Hidden Charged Dark Matter

TL;DR

The paper examines dark matter that is SM-neutral but charged under an exact hidden U(1) gauge symmetry, within a WIMPless framework. It analyzes the hidden sector's particle content, relic density, kinetic decoupling, and the impact on the matter power spectrum, along with non-perturbative effects such as bound-state formation and Sommerfeld-enhanced annihilation, across both early-universe and non-linear structure scales. The study finds that hidden charged DM with masses from 1 GeV to 10 TeV can naturally reproduce the observed relic density and remains compatible with astrophysical constraints, while predicting distinctive small-scale structure features like cores in dwarf halos and velocity-dependent self-interactions. The work highlights that, although current observations do not distinguish this scenario from neutral DM in many aspects, the model yields testable predictions for halo morphologies and protohalo annihilation, and could be further probed by introducing connector sectors linking the hidden and visible sectors.

Abstract

Can dark matter be stabilized by charge conservation, just as the electron is in the standard model? We examine the possibility that dark matter is hidden, that is, neutral under all standard model gauge interactions, but charged under an exact U(1) gauge symmetry of the hidden sector. Such candidates are predicted in WIMPless models, supersymmetric models in which hidden dark matter has the desired thermal relic density for a wide range of masses. Hidden charged dark matter has many novel properties not shared by neutral dark matter: (1) bound state formation and Sommerfeld-enhanced annihilation after chemical freeze out may reduce its relic density, (2) similar effects greatly enhance dark matter annihilation in protohalos at redshifts of z ~ 30, (3) Compton scattering off hidden photons delays kinetic decoupling, suppressing small scale structure, and (4) Rutherford scattering makes such dark matter self-interacting and collisional, potentially impacting properties of the Bullet Cluster and the observed morphology of galactic halos. We analyze all of these effects in a WIMPless model in which the hidden sector is a simplified version of the minimal supersymmetric standard model and the dark matter is a hidden sector stau. We find that charged hidden dark matter is viable and consistent with the correct relic density for reasonable model parameters and dark matter masses in the range 1 GeV < m_X < 10 TeV. At the same time, in the preferred range of parameters, this model predicts cores in the dark matter halos of small galaxies and other halo properties that may be within the reach of future observations. These models therefore provide a viable and well-motivated framework for collisional dark matter with Sommerfeld enhancement, with novel implications for astrophysics and dark matter searches.

Figures (6)

• Figure 1: Allowed regions in $(m_X, \alpha_X)$ plane, where $m_X$ is the mass of the dark matter charged under the unbroken hidden sector $\text{U(1)}_{\text{EM}}$ with fine-structure constant $\alpha_X$. Contours for fixed dark matter cosmological relic density consistent with WMAP results, $\Omega_X h^2 = 0.11$, are shown for $(\tan\theta_W^h, \xi_{\text{RH}}) = (\sqrt{3/5}, 0.8)$, $(\sqrt{3/5},0.1)$, $(10, 0.1)$ (dashed), from top to bottom, as indicated. The shaded regions are disfavored by constraints from the Bullet Cluster observations on self-interactions (dark red) and the observed ellipticity of galactic dark matter halos (light yellow). The Bullet Cluster and ellipticity constraints are derived in Secs. \ref{['sec:bullet']} and \ref{['sec:halo']}, respectively.
• Figure 2: Kinetic decoupling temperatures as functions of dark matter mass $m_X$ for $(\tan\theta_W^h, \xi_{\text{RH}}) = (10, 0.1)$, $(\sqrt{3/5},0.1)$, $(\sqrt{3/5}, 0.8)$, from top to bottom, as indicated. For each case we plot both the hidden sector photon temperature $T^h_{\text{kd}}$ (dashed) and the corresponding visible sector photon (CMB) temperature $T_{\text{kd}}$ (solid) at the time of kinetic decoupling.
• Figure 3: The normalized amplitudes of dark matter fluctuation for different modes with comoving wavenumbers $x_d=0.01,7.5,75,16$ as functions of $x=k\eta$, where $\eta$ is the conformal time. We fix $(\tan\theta_W^h, \xi_{\text{RH}}) = (\sqrt{3/5}, 0.8)$ for this plot.
• Figure 4: Transfer functions of the normalized dark matter density perturbation amplitude for $(\tan\theta_W^h, \xi_{\text{RH}}) = (\sqrt{3/5}, 0.8)$ (top), $(\sqrt{3/5},0.1)$ (middle), and $(10, 0.1)$ (bottom).
• Figure 5: Mass of the smallest virialized dark matter structure that can form, as a function of the dark matter mass $m_X$ for $(\tan\theta_W^h, \xi_{\text{RH}}) = (\sqrt{3/5}, 0.8), (\sqrt{3/5},0.1), (10, 0.1)$, as indicated.