Opening the 1$\,$Hz axion window

TL;DR

This paper proposes a two-axion model where a 1 Hz oscillating ALP accounts for all dark matter, while the QCD axion solves the strong CP problem, with testable predictions for upcoming experiments.

Contribution

It introduces a novel two-axion framework that explains the 1 Hz ALP as dark matter and addresses the strong CP problem simultaneously.

Findings

The 1 Hz ALP can constitute all dark matter in the universe.

The model predicts an oscillating strong CP phase within experimental reach.

The 1 Hz axion is detectable by future laboratory and astrophysical searches.

Abstract

An axion-like particle (ALP) with mass $m_\phi \sim 10^{-15}$eV oscillates with frequency $\sim$1 Hz. This mass scale lies in an open window of astrophysical constraints, and appears naturally as a consequence of grand unification (GUT) in string/M-theory. However, with a GUT-scale decay constant such an ALP overcloses the Universe, and cannot solve the strong CP problem. In this paper, we present a two axion model in which the 1 Hz ALP constitutes the entirety of the dark matter (DM) while the QCD axion solves the strong CP problem but contributes negligibly to the DM relic density. The mechanism to achieve the correct relic densities relies on low-scale inflation ($m_\phi \lesssim H_{\rm inf}\lesssim 1$ MeV), and we present explicit realisations of such a model. The scale in the axion potential leading to the 1 Hz axion generates a value for the strong CP phase which oscillates around $\bar{\theta}_{\rm QCD}\sim 10^{-12}$, within reach of the proton storage ring electric dipole moment experiment. The 1 Hz axion is also in reach of near future laboratory and astrophysical searches.