Axino Light Dark Matter and Neutrino Masses with R-parity Violation

Motivated by the recent observation of the 511 keV $\gamma$-ray line emissions from the galactic bulge and an explanation for it by the decays of light dark matter particles, we consider the light axino whose mass can be in the 1-10 MeV range, particularly, in the context of gauge-mediated supersymmetry breaking models. We discuss the production processes and cosmological constraints for the light axino dark matter. It is shown that the bilinear R-parity violating terms provide an appropriate mixing between the axino and neutrinos so that the light axino decays dominantly to $e^+ e^- \nu$. We point out that the same bilinear R-parity violations consistently give both the lifetime of the axino required to explain the observed 511 keV $\gamma$-rays and the observed neutrino masses and mixing.