Abstract
We present a 5D axion-neutrino model that explains the Standard Model fermion mass hierarchy and flavor structure, while simultaneously generating a high-quality axion. The axion and right-handed neutrinos transform under a 5D Peccei-Quinn gauge symmetry, and have highly suppressed profiles on the UV brane where the symmetry is explicitly broken. This setup allows neutrinos to be either Dirac, or Majorana with hierarchically small sterile neutrino masses. The axion decay constant originates from the IR scale, which in the holographically dual 4D description corresponds to the confinement scale of some new strong dynamics with a high-quality global Peccei-Quinn symmetry that produces a composite axion and light, composite sterile neutrinos. The sterile neutrinos could be observed in astrophysical or laboratory experiments, and the model predicts specific axion--neutrino couplings.
Highlights
Two unsettled issues in the Standard Model are neutrino masses and the strong CP problem
The most popular solution to the strong CP problem is the Peccei-Quinn (PQ) mechanism [6], where the axion is a pseudo-NambuGoldstone boson [7,8] that results from the spontaneous breaking of a global Uð1ÞPQ symmetry
Any axion solution to the strong CP problem must address the axion quality problem, which requires extraneous, explicit violations of the global PQ symmetry to be sufficiently suppressed compared to that arising from nonperturbative QCD
Summary
Two unsettled issues in the Standard Model are neutrino masses and the strong CP problem. The most popular solution to the strong CP problem is the Peccei-Quinn (PQ) mechanism [6], where the axion is a pseudo-NambuGoldstone boson [7,8] that results from the spontaneous breaking of a global Uð1ÞPQ symmetry These two solutions appear to be unrelated; the similarity of the PQbreaking and Majorana mass scales suggests there could be an underlying mechanism responsible for both neutrino masses and the axion. Explicit PQ (and lepton number (L)) violating terms are allowed on the UV brane, and the fundamental Majorana mass scale is tied to explicit Planck-scale PQ (and L) violation Despite this connection, the sterile neutrino mass eigenstates can be naturally light. The sterile neutrino mass eigenstates can be naturally light This is because the right-handed neutrino profiles can be localized towards the IR brane, away from the explicit symmetry violation. While right-handed neutrinos propagating in an extra dimension were previously considered in Refs. [22–33], our setup is the first model to amalgamate neutrinos with axion physics
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