton lifetime and two are disfavored by a failure to unify the gauge couplings. The remaining three unify at a too low scale, but can be salvaged by various amounts of threshold corrections. We parametrize this and thereby rank the models by the size of the threshold corrections required to save them.
Highlights
A prerequisite of grand unification is that the evolution of the Standard Model (SM) gauge couplings with energy scale, governed by the renormalization group equations (RGEs), must be such that they unify
It is well-known that the gauge couplings do not unify in non-supersymmetric models unless intermediate symmetry breaking scales or fields with intermediate-scale masses are added, but that successful gauge coupling unification can be achieved in the minimal supersymmetric SM [12,13,14]
Since the SO(10) group is of rank five, which is one larger than the SM gauge group, the symmetry breaking may occur in multiple steps [15,16,17,18,19,20,21,22,23,24,25,26,27,28]
Summary
A prerequisite of grand unification is that the evolution of the SM gauge couplings with energy scale, governed by the renormalization group equations (RGEs), must be such that they unify. Since the SO(10) group is of rank five, which is one larger than the SM gauge group, the symmetry breaking may occur in multiple steps [15,16,17,18,19,20,21,22,23,24,25,26,27,28] This modifies the evolution of the gauge couplings in a way that can allow their unification even in non-SUSY models.
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