Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Atomic Dark Matter

David E. Kaplan, Gordan Z. Krnjaic, Keith R. Rehermann, Christopher M. Wells

TL;DR

This work proposes that dark matter may be predominantly atomic, formed from a dark proton and dark electron bound by a hidden U(1) force, with a dark asymmetry setting the abundance. By mapping the parameter space, the authors show dark recombination can yield a residual ionization fraction $X_e$ that delays kinetic decoupling and damps small-scale structure, potentially alleviating tensions with observed subhalo populations. They develop a direct-detection framework in which inelastic dark-atom–nucleus scattering, driven by a broken axial U(1) that mixes with the SM, can produce DAMA-like modulated signals while evading other experiments; this requires hyperfine splittings of order $\mathcal{O}(100\,\mathrm{keV})$ and constrains the mediator mass $M_X$ and mixing $\epsilon$. The analysis connects cosmology, structure formation, and direct detection in a cohesive atomic dark matter scenario and discusses avenues for UV completions, potential dark-sector cooling channels, and observational consequences in halos and gamma-ray lines.

Abstract

We propose that dark matter is dominantly comprised of atomic bound states. We build a simple model and map the parameter space that results in the early universe formation of hydrogen-like dark atoms. We find that atomic dark matter has interesting implications for cosmology as well as direct detection: Protohalo formation can be suppressed below $M_{proto} \sim 10^3 - 10^6 M_{\odot}$ for weak scale dark matter due to Ion-Radiation interactions in the dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine splittings of order $100 \kev$, consistent with the inelastic dark matter interpretation of the DAMA data while naturally evading direct detection bounds.

Atomic Dark Matter

TL;DR

This work proposes that dark matter may be predominantly atomic, formed from a dark proton and dark electron bound by a hidden U(1) force, with a dark asymmetry setting the abundance. By mapping the parameter space, the authors show dark recombination can yield a residual ionization fraction that delays kinetic decoupling and damps small-scale structure, potentially alleviating tensions with observed subhalo populations. They develop a direct-detection framework in which inelastic dark-atom–nucleus scattering, driven by a broken axial U(1) that mixes with the SM, can produce DAMA-like modulated signals while evading other experiments; this requires hyperfine splittings of order and constrains the mediator mass and mixing . The analysis connects cosmology, structure formation, and direct detection in a cohesive atomic dark matter scenario and discusses avenues for UV completions, potential dark-sector cooling channels, and observational consequences in halos and gamma-ray lines.

Abstract

We propose that dark matter is dominantly comprised of atomic bound states. We build a simple model and map the parameter space that results in the early universe formation of hydrogen-like dark atoms. We find that atomic dark matter has interesting implications for cosmology as well as direct detection: Protohalo formation can be suppressed below for weak scale dark matter due to Ion-Radiation interactions in the dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine splittings of order , consistent with the inelastic dark matter interpretation of the DAMA data while naturally evading direct detection bounds.

Paper Structure

This paper contains 13 sections, 53 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Cosmology
  3. Dark Recombination and Halo Constraints
  4. Protohalos and Radiation Damping
  5. Direct Detection
  6. Non-relativistic Interaction Hamiltonian
  7. Inelastic Dark Atom - Nucleus Scattering
  8. Modulated Nuclear Recoil Rate
  9. Mixing With The Standard Model:
  10. Sufficient Recombination:
  11. Energy Level Corrections Due to the Axial U(1):
  12. Breaking the Axial U(1):
  13. Discussion

Figures (3)

  • Figure 1: The allowed parameter space in $\alpha_D-m_{\textbf{\scriptsize{e}}}$ for a given $m_{\textbf{\scriptsize{p}}}$ and as a function of the residual ionization fraction, $X_{\textbf{\scriptsize{e}}}$. Atom dark matter is viable in the colored regions, which correspond to $10^{-2}<X_{\textbf{\scriptsize{e}}}<10^{-1}$ (red circles), $10^{-3}<X_{\textbf{\scriptsize{e}}}<10^{-2}$ (blue triangles), $\,10^{-4}<X_{\textbf{\scriptsize{e}}}<10^{-3}$ (magenta boxes) and $X_{\textbf{\scriptsize{e}}}<10^{-4}$ (green stars). The striped region is ruled out by Eq. \ref{['eq:sigmaMbound']} with $\kappa = 3$ and this region extends to the dashed black line for $\kappa = 10$. The black-crossed region is ruled out because $X_{\textbf{\scriptsize{e}}}> 10\%$.
  • Figure 2: Examples of the modulated spectrum at DAMA defined in Eq. (\ref{['eq:dRdER_def']}) complete with the data points from the DAMA and DAMA/LIBRA experiments. The curves correspond to the following choice of parameters from left to right in order of their rise from zero (black, red and green in color order) $m_p = 200, 100, 70~$ GeV ; $m_e \simeq 2.1, 1.7, 1.7~$ GeV ; $f_{eff} \simeq 103, 92, 67~$ GeV. The hyperfine splittings are all about $111~$ keV.
  • Figure 3: The allowed parameter space for $M_X$ and $g_5$ for two values of the dark proton mass. The other atomic parameters -- $\alpha_D$ and $m_{\textbf{\scriptsize{e}}}$ -- have been chosen so that $E_{hf} \simeq 111\,{\rm keV}$ and $X_{\textbf{\scriptsize{e}}} \leq 10^{-4}$. On each plot, the displayed values of $\epsilon^2$ are $10^{-6},\,10^{-5}\,\textrm{and}\,10^{-4}$ from left to right (green, red and blue in color order). To be consistent with constraints discussed in Ref. Pospelov:2008zw, $M_{X}\geq 0.5\,{\rm GeV}$ for $\epsilon^{2} = 10^{-4}$. The allowed points have an average rate, in the 2 to 6 keVee bins at DAMA, between $0.99\times10^{-2}$ and $1.63\times10^{-2}$ cpd/kg/keVee. The excluded regions correspond to choices of parameters which do not satisfy Eq. (\ref{['eq:massspec']}); the solid black line is for $\Lambda = 1\,{\rm TeV}$ and the dashed line is for $\Lambda = 10\,{\rm TeV}$.