The phenomenon of the axion kinetic misalignment with a generic PQ-breaking operator

TL;DR

This paper explores how generic PQ-breaking operators affect axion dark matter, early universe cosmology, and gravitational wave signals, revealing suppressed GW signals and identifying viable parameter regions.

Contribution

It introduces a comprehensive analysis of the effects of generic PQ-breaking operators on axion phenomenology, including relic density, cosmological evolution, and GW signals, with a detailed parameter space scan.

Findings

Nonzero initial axion velocity causes brief early matter and kinetic domination epochs.

Gravitational wave signals from cosmic strings are highly suppressed and undetectable.

Identifies parameter regions consistent with all experimental constraints.

Abstract

We investigate the phenomenology induced by generic PQ-breaking operators within the axion kinetic misalignment framework. We analyze their impact on the relic density of axion dark matter (DM), the PQ quality problem, axion-mediated fifth-force, as well as Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) constraints. A nonzero initial axion velocity gives rise to brief periods of early matter domination and axion kinetic domination, leading to a nonstandard cosmological evolution. We compute the resulting gravitational wave (GW) signal from global cosmic strings and find that, because these nonstandard epochs are extremely short, the signal is highly suppressed and beyond the reach of existing experiments. Finally, we perform a parameter space scan, identify the regions and benchmark point that are consistent with all experimental constraints.