Galactic 511 keV line from MeV millicharged dark matter

Abstract

We present a possible explanation of the recently observed 511 keV $γ$-ray anomaly with a new ``millicharged'' fermion. The new fermion is light (${\cal O}({\rm MeV})$) but has never been observed by any collider experiments mainly because of its tiny electromagnetic charge $εe$. We show that constraints from its relic density in the Universe and collider experiments allow a parameter range such that the 511 keV cosmic $γ$-ray emission from the galactic bulge may be due to positron production from this millicharged fermion.

Figures (3)

• Figure 1: The "millicharge" annihilation diagrams to, (a) $e^+e^-$ and (b-1) $2\gamma_{\rm ex}$; (b-2) the bremsstrahlung diagram related to (b-1). The cross diagram in (b-1) is not shown.
• Figure 2: The plot for $\alpha_{\rm ex}/\alpha$ versus $\varepsilon$. The lower left corner (yellow shaded regions) is excluded by the DM relic density constraint: the lines correspond to $\Omega_{\chi}h^2=0.11$ and $m_{\chi}=1, 3,$ and $10$ MeV, respectively. The vertical bands (gray shaded regions) are the allowed range of $\varepsilon$ that will be given by the 511 keV $\gamma$-ray flux constraint analysis: the regions correspond to $m_{\chi}=1, 3,$ and $10$ MeV, respectively. The region excluded by the Debye screening is shown from the central (pink shaded) region to the lower right corner marked by Debye screening. The (green) region $\alpha_{\rm ex}/\alpha >0.01 (0.1)$ is excluded since more than $1 (10)\%$ diffuse gamma ray flux compared to the 511 keV flux is expected.
• Figure 3: The plot for $\varepsilon$ versus $m_{\chi}$. The dark (blue) shaded region is excluded by the SLAC search of millicharged particles Prinz:1998ua. We plot the Navarro-Frenk-White profile Navarro model line (dotted) using the fitting parameter ($R=20$ kpc, $\rho_0 = 0.347$ GeV/${\rm cm}^3$) and the lightly shaded (yellow) region for the uncertainty range $\overline{J}(0.0086{\rm\ sr}) \sim 50-500$. After considering the recently given strong constraint on the light dark matter mass lowerm, the allowed region is $m_\chi \lesssim 4$ MeV (bottom).