Explanation of the 511 keV line: Cascade annihilating dark matter with the ${}^8$Be anomaly

Abstract

A possible dark matter (DM) explanation about the long-standing issue of the Galactic 511 keV line is explored in this paper. For DM cascade annihilations of concern, a DM pair ${\pi }_d^{+}{\pi }_d^{-}$ annihilates into unstable ${\pi }_d^0{\pi }_d^0$, and ${\pi }_d^0$ decays into $e^{+}{e}^{-}$ with new interactions suggested by the ${}^8$Be anomaly. Considering the constraints from the effective neutrino number $N_{\mathrm{eff}}$ and the 511 keV gamma-ray emission, a range of DM is obtained, $11.6\lesssim m_{{\pi }_d^{\pm }}\lesssim 15$ MeV. The typical DM annihilation cross section today is about 3.3 $\times$ $10^{-29}$ cm${}^3$ s${}^{-1}$, which can give an explanation about the 511 keV line. The MeV scale DM may be searched by the DM-electron scattering, and the upper limit set by the CMB s-wave annihilation is derived in DM direct detections.

Comments: 7 pages, 3 figures, in twocolumn