Boomerang Mechanism Explaining Excess Radio Background

We propose a {\em boomerang mechanism} for the explanation of the excess radio background detected by ARCADE. In an early stage, at a temperature $T \sim 100\,{\rm keV}$, a fraction of relic neutrinos is resonantly converted into dark neutrinos by mixing induced by a pre-existing lepton asymmetry. Dark neutrinos decay much later into a dark photon, mixed with photon, and a dark fermion, with a lifetime longer than the age of the Universe, as required by a solution to the excess radio background. This scenario circumvents the upper bound on the neutrino magnetic moment but still implies a testable lower bound.