Abstract
We investigate the cosmological stability of light bosonic dark matter carrying a tiny electric charge. In the wave-like regime of high occupation numbers, annihilation into gauge bosons can be drastically enhanced by parametric resonance. The millicharged particle can either be minimally coupled to photons or its electromagnetic interaction can be mediated via kinetic mixing with a massless hidden photon. In the case of a direct coupling current observational constraints on the millicharge are stronger than those arising from parametric resonance. For the (theoretically preferred) case of kinetic mixing large regions of parameter space are affected by the parametric resonance leading at least to a fragmentation of the dark matter field if not its outright destruction.
Highlights
Very light dark matter (DM) must consist of bosonic particles
While this naively rules out any sizable electric charge assigned to DM particles, it is still possible that their charge is tiny, thereby strongly suppressing interactions with photons
We have investigated the cosmological longevity of such DM particles in the sub-eV mass regime
Summary
Very light (sub-eV) dark matter (DM) must consist of bosonic particles. Common examples are axions, axionlike particles, or dark photons [1,2,3,4,5]. In significant parts of the parameter space, the stability of axionlike particles toward their decay into photons requires a nontrivial interplay of the expansion of the Universe, as well as plasma effects [1,2,10] (cf [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30], for some situations where Bose enhancement from high occupation numbers may lead to interesting signatures for axionlike particles) These particles do not carry a conserved charge that would naturally render them stable toward decay.
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