Asymmetric Dark Matter: Theories, Signatures, and Constraints

Abstract: We review theories of Asymmetric Dark Matter (ADM), their cosmological implications and detection. While there are many models of ADM in the literature, our review of existing models will center on highlighting the few common features and important mechanisms for generation and transfer of the matter-anti-matter asymmetry between dark and visible sectors. We also survey ADM hidden sectors, the calculation of the relic abundance for ADM, and how the DM asymmetry may be erased at late times through oscillations. We consider cosmological constraints on ADM from the cosmic microwave background, neutron stars, the Sun, and brown and white dwarves. Lastly, we review indirect and direct detection methods for ADM, collider signatures, and constraints.

• The paper demonstrates that ADM models naturally link dark and baryon densities through symmetry transfer and simultaneous generation mechanisms.
• It establishes that ADM yields distinct detection signals, such as nuclear recoils and gamma-ray emissions, setting it apart from traditional WIMP scenarios.
• It highlights astrophysical constraints from systems like neutron stars and white dwarfs, outlining experimental challenges and future prospects for ADM research.

Overview of Asymmetric Dark Matter: Theories, Signatures, and Constraints

The paper under review offers an extensive exploration of Asymmetric Dark Matter (ADM) models, emphasizing their theoretical underpinnings, observational signatures, and cosmological constraints. It serves as a thorough examination of the mechanisms that tie the dark matter (DM) density to the baryon density, a feature not intrinsically addressed by the traditional Weakly Interacting Massive Particle (WIMP) paradigm.

Theoretical Foundations and Mechanisms

ADM theory investigates how a primordial asymmetry between matter and antimatter in the dark sector could mirror the observable asymmetry in baryonic matter. The main proposition is that the abundance similarity between DM and baryons could naturally emerge from shared processes in the early universe. This connection might be established via transfer mechanisms that communicate the asymmetry between sectors, or through shared-generation mechanisms where both asymmetries arise contemporaneously via common dynamics.

Key mechanisms for ADM include:

1. Asymmetry Transfer:
   • Electroweak Sphalerons: These allow for the transfer of asymmetries due to their violation of baryon (B) and lepton (L) numbers.
   • Higher-Dimension Operators: These operators create effective interactions between visible-sector baryons/leptons and dark-sector particles, allowing the alignment of chemical potentials between sectors.
2. Asymmetry Generation:
   • Cogenesis: Entails simultaneous generation of visible and dark sector asymmetries from a unified process. Examples include decay of heavy particles and adaptations of Affleck-Dine mechanisms in supersymmetry.
   • Darkogenesis: Refers to symmetry breaking solely in the dark sector, where dark-sector sphalerons generate an asymmetry independent of visible baryogenesis.

Implications and Observational Signatures

The paper stresses the need to annihilate the symmetric DM component to ensure that the observed relic density is determined by the asymmetry. This is typically achieved via annihilations into lighter force carriers within hidden dark sectors, which often have complex dynamics similar to the visible sector.

ADM models predict observable effects different from traditional WIMP scenarios due to their complex interactions and enhanced connections to the visible sector processes. These include:

• Direct and Indirect Detection: Interactions with baryonic matter via higher-dimension operators or dark forces could lead to detectable signals in direct detection experiments, such as those involving nuclear recoils. Indirect detection could reveal ADM via continuous self-annihilations of any residual symmetric DM components, observable through gamma rays or other decay products.
• Astrophysical Constraints: Neutron stars, white dwarfs, and even the Sun act as cosmic laboratories, where DM accumulation might lead to observable deviations in stellar evolution.

Future Directions and Challenges

The analysis underscores that while ADM provides a compelling framework to resolve some cosmological observations inconsistencies, several challenges remain. These include determining the precise scale and nature of new physics beyond the Standard Model, particularly the detailed behavior of dark sector forces and their mixing with visible sector components. Furthermore, the potential for late-time oscillations between dark matter and anti-dark matter introduces complications in maintaining an asymmetry to the present day.

The review speculates on future advancements and experimental prospects in identifying signals exclusive to ADM through improved detection capabilities and novel astrophysical measurements. These will be critical to confirming the viability of ADM as an alternative to the conventional WIMP paradigm.

The review emphasizes that the rigorous exploration of ADM models is not only crucial for understanding the apparent baryon-DM coincidence but also advances a broader inquiry into the nature of dark matter and its interactions with the visible universe.