Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Millicharged Atomic Dark Matter

James M. Cline, Zuowei Liu, Wei Xue

TL;DR

This work investigates a minimal atomic dark matter model in which a massless hidden photon $\gamma'$ kinetically mixes with the Standard Model photon, giving dark constituents a millicharge $\epsilon e$. It derives direct-detection cross sections for both elastic scattering (dominant when $m_{\mathbf e} \ll m_{\mathbf p}$) and inelastic hyperfine transitions (dominant when $m_{\mathbf e} = m_{\mathbf p}$), and maps the viable parameter space against cosmological constraints and Xenon100 bounds. The study finds that elastic scattering can yield detectable signals near current limits over a wide mass range, while the equal-mass case can explain CoGeNT via keV-scale hyperfine transitions with $\varepsilon \sim 10^{-2}$ and $\alpha' \sim 0.06$. The model remains consistent with several astrophysical and cosmological bounds due to screening and small ionization fractions, and it offers clear, testable predictions for near-term direct-detection experiments and future cosmological/astrophysical probes.

Abstract

We present a simplified version of the atomic dark matter scenario, in which charged dark constituents are bound into atoms analogous to hydrogen by a massless hidden sector U(1) gauge interaction. Previous studies have assumed that interactions between the dark sector and the standard model are mediated by a second, massive Z' gauge boson, but here we consider the case where only a massless gamma' kinetically mixes with the standard model hypercharge and thereby mediates direct detection. This is therefore the simplest atomic dark matter model that has direct interactions with the standard model, arising from the small electric charge for the dark constituents induced by the kinetic mixing. We map out the parameter space that is consistent with cosmological constraints and direct searches, assuming that some unspecified mechanism creates the asymmetry that gives the right abundance, since the dark matter cannot be a thermal relic in this scenario. In the special case where the dark "electron" and "proton" are degenerate in mass, inelastic hyperfine transitions can explain the CoGeNT excess events. In the more general case, elastic transitions dominate, and can be close to current direct detection limits over a wide range of masses.

Millicharged Atomic Dark Matter

TL;DR

This work investigates a minimal atomic dark matter model in which a massless hidden photon kinetically mixes with the Standard Model photon, giving dark constituents a millicharge . It derives direct-detection cross sections for both elastic scattering (dominant when ) and inelastic hyperfine transitions (dominant when ), and maps the viable parameter space against cosmological constraints and Xenon100 bounds. The study finds that elastic scattering can yield detectable signals near current limits over a wide mass range, while the equal-mass case can explain CoGeNT via keV-scale hyperfine transitions with and . The model remains consistent with several astrophysical and cosmological bounds due to screening and small ionization fractions, and it offers clear, testable predictions for near-term direct-detection experiments and future cosmological/astrophysical probes.

Abstract

We present a simplified version of the atomic dark matter scenario, in which charged dark constituents are bound into atoms analogous to hydrogen by a massless hidden sector U(1) gauge interaction. Previous studies have assumed that interactions between the dark sector and the standard model are mediated by a second, massive Z' gauge boson, but here we consider the case where only a massless gamma' kinetically mixes with the standard model hypercharge and thereby mediates direct detection. This is therefore the simplest atomic dark matter model that has direct interactions with the standard model, arising from the small electric charge for the dark constituents induced by the kinetic mixing. We map out the parameter space that is consistent with cosmological constraints and direct searches, assuming that some unspecified mechanism creates the asymmetry that gives the right abundance, since the dark matter cannot be a thermal relic in this scenario. In the special case where the dark "electron" and "proton" are degenerate in mass, inelastic hyperfine transitions can explain the CoGeNT excess events. In the more general case, elastic transitions dominate, and can be close to current direct detection limits over a wide range of masses.

Paper Structure

This paper contains 6 sections, 11 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Direct detection
  3. $m_{\mathbf e} \ll m_{\mathbf p}$ case
  4. Special case $m_{\mathbf e} = m_{\mathbf p}$
  5. Other Constraints
  6. Conclusions

Figures (2)

  • Figure 1: Diagonal lines: contours of constant $\beta$, eq. (\ref{['betaeq']}), in the $m_{\mathbf H}$-$\sigma_{p,\rm eff}$ plane. Lines are labeled by the value of $\log_{10}\beta$. Background shows limits from the Xenon100 experiment Aprile:2011hi.
  • Figure 2: Lowest curve: values of $\epsilon$ that saturate the Xenon100 direct detection bound, as a function of $m_{\mathbf H}$, for elastic scattering of atoms in the model with $\alpha'=m_{\mathbf e} /m_{\mathbf p} = 0.1$. Upper curves: upper limits on $\epsilon$ from CMB (ref. McDermott:2010pa), capability of ${\mathbf p}$ ions to penetrate 1 km of rock (this work), CoGeNT limit (adapted from McDermott:2010pa) and Xenon100 limit (our estimate) from detection of ${\mathbf p}$ ions. Note that the ion detection limits may not apply (see section 2.1).