Abstract
We study the Witten effect of hidden monopoles on the QCD axion dynamics, and show that its abundance as well as isocurvature perturbations can be significantly suppressed if there is a sufficient amount of hidden monopoles. When the hidden monopoles make up a significant fraction of dark matter, the Witten effect suppresses the abundance of axion with the decay constant smaller than 1012GeV. The cosmological domain wall problem of the QCD axion can also be avoided, relaxing the upper bound on the decay constant when the Peccei–Quinn symmetry is spontaneously broken after inflation.
Highlights
The smallness of the strong CP phase is an outstanding mystery in particle physics, and in particular, it lacks any obvious anthropic explanation
In this Letter, we have proposed a novel mechanism to suppress the axion abundance based on the Witten effect
In particular we have focused on the case in which the Witten effect becomes important before the QCD phase transition
Summary
The smallness of the strong CP phase is an outstanding mystery in particle physics, and in particular, it lacks any obvious anthropic explanation. When the PQ symmetry is spontaneously broken before inflation, the abundance of the QCD axion dark matter depends on the axion decay constant fa and the initial misalignment angle θi [8]: Ωah. When the PQ symmetry is spontaneously broken after inflation, an axionic string and wall system appears at the QCD phase transition [31]. We focus on hidden magnetic monopoles that are much less constrained, and study their Witten effect on the QCD axion dynamics, taking account of the adiabatic suppression mechanism as well as various theoretical/cosmological possibilities. The hidden monopoles (as well as vector fields) may make up the significant fraction of DM They have non-negligible self-interactions, which could ameliorate small-scale tensions of the ΛCDM [37]
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