Non-Abelian dark matter and dark radiation

TL;DR

This paper introduces a non-Abelian dark sector in which WIMP dark matter carries dark color under SU($N$)$_d$ and interacts with massless dark gluons that act as dark radiation. Dark matter multiplicity alters relic abundance calculations and enhances collider production while leaving direct detection largely unchanged; the dark radiation component contributes a small ΔN_eff and behaves as a perfect fluid with zero viscosity, imprinting distinctive CMB signatures. DM–DR interactions induce a drag that yields a smooth, scale-spanning suppression of the linear matter power spectrum, offering a potential route to alleviate tensions in $H_0$ and $\sigma_8$ within ΛCDM. The authors provide analytic relations for the relic abundance, outline experimental prospects across direct/indirect detection and colliders, and discuss cosmological implications, highlighting regions of parameter space that could be probed by future observations.

Abstract

We propose a new class of dark matter models with unusual phenomenology. What is ordinary about our models is that dark matter particles are WIMPs, they are weakly coupled to the Standard Model and have weak scale masses. What is unusual is that they come in multiplets of a new "dark" non-Abelian gauge group with milli-weak coupling. The massless dark gluons of this dark gauge group contribute to the energy density of the universe as a form of weakly self-interacting dark radiation. In this paper we explore the consequences of having i.) dark matter in multiplets ii.) self-interacting dark radiation and iii.) dark matter which is weakly coupled to dark radiation. We find that i.) dark matter cross sections are modified by multiplicity factors which have significant consequences for collider searches and indirect detection, ii.) dark gluons have thermal abundances which affect the CMB as dark radiation. Unlike additional massless neutrino species the dark gluons are interacting and have vanishing viscosity and iii.) the coupling of dark radiation to dark matter represents a new mechanism for damping the large scale structure power spectrum. A combination of additional radiation and slightly damped structure is interesting because it can remove tensions between global $Λ$CDM fits from the CMB and direct measurements of the Hubble expansion rate ($H_0$) and large scale structure ($σ_8$).

Figures (8)

• Figure 1: Annihilation of dark matter into $SU(2)_{weak}\ $ gauge bosons or SM fermions. Since the mass splitting between members of $SU(2)_{weak}\ $ multiplets is small compared with energy transfer in the annihilation diagrams (i.e. twice the $\chi$ mass) we compute the co-annihilation of full $SU(2)_{weak}\ $ multiplets and ignore the mass splittings.
• Figure 2: Color factors for the different types of DM search experiments. The different multiplicity factors can be easily understood from the color flow in the figure. For direct detection the color of the incoming dark matter is the same as of the outgoing and so there is no multiplicity factor. For indirect detection it is an annihilation diagram, so just as for the thermal relic calculation there is a $1/2N$ suppression because a DM particle can only annihilate if it finds the anti-particle with the right anti-dark color. For colliders there is an $2N$ enhancement because any of the different N colors can be created and an extra 2 from Dirac vs Majorana.
• Figure 3: Indirect detection constraints from gamma ray line searches from H.E.S.S. (blue region) and projected sensitivity for gamma ray lines at CTA (gray region) assuming an NFW profile for the dark matter distribution in the galactic center, both taken from Ovanesyan:2014fwa. The red dots are the cross sections for dark matter annihilation into gamma rays in our models for $N=2$ up to $N=10$, which were obtained by appropriately rescaling the NLL cross section with Sommerfeld enhancement from Ovanesyan:2014fwa. For comparison we plotted in black the cross-section for the annihilation cross-section to photons in the standard "wino" model as a function of mass, also from Ovanesyan:2014fwa.
• Figure 4: Expected significance of missing energy (MET) searches for DM at the LHC and a future 100 TeV collider. The solid black lines in each plot correspond to the sensitivity of the collider to "wino"-like DM, an $SU(2)_{weak}\ $ triplet Majorana fermion. The colored dots labeled by different $N$-values correspond to our models in which the DM is a Dirac fermion with multiplicity $N$ and mass chosen to yield the correct abundance from thermal freeze-out.
• Figure 5: The process through which dark gluons maintain equilibrium with the DM and the SM plasma for small $\alpha_d$.