Axionic Hot Dark Matter in the Hadronic Axion Window
Takeo Moroi, Hitoshi Murayama
TL;DR
The paper investigates whether a hadronic (KSVZ) axion with $f_a \sim 10^{6}{\rm GeV}$ can act as the hot dark matter component in a mixed dark matter framework. It combines particle-physics modeling of the axion with astrophysical and cosmological constraints, showing that thermal production yields a relic density $\Omega_a \sim 0.1$–$0.2$ for $m_a \sim { m a few\,eV}$, consistent with HDM requirements if the axion-photon coupling is sufficiently suppressed. It identifies the viable parameter window $3\times 10^5{\rm GeV} \lesssim f_a \lesssim 2\times 10^6{\rm GeV}$ and discusses how SN1987A, HB stars, red giants, and UV background limits shape this region, while highlighting uncertainties in $z$ and $E_{\rm PQ}/N$. The work proposes experimental tests, notably resonant solar axion absorption via $^{57}$Fe and the search for axion bursts from future supernovae in large water Cherenkov detectors, providing concrete paths to falsify or confirm hadronic axion HDM in the coming decades.
Abstract
Mixed dark matter scenario can reconcile the COBE data and the observed large scale structure. So far the massive neutrino with a mass of a few eV has been the only discussed candidate for the hot dark matter component. We point out that the hadronic axion in the so-called hadronic axion window, f_a \sim 10^6 GeV, is a perfect candidate as hot dark matter within the mixed dark matter scenario. The current limits on the hadronic axion are summarized. The most promising methods to verify the hadronic axion in this window are the resonant absorption of almost-monochromatic solar axions from M1 transition of the thermally excited $^{57}$Fe in the Sun, and the observation of the ``axion burst'' in water Čerenkov detectors from another supernova.