Reduced axion abundance from an extended symmetry

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 grand unified theory (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 toward alleviating the isocurvature problem from inflation. Furthermore, instead of possible overclosure from cosmic strings, the extended symmetry implies the formation of nontopological textures which provide a relatively small abundance.