Post-inflationary axions: a minimal target for axion haloscopes

TL;DR

This paper explores the potential of post-inflationary axion-like particles with domain wall number greater than one to serve as dark matter, highlighting their possible detectability through photon coupling and discussing formation dynamics and constraints.

Contribution

It provides the first detailed analysis of domain wall formation and dynamics for ALPs with N>1, including numerical simulations and implications for dark matter abundance and detectability.

Findings

ALPs with N>1 can account for dark matter with small decay constants.

Number of domain walls per Hubble patch can be estimated through simulations.

Dark matter ALPs may produce large isocurvature fluctuations.

Abstract

An axion-like-particle (ALP) in the post-inflationary scenario with domain wall number $N>1$ can be dark matter if the residual $\mathbb{Z}_N$ symmetry has a small explicit breaking. Although we cannot determine the full dynamics of the system reliably, we provide evidence that such an ALP can account for the observed dark matter abundance while having a relatively small decay constant and consequently a possibly large coupling to photons. In particular, we determine the number of domain walls per Hubble patch around the time when they form using numerical simulations and combine this with analytic expectations about the subsequent dynamics. We show that the strongest constraint on the decay constant is likely to come from the dark matter ALPs being produced with large isocurvature fluctuations at small spatial scales. We also comment on the uncertainties on the dark matter small-scale structure that might form from these overdensities, in particular pointing out the importance of quantum pressure in the $N=1$ case.