We propose a GeV-scale self-interacting dark matter (SIDM) candidate within a dark U(1)D gauged extension of the Standard Model (SM), addressing small-scale structure issues in ΛCDM while predicting an observable contribution to ΔNeff in the form of dark radiation. The model introduces a fermionic DM candidate χ and a scalar ϕ, both charged under an unbroken U(1)D gauge symmetry. The self-interactions of χ are mediated by a light vector boson Xμ, whose mass is generated via the Stueckelberg mechanism. The relic abundance of χ is determined by thermal freeze-out through annihilations into Xμ, supplemented by a nonthermal component from the late decay of ϕ. Crucially, ϕ decays after the big bang nucleosynthesis (BBN) but before the cosmic microwave background (CMB) epoch, producing additional χ and a dark radiation species (νS). This late-time production compensates for thermal underabundance due to efficient annihilation into light mediators, while remaining consistent with structure formation constraints. The accompanying dark radiation yields a detectable ΔNeff, compatible with Planck 2018 bounds and within reach of next-generation experiments such as SPT-3G, CMB-S4, and CMB-HD.
Borah et al. (Wed,) studied this question.