Reduced Axion Abundance from an Extended Symmetry

TL;DR

This paper introduces an extended symmetry framework that naturally reduces axion relic abundance, allowing for higher PQ scales and addressing isocurvature issues, with implications for dark matter and early universe cosmology.

Contribution

It develops a model with an extended $SO(N)$ symmetry that naturally suppresses axion relic abundance and permits large PQ scales, including near GUT scale.

Findings

Reduces axion relic abundance through extended symmetry.

Allows large PQ scales, approaching GUT scale.

Addresses isocurvature problem and cosmic string over-closure.

Abstract

In recent work we showed that the relic dark matter abundance of QCD axions can be altered when the Peccei-Quinn (PQ) field is coupled to very light scalar/s, rendering the effective axion mass dynamical in the early universe. In this work we develop this framework further, by introducing a new extended symmetry group to protect the new particles' mass. We find that with a new global $SO(N)$ symmetry, with large $N$, we can not only account for the lightness of the new scalars, but we can reduce the axion relic abundance in a technically natural way. This opens up the possibility of large PQ scales, including approaching the GUT scale, and still naturally producing the correct relic abundance of axions. Also, in these models the effective PQ scale is relatively small in the very early universe, and so this can help towards alleviating the isocurvature problem from inflation. Furthermore, instead of possible over-closure from cosmic strings, the extended symmetry implies the formation of non-topological textures which provide a relatively small abundance.