Implications of Higgs discovery for the strong CP problem and unification

Abstract

A Z2 symmetry that extends the weak interaction, SU(2)L → SU(2)L ×SU(2)′, and the Higgs sector, H(2) → H(2, 1) + H′(1, 2), yields a Standard Model quartic coupling that vanishes at scale v′ = 〈H′〉 ≫ 〈H〉. Near v′, theories either have a “prime” sector, or possess “Left-Right” (LR) symmetry with SU(2)′ = SU(2)R. If the Z2 symmetry incorporates spacetime parity, these theories can solve the strong CP problem. The LR theories have all quark and lepton masses arising from operators of dimension 5 or more, requiring Froggatt-Nielsen structures. Two-loop contributions to θ¯ are estimated and typically lead to a neutron electric dipole moment of order 10−27e cm that can be observed in future experiments. Minimal models, with gauge group SU(3) × SU(2)L × SU(2)L × U(1)B−L, have precise gauge coupling unification for v′ = 1010±1 GeV, successfully correlating gauge unification with the observed Higgs mass of 125 GeV. With SU(3) × U(1)B−L embedded in SU(4), the central value of the unification scale is reduced from 1016−17 GeV to below 1016 GeV, improving the likelihood of proton decay discovery. Unified theories based on SO(10) × CP are constructed that have H + H′ in a 16 or 144 and generate higher-dimensional flavor operators, while maintaining perturbative gauge couplings.