Semi-annihilation of Dark Matter

Abstract: We show that the thermal relic abundance of dark matter can be affected by a new type of reaction: semi-annihilation. Semi-annihilation takes the schematic form X_i X_j -> X_k phi, where X_i are stable dark matter particles and phi is an unstable state. Such reactions are generically present when dark matter is composed of more than one species with "flavor" and/or "baryon" symmetries. We give a complete set of coupled Boltzmann equations in the presence of semi-annihilations, and study two toy models featuring this process. Semi-annihilation leads to non-trivial dark matter dynamics in the early universe, often dominating over ordinary annihilation in determining the relic abundance. This process also has important implications for indirect detection experiments, by enriching the final state spectrum from dark matter (semi-)annihilation in the Milky Way.

• The paper introduces semi-annihilation as a novel dark matter process that modifies thermal relic density and challenges conventional models.
• It employs coupled Boltzmann equations and toy models to quantitatively assess the impact of semi-annihilation compared to standard annihilation.
• The study highlights indirect detection implications by predicting enriched cosmic particle spectra, offering new avenues for experimental searches.

Semi-annihilation of Dark Matter: Implications and Calculations

The study in the paper titled "Semi-annihilation of Dark Matter" reveals a novel interaction that fundamentally alters the thermal relic density of dark matter, which challenges and complements existing models in particle physics. The concept of semi-annihilation as outlined in the paper is a significant area of interest due to its deviation from traditional annihilation scenarios and its implications for our understanding of dark matter dynamics in the universe.

Introduction to Semi-annihilation

Semi-annihilation is a process where two stable dark matter particles interact to produce a third dark matter particle and a secondary state. This contrasts with traditional annihilation where dark matter particles completely transform into standard model (SM) particles. The reaction form discussed is $\psi_i \psi_j \rightarrow \psi_k \phi$, where $\psi_i$ and $\psi_j$ are stable dark matter particles, $\psi_k$ is another dark matter particle, and $\phi$ is an unstable particle that eventually decays to SM states. This interaction is found in scenarios where dark matter includes multiple species with flavor or baryon symmetries, expanding the dynamics that need to be considered.

Coupled Boltzmann Equations and Toy Models

The paper provides a thorough exploration of semi-annihilation processes by developing a complete set of coupled Boltzmann equations to account for these interactions, supplementing conventional annihilation equations. Two toy models were investigated to observe the impact of semi-annihilation on dark matter dynamics, demonstrating that semi-annihilation can surpass ordinary annihilation in affecting relic abundance for certain parameters. The semi-annihilation reactions necessitate considering additional conservation and symmetry principles, such as larger stabilizing symmetries of $Z_3$ or multi-flavor/baryon symmetries found in supersymmetric models.

Implications and Detection

This study suggests notable implications for dark matter detection, particularly for indirect detection methods. Semi-annihilation enriches the final spectra of detectable particles from dark matter interactions, such as those observable in the Milky Way, influencing predictions for experiments seeking indirect evidence of dark matter through cosmic anomalies. Although it does not contribute new channels for direct detection, the enriched spectra from semi-annihilation provide additional data points which could refine experimental analyses and add complexity to indirect detection methods.

Future Directions

The study invites further exploration of multi-component dark matter models where semi-annihilation processes could provide a fourth type of exception in predicting relic abundance alongside established phenomena like co-annihilation. The unique characteristics of semi-annihilation merit expanded research into dark matter models with higher symmetries and complex particle interactions, potentially unveiling undiscovered facets of dark matter physics.

In conclusion, by introducing semi-annihilation into the space of potential dark matter interactions, the paper broadens the theoretical landscape and invites reconsideration of dark matter dynamics and detection strategies. Researchers can look forward to further investigations into the implications of semi-annihilation for particle cosmology and the quest for a comprehensive understanding of dark matter.