Axion Dark Matter Archaeology with Primordial Gravitational Waves

TL;DR

This paper explores how primordial gravitational waves can provide unique insights into post-inflationary axion dark matter models, especially those with heavy quark domination, by proposing blue-tilted GW signals as a probe.

Contribution

It introduces the use of blue-tilted inflationary gravitational waves to distinguish post-inflationary PQ breaking axion models with heavy quark decay effects.

Findings

Axion mass can be as low as 10^{-8} eV in certain models.

Blue-tilted GWs can constrain heavy quark mass and decay constant.

Future GW detectors can probe axion decay constants above 10^{14} GeV.

Abstract

We investigate the complementary information to be gained from inflationary gravitational wave (IGW) signals and searches for QCD axion dark matter. We focus on post-inflationary Peccei-Quinn (PQ) breaking axion models that are cosmologically safe. Recent work has shown that a greater number of such models exist. This is because the heavy quarks required for the colour anomaly can provoke a period of heavy quark domination (HQD), which, through decay, dilutes the axion abundance. In this work we show for the first time that the axion dark matter mass can be as low as $m_a\sim10^{-8}\,{\rm eV}$ for models where the heavy quarks decay via dimension 6 terms. This is achieved by allowing the mass of the heavy quarks to differ from the axion decay constant, $m_Q\neq f_a$. Consequently, the observables that would distinguish between pre- and post-inflationary PQ breaking, $m_a$ and the additional relativistic degrees of freedom $\Delta N_{\rm eff}$, now become indiscernible. To solve this, we propose using blue-tilted IGWs to probe HQD. In scenarios where such a blue tilt is present, the enhanced GW signal allows future interferometers to place non-trivial constraints on the parameters $m_Q$ and $f_a$, thereby complementing haloscope searches. While some degeneracies with other parameters such as $m_Q$ remain, detectors such as BBO and ET will be able to optimistically probe $f_a\gtrsim 10^{14}\,{\rm GeV}$.