Freeze-in Production of Non-Abelian Millicharged Vector Dark Matter

Abstract

We present the first predictive realization, to our knowledge, of vector freeze-in dark matter from a hidden non-Abelian $SU(2)$ gauge sector spontaneously broken by a Higgs triplet to a residual $U(1)$ symmetry with a massless dark photon mediator. A massive dark vector particle-antiparticle pair acquires small millicharges through a dimension-4 kinetic-mixing term with an induced coefficient $ϵ$, generated by an effective dimension-5 operator involved the Higgs triplet. The dark sector interactions are governed by the hidden gauge coupling $g_D$, providing a weak connection to the Standard Model to realize the freeze-in dark matter production mechanism. Solving the two-temperature Boltzmann evolution including plasmon decay, we find a wide region of parameter space consistent with the observed relic abundance while satisfying astrophysical and cosmological constraints. This minimal framework connects non-Abelian vector dynamics with long-range dark forces and may be probed by upcoming sub-GeV dark matter direct-detection experiments.

Comments: 39 pages, 7 figures, v2: journal version