Abstract
We propose a scenario where the spontaneous breakdown of the Peccei-Quinn symmetry leads to monopole production. Both the axion and the monopole contribute to dark matter, and the Witten effect on the axion mass is a built-in feature. In the KSVZ-type axion model, seemingly different vacua are actually connected by the hidden gauge symmetry, which makes the axionic string unstable and separate into two Alice strings. The Alice string is attached to a single domain wall due to the QCD instanton effect, solving the domain-wall problem. This is in the same spirit of the Lazarides-Shafi mechanism, although the discrete Peccei-Quinn symmetry is not embedded into the center of the original gauge symmetry. In the DFSZ-type axion model, the domain-wall problem is avoided by the Witten effect. If the Peccei-Quinn symmetry is explicitly broken by a small amount, monopoles acquire a tiny electric charge and become mini-charged dark matter. Interestingly, the quality of the Peccei-Quinn symmetry is closely tied to darkness of dark matter.
Highlights
The origin of dark matter (DM) remains a mystery in both cosmology and particle physics
If the PQ symmetry gets spontaneously broken after inflation, on the other hand, axions are produced by the decay of domain walls and cosmic strings
Motivated by the coincidence of the energy scales, we have pursued the possibility of unifying the PQ symmetry breaking and the production of the monopole DM
Summary
The origin of dark matter (DM) remains a mystery in both cosmology and particle physics. One example is a hidden monopole of which the stability is guaranteed by its topological charge [1,2,3] Another is the QCD axion [4,5,6,7] or axionlike particles of which the mass and couplings are suppressed by the decay constant. If the PQ symmetry gets spontaneously broken after inflation, on the other hand, axions are produced by the decay of domain walls and cosmic strings In this case, the axions explain the observed DM abundance if the PQ breaking scale is of Oð1010Þ GeV [20] or slightly larger [21]. The vacuum expectation value (VEV) of Φ is given as hΦi 1⁄4 pvffiffi 1 This spontaneously breaks SUð2ÞH gauge symmetry down to Uð1ÞH. The coupling to hidden photons usually does not give any mass to the axion, but in the presence of hidden monopoles, it generates the effective axion mass that depends on the monopole number density via the Witten effect [25,26]
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