GUT monopoles, the Witten effect and QCD axion phenomenology

TL;DR

This paper explores the complex interaction between magnetic monopoles and axions via the Witten effect, revealing how monopoles influence axion potentials and potentially suppress dark matter and isocurvature signals in the early universe.

Contribution

It demonstrates that monopoles modify axion potentials through the Witten effect, affecting axion phenomenology and dark matter abundance.

Findings

Monopoles induce a non-exponentially suppressed axion potential.

A population of GUT monopoles can suppress axion dark matter.

Monopoles can reduce CMB isocurvature fluctuations.

Abstract

Taken individually, magnetic monopoles and axions are both well-motivated aspects of physics beyond the Standard Model. We demonstrate that by virtue of the Witten effect, their interplay is furthermore nontrivial; monopoles break the corresponding axial symmetries explicitly and result in an axion potential lacking the usual instanton-derived exponential suppression factor. As axions are distinguished by their extremely weak interactions, any regime where their effects are enhanced may be of interest. As an application of these findings, we demonstrate that a phenomenologically acceptable population of grand-unification monopoles in the early Universe can efficiently suppress both the QCD axion dark matter abundance and CMB isocurvature contribution.