Probing 5.49 MeV Solar Axions at Xenon Experiments

Abstract

The monochromatic 5.49 MeV solar axions induced by the isovector coupling $g_{3aN}$ can be searched for at the dark matter direct detection experiments. In this paper we estimate the prospects of the relevant axion couplings for axion mass $m_a<$ 1 MeV with xenon targets. Given the axion-electron coupling $g_{ae}$, the signal is dominated by the axion-induced $e^{+}{e}^{-}$ pair production whose cross section is largely enhanced when the axion mass approaches twice of the electron mass. Furthermore, the cross section depends on the atomic number squared $Z^2$. This allows the next-generation xenon experiments to surpass the current Borexino constraints and provide sensitivities competitive with those of the large neutrino detectors such as JUNO and Hyper-Kamiokande. With an exposure of 200 and 1000 ton$\cdot$yr, the couplings $|g_{3aN}{g}_{ae}|$ can be probed down to $1.59\times 10^{-14}$ and $7.12\times 10^{-15}$ at 90% C.L., respectively. If the axion couples to photons, the projected sensitivities on $|g_{3aN}{g}_{a\gamma }|$ can touch down to $6.76\times 10^{-12}$ GeV${}^{-1}$ and $3.02\times 10^{-12}$ GeV${}^{-1}$, respectively.

Comments: 27 pages, 8 figures and 2 tables