Abstract
New gauge bosons at the MeV scale with tiny gauge couplings (so-called dark photons) can be responsible for the freeze-in production of dark matter and provide a clear target for present and future experiments. We study the effects of thermal mixing between dark photons and Standard Model gauge bosons and of the resulting plasmon decays on dark matter production before and after the electroweak phase transition. In the parameter regions preferred by the observed dark matter relic abundance, the dark photon is sufficiently long-lived to be probed with fixed-target experiments and light enough to induce direct detection signals. Indeed, current limits from XENON1T already constrain Dirac fermion dark matter in the GeV to TeV range produced via the freeze-in mechanism. We illustrate our findings for the case of a $U(1)_{B-L}$ gauge extension and discuss possible generalisations.
Highlights
Given our complete ignorance of the nature of dark matter (DM), it is sensible to construct DM models in analogy to phenomena known from the Standard Model (SM)
Both the hypercharge gauge boson and the photon mix with the A0 boson, leading to two mass eigenstates that can mediate processes involving DM particles. This finding is relevant if the thermal mass of one of these eigenstates is large enough to allow for direct decays into DM particles. Such plasmon decays have recently been found to give a relevant contribution to the freeze-in production of sub-MeV DM [25,26], and we extend the discussion there to more general gauge groups and to temperatures above the electroweak phase transition (EWPT)
We have studied the cosmology and phenomenology of a model of DM produced via the freeze-in mechanism with an MeV-scale Uð1Þ0 gauge boson
Summary
Given our complete ignorance of the nature of dark matter (DM), it is sensible to construct DM models in analogy to phenomena known from the Standard Model (SM). The density of A0 bosons is given by an equilibrium distribution, opening up a new way to produce DM particles: A0A0 → χχ This channel depends in a different way on the specific Uð1Þ0 charge assignments than processes involving both DM and SM particles and opens up additional parameter space in the translation between the relic density requirement and direct detection experiments. If the A0 boson is in thermal equilibrium, its mass must be larger than a few MeV in order to satisfy constraints on the number of relativistic degrees of freedom during big bang nucleosynthesis (BBN) [13] and on exotic sources of energy injection [14,15] In this mass range and for the required coupling strength, the dark photons are long-lived and can be searched for in a number of ways at the intensity frontier [16], using for example fixed-target.
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