Dark Matter and Dark Radiation

TL;DR

This paper investigates a dark sector in which an unbroken U(1)_D gauge symmetry (dark electromagnetism) couples only to dark matter, forming a neutral DM plasma of χ and χ̄. The authors quantify cosmological and astrophysical constraints, finding that a minimal χ–γ̂ model cannot simultaneously yield the correct relic abundance and maintain collisionless galactic dynamics, unless DM is heavy or additional annihilation channels are present. They show that incorporating weak interactions (SU(2)_L) allows viable relic densities with μlarger α̂, while keeping galactic dynamics safe; they also discuss the role of dark radiation in structure formation and potential plasma-instability effects, such as Weibel instabilities, which could alter halo assembly. The work suggests that rich phenomenology may reside in the dark sector, including dark atoms and indirect signatures in galactic dynamics, and calls for further simulations to understand collective dark-plasma behavior.

Abstract

We explore the feasibility and astrophysical consequences of a new long-range U(1) gauge field ("dark electromagnetism") that couples only to dark matter, not to the Standard Model. The dark matter consists of an equal number of positive and negative charges under the new force, but annihilations are suppressed if the dark matter mass is sufficiently high and the dark fine-structure constant $\hatα$ is sufficiently small. The correct relic abundance can be obtained if the dark matter also couples to the conventional weak interactions, and we verify that this is consistent with particle-physics constraints. The primary limit on $\hatα$ comes from the demand that the dark matter be effectively collisionless in galactic dynamics, which implies $\hatα\lesssim 10^{-4}$ for TeV-scale dark matter. These values are easily compatible with constraints from structure formation and primordial nucleosynthesis. We raise the prospect of interesting new plasma effects in dark matter dynamics, which remain to be explored.

Figures (6)

• Figure 1: The allowed values of dark $g_{\rm light}$ (those degrees of freedom relativistic at $T_{\rm BBN}$ ) and $g_{\rm heavy}$ (the remaining dark degrees of freedom) arising from BBN constraints Eqs. (\ref{['eq:BBNbound']}) and (\ref{['eq:BBNbound2']}). The allowed regions correspond to 95% confidence levels for $\xi(T_{\rm RH})=1$ and a visible sector $g_{*\rm vis} = 106.75$ (red), $\xi(T_{\rm RH})=1$ and $g_{*\rm vis} =228.75$ (corresponding to MSSM particle content, in blue), and $\xi(T_{\rm RH})=1.4(1.7)$ and $g_{*\rm vis} = 106.75(228.75)$ (in yellow). The minimal dark sector model of this paper is noted by a black star at $g_{\rm light} = 2$ and $g_{\rm heavy} = 3.5$.
• Figure 2: Pair annihilation/creation of dark matter $\chi$ into dark photons $\hat{\gamma}$ via $t$ and $u$-channel exchange diagrams. These processes keep the dark sector in thermal equilibrium until the $\chi$ particles become non-relativistic.
• Figure 3: The allowed regions of $\hat{\alpha}$ vs. $m_\chi$ parameter space. The relic abundance allowed region applies to models in which $U(1)_D$ is the only force coupled to the dark matter; in models where the DM is also weakly interacting, this provides only an upper limit on $\hat{\alpha}$. The thin yellow line is the allowed region from correct relic abundance assuming $\Omega_{\rm DM}h^2 = 0.106\pm 0.08$, $\xi(T_{\rm RH}) = 1$, $g_{*\rm vis} \approx 100$, and $g_{\rm heavy}+g_{\rm light}= 5.5$ while the surrounding blue region is $g_{*\rm vis} = 228.75(60)$, $\xi(T_{\rm RH}) = 1(0.1)$, and $g_{\rm heavy}+g_{\rm light}= 100(5.5)$ at the lower(upper) edge. The diagonal green line is the upper limit on $\hat{\alpha}$ from effects of hard scattering on galactic dynamics; in the red region, even soft scatterings do not appreciably affect the DM dynamics. We consider this to be the allowed region of parameter space.
• Figure 4: Feynman diagrams leading to $\gamma/\hat{\gamma}$ mixing. The vertex in a) can be expanded into that shown in b), as the only particle to which the $\hat{\gamma}$ couples is $\chi/\bar{\chi}$. Since the mass and $SU(2)_L$ charge of these two particles are the same, yet they possess opposite $U(1)_D$ charge, the sum of the $\chi$ and $\bar{\chi}$ diagrams in b) is zero, and the overall mixing vanishes.
• Figure 5: Feynman diagram leading to $\hat{\gamma}$ interactions with SM fermions $f$. The vertex in a) can be expanded into that shown in b), as the only particle with an interaction with $\hat{\gamma}$ is the $\chi/\bar{\chi}$. Since the mass and $SU(2)_L$ charge of these two particles are the same, yet the $U(1)_D$ charges are opposite, the sum of the $\chi$ and $\bar{\chi}$ diagrams in b) is zero, and the overall coupling of $f$ to $\hat{\gamma}$ is therefore zero as well.