Non-supersymmetric Grand Unified SO(10) × U(1) PQ models have all the ingredients to solve several fundamental problems of particle physics and cosmologyneutrino masses and mixing, baryogenesis, the non-observation of strong CP violation, dark matter, inflation -in one stroke. The axion -the pseudo Nambu-Goldstone boson arising from the spontaneous breaking of the U(1) PQ Peccei-Quinn symmetry -is the prime dark matter candidate in this setup. We determine the axion mass and the low energy couplings of the axion to the Standard Model particles, in terms of the relevant gauge symmetry breaking scales. We work out the constraints imposed on the latter by gauge coupling unification. We discuss the cosmological and phenomenological implications.
We summarize recent studies of realistic nonsupersymmetric Grand Unified Theories (GUTs) extended with a global U(1) PQ symmetry, so as to accommodate the axion solution to the strong CP problem. Aside from solving the CP problem and unifying the gauge structure of the SM, these models can also yield realistic spectra and mixings, including neutrino masses, and allowing for a consistent cosmological history that accounts for inflation, dark matter and baryogenesis. In our studies of SO(10) and SU(5) theories, we determined the mass and couplings of the axion in terms of the relevant threshold scales, and assessed how the former are constrained from the requirements of gauge coupling unification, proton decay searches and collider bounds. The axion mass ends up being rather constrained for GUT scale axions, particularly in the case of SU(5), and could be probed by upcoming dark matter experiments, such as ABRACADABRA and CASPEr-Electric.
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