Is the Turner Window Open? Seeking Closure with Resonant Absorption of Galactic Axions in NaI Dark Matter Detectors

Abstract

Motivated by the DAMA/LIBRA annual modulation signal, the dark matter community has invested heavily in ultra-clean underground NaI detectors to search for light WIMPs. We point out a new target of opportunity for these detectors -- axions produced by the carbon-burning stars within our galaxy. These stars synthesize large quantities of $^{23}$Na, keeping it at temperatures $\sim 10^9$K for periods up to tens of thousands of years. Under these conditions, $^{23}$Na radiates 440 keV axions through repeated photo-excitation and axio-deexcitation of its first excited state. Upon reaching a NaI detector, the process is reversed: the axion is resonantly absorbed, producing a 440 keV deexcitation photon. NaI thus serves as both $\gamma$ source and $\gamma$ detector. We find that existing NaI detectors can probe axion-nucleon couplings $|g_{aNN}^\mathrm{eff~^{23}Na}| \approx g_{app} \sim 10^{-6}$--$10^{-2}$, including QCD axions with $m_a \gtrsim 10$ eV. While there are several astrophysical constraints on axions with these couplings, our re-examination of these bounds shows that substantial gaps remain, providing strong motivation for the proposed searches.