Residual Non-Abelian Dark Matter and Dark Radiation

TL;DR

The paper addresses the origin of dark matter and dark radiation within a shared non-Abelian dark sector. It proposes a concrete $SU(3)$→$SU(2)$ breaking scenario in which massless gauge bosons $A^{1,2,3}$ act as DR and massive bosons $A^{4..8}$ serve as vector dark matter (VDM); their interactions arise from non-Abelian gauge dynamics and enable DM–DR scattering with a cross section scaling as $\sigma_{DM-DR}\sim g^4/T^2$. DM relic abundance is achieved via freeze-in through $\phi+\phi\to A^{m}+A^{m}$, with entropy dilution from late $\phi$ decay and a Higgs portal allowing $\delta N_{\mathrm{eff}}$ to be in the observed range, typically $\delta N_{\mathrm{eff}}\sim 0.5$. Numerical results show a suppressed matter power spectrum and a reduced $\sigma_8$ (e.g., $\sigma_8\approx 0.746$ for $m_\chi\simeq 10$ TeV and $g_X^2\sim 10^{-7}$), offering a potential resolution to Planck–HST and small-scale structure tensions. Overall, the framework links DM and DR in a testable non-Abelian context with clear cosmological signatures.

Abstract

We propose a novel particle physics model in which vector dark matter (VDM) and dark radiation (DR) originate from the same non-Abelian dark sector. We show an illustrating example where dark $SU(3)$ is spontaneously broken into $SU(2)$ subgroup by the nonzero vacuum expectation value of a complex scalar in fundamental representation of $SU(3)$. The massless gauge bosons associated with the residual unbroken $SU(2)$ constitute DR and help to relieve the tension in Hubble constant measurements between $\textit{Planck}$ and Hubble Space Telescope. In the meantime, massive dark gauge bosons associated with the broken generators are VDM candidates. Intrinsically, this non-Abelian VDM can interact with non-Abelian DR in the cosmic background, which results in a suppressed matter power spectrum and leads to a smaller $σ_8$ for structure formation.

Figures (6)

• Figure 1: An example Feynman diagram for decay $\phi\rightarrow A^a A^a$.
• Figure 2: Feynman diagram for DM-DR scattering in the $U(1)$ case.
• Figure 3: Typical Feynmann diagrams for DM-DR (left) and DM-DM (right) scattering.
• Figure 4: $\lambda_{\phi H}$ as function of DM mass $m_A$. The solid, dot-dashed and dashed lines correspond to $m_\phi=1,2,5\mathrm{TeV}$, respectively.
• Figure 5: Schematic picture for thermal history of DM $A^m$, DR $A^a$, dark Higgs boson $\phi$ and SM.