- The paper introduces a novel model where dark matter is composed of hydrogen-like atoms formed by dark protons and dark electrons under a hidden U(1) gauge force.
- It details the early universe conditions required for dark recombination and atomic binding, impacting cosmic structure formation and suppressing small-scale protohalo growth.
- The model predicts hyperfine splitting-induced inelastic scattering processes that may explain direct detection anomalies reported by experiments such as DAMA versus CDMS and XENON10.
An Expert Overview of "Atomic Dark Matter"
The paper "Atomic Dark Matter" by Kaplan et al. ventures into an intriguing alternative model to explain the nature of dark matter, proposing that it could predominantly consist of atomic bound states akin to hydrogen atoms. This hypothesis diverges from traditional models, such as the Weakly Interacting Massive Particles (WIMPs), by introducing complexity in the dark matter sector that encompasses atomic structures bound by a hidden U(1) gauge symmetry.
Introduction
Dark matter is renowned for comprising more than 80% of the universe's mass, yet its properties and interactions remain largely enigmatic. Traditional hypotheses often simplify dark matter into single particle species with weak-scale mass, like neutralinos in supersymmetry. While successful in cosmological abundance predictions, these models face challenges such as discrepancies in galactic simulations and experimental results from direct detection experiments like DAMA, which detects positive signals inconsistent with null results from other studies like CDMS and XENON10.
Model Proposal
The authors propose a model where dark matter forms atomic bound states, termed "atomic dark matter," comprising dark protons and dark electrons interacting via a hidden U(1) gauge force. This model maps the parameter space for the formation of stable hydrogen-like dark atoms in the early universe. The paper details the conditions under which dark sector recombination and atomic binding occur effectively, exploring the implications of such atomic configurations on both cosmological scales and direct detection experiments.
Cosmological Implications
One remarkable implication of atomic dark matter is its effect on cosmological structure formation. The presence of a residual ionized component in dark matter can lead to prolonged interaction with "dark radiation," which suppresses the formation of small-scale structures, addressing discrepancies in observed galaxy substructures. The potential suppression of protohalo growth below masses of ~10³ - 10⁶ solar masses is achieved through ion-radiation interactions, making this atomic model conducive to observations suggesting reduced small-scale structures in dark matter distributions.
Moreover, the research suggests that dark matter with weak-scale atomic characteristics might reconcile the hyperfine structure splittings (~100 keV), which support the inelastic dark matter interpretation compatible with DAMA's observed signals.
Direct Detection and Constraints
The authors detail how atomic dark matter, specifically through hyperfine level transitions, could lead to inelastic scattering processes observable in direct detection experiments. This relates to the DAMA results, where energy quantization in atomic states could yield the necessary energy scale for observed scattering events. However, this model must still align with constraints from other experiments like CDMS, which necessitate a smaller observational ionized fraction of dark atoms.
Theoretical and Practical Implications
While providing an innovative view on dark matter properties, this hypothesis opens intriguing avenues in the field. It suggests the dark sector could exhibit complex dynamics reminiscent of baryonic matter, such as the potential for molecular formations leading to "dark stars." Enhanced scattering cross-sections could signal new interactions within galaxy clusters or dark matter structures.
Conclusion
This paper sets a foundation for further studies exploring the intricate interplay between atomic dark matter models and existing astrophysical observations. Verifying these models will involve both theoretical work and likely complex simulations, facilitating an evaluation of how these atomic interactions influence our broader cosmological understanding. In parallel, experimental endeavors could advance these theories, allowing for the tuning of parameters and interactions to verify these speculative yet compelling ideas. This research signifies a step towards uncovering potential layers of complexity in the nature of dark matter beyond the classical WIMP paradigm.