The meV mass frontier of axion physics

We explore consequences of the idea that the cooling speed of white dwarfs can be interpreted in terms of axion emission. In this case, the Yukawa coupling to electrons has to be $g_{ae}\sim 1\times {10}^{-13}$, corresponding to an axion mass of a few meV. Axions then provide only a small fraction of the cosmic cold dark matter, whereas core-collapse supernovae release a large fraction of their energy in the form of axions. We estimate the diffuse supernova axion background in the Universe, consisting of 30 MeV-range axions with a radiation density comparable to the extragalactic background light. The diffuse supernova axion background would be challenging to detect. However, axions with white-dwarf-inspired parameters can be accessible in a next-generation axion helioscope.

Figure 1

Diffuse backgrounds of SN neutrinos and axions, assuming that either one carries away the full SN energy ($3\times {10}^{53}\mathrm{erg}$). The width of the bands reflects only the uncertainty in the core-collapse rate $R_{\mathrm{cc}}$. For ${\overline{\nu }}_e$, a thermal spectrum with $T=4\mathrm{MeV}$ is assumed, carrying away $1/6$ of the total energy, whereas for axions we use the bremsstrahlung-inspired spectrum of Eq. (8) with $T_{\mathrm{core}}=30\mathrm{MeV}$.