Axion Dark Matter Archaeology with Primordial Gravitational Waves

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. By leveraging the features of the GW signal, future interferometers can probe $m_Q$ and $f_a$ complementing the sensitivity of haloscope experiments, potentially pinning down all relevant parameter that describe the physics. Specifically, we find that BBO and ET GW detectors will both be able to optimistically probe $f_a\gtrsim 10^{14}\,{\rm GeV}$.