The Spectrum of Global Axion Strings

TL;DR

This paper analyzes the spectrum of axions emitted from global strings in the early universe using large-scale simulations, addressing systematic uncertainties to refine the predicted axion dark matter mass range relevant for upcoming experiments.

Contribution

It provides a detailed analysis of axion spectra from global strings, incorporating systematic effects and extrapolations to improve mass predictions.

Findings

Predicted axion mass range: 95 μeV to 450 μeV.

Identified systematic effects affecting previous estimates.

Refined predictions will guide future axion detection experiments.

Abstract

The post-inflationary Peccei-Quinn (PQ) symmetry breaking scenario provides a unique opportunity to pinpoint the QCD axion dark matter mass, which is a crucial input for laboratory experiments that are designed for probing specific mass ranges. Predicting their mass requires a precise knowledge of how axions are produced from the decay of topological defects in the early Universe that are inevitably formed. In this contribution, we present recent results on the analysis of the spectrum of axions radiated from global strings based on large scale numerical simulations of the cosmological evolution of the PQ field on a static lattice. We highlight several systematic effects that have been overlooked in previous works, such as the dependence on the initial conditions, contaminations due to oscillations in the spectrum, and discretisation effects; some of which could explain the discrepancy in the current literature. Taking these uncertainties into account and performing the extrapolation to cosmologically relevant string tensions, we find that the dark matter mass is predicted to be in the range of $95\,\mu\text{eV} \lesssim m_a \lesssim 450 \, \mu\text{eV}$, which will be probed by some of the next generation direct detection experiments.