Galactic 511 keV line from MeV millicharged dark matter

Abstract

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