Self-interacting dark matter with observable ΔN_{\rm eff}

2025/04/23 by Borah, Debasish, Mahapatra, Satyabrata, Sahu, Narendra +1
#Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph)

paper · doi:10.48550/arxiv.2504.16910

Abstract

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 ΔN_{\rm eff} 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 X^μ, whose mass is generated via the Stueckelberg mechanism. The relic abundance of χ is determined by thermal freeze-out through annihilations into X^μ, supplemented by a non-thermal 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 (ν_S). 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 ΔN_{\rm eff}, compatible with Planck 2018 bounds and within reach of next-generation experiments such as SPT-3G, CMB-S4, and CMB-HD.

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