The SIMP Miracle (1402.5143v2)

Abstract: We present a new paradigm for achieving thermal relic dark matter. The mechanism arises when a nearly secluded dark sector is thermalized with the Standard Model after reheating. The freezeout process is a number-changing 3->2 annihilation of strongly-interacting-massive-particles (SIMPs) in the dark sector, and points to sub-GeV dark matter. The couplings to the visible sector, necessary for maintaining thermal equilibrium with the Standard Model, imply measurable signals that will allow coverage of a significant part of the parameter space with future indirect- and direct-detection experiments and via direct production of dark matter at colliders. Moreover, 3->2 annihilations typically predict sizable 2->2 self-interactions which naturally address the core vs. cusp' andtoo-big-to-fail' small structure problems.

• The paper introduces a novel 3→2 annihilation mechanism in a dark sector that produces thermal relic dark matter, offering a clear alternative to traditional WIMP models.
• It demonstrates that maintaining thermal equilibrium between the dark and Standard Model sectors is crucial for achieving the correct relic abundance and addressing small-scale cosmic structure issues.
• The study presents a weakly-coupled toy model with a global symmetry that predicts sub-GeV dark matter and outlines potential experimental signals in indirect, direct, and collider searches.

Overview of the SIMP Miracle Paper

The paper entitled "The SIMP Miracle" introduces a novel approach for achieving thermal relic dark matter (DM) through a unique mechanism involving a secluded dark sector that thermalizes with the Standard Model (SM) post-reheating. The primary focus is on the freeze-out process driven by number-changing $3\rightarrow 2$ annihilation of strongly-interacting-massive-particles (SIMPs) in the dark sector. This mechanism indicates the potential presence of sub-GeV dark matter and suggests measurable signals that could be detected via future experiments, both indirect and direct, along with potential direct production at collider facilities.

Key Mechanisms and Theoretical Insights

The SIMP paradigm proposes a shift from the conventional weakly-interacting-massive-particle (WIMP) model, emphasizing that the requisite processes for DM relic creation can be dominated by $3\rightarrow 2$ interactions within a strongly self-interacting dark sector. The DM mass predicted by this mechanism is based on a generalized geometric mean, resulting in a mass scale around 100 MeV to sub-GeV levels, driven by self-interaction strengths denoted as $\alpha_{\text{eff}}$. The paper outlines that effective parameters and couplings established in these interactions can resolve known small-scale structure problems in cosmology, such as the core vs. cusp' andtoo-big-to-fail' problems due to significant $2\to2$ self-interactions.

Methodology and Results

The authors present a detailed solution to Boltzmann equations, verifying the feasibility of the $3\to 2$ mechanism by illustrating a nearly linear relationship between the coupling strength $\alpha_{\rm eff}$ and the mass of the DM. They observe that the freeze-out occurs slightly later than in conventional scenarios but reaches the final relic abundance more quickly due to the characteristics of the $3\to 2$ interactions. The paper emphasizes that maintaining thermal equilibrium between the dark sector and the SM is critical to prevent the dark sector from overheating, thereby setting rigorous conditions on interaction rates and coupling strengths.

In exploring model implementations, the authors introduce a weakly-coupled toy model embodying the SIMP principles, leveraging a global symmetry to stabilize DM and employing minimal interaction couplings with the SM. The model highlights potential experimental signatures as well as constraints arising from collider data and cosmic observations.

Implications and Future Directions

This paradigm offers significant implications for future DM research, particularly in probing the sub-GeV mass region. The approach predicts measurable couplings with the SM sector and significant DM self-interactions, suggesting that the paradigm may have observable consequences in astrophysical phenomena and collider experiments.

The researchers note that exploring concrete realizations of this mechanism and addressing experimental constraints remain crucial tasks. By achieving such a strong link between theoretical predictions and potential experimental validation, the SIMP mechanism stands to refine our understanding of dark matter's nature and interaction characteristics.

Future studies could dive deeper into model-specific scenarios and investigate further experimental routes to validate SIMP predictions, guiding the search for robust DM candidates in the parameter space defined by self-interaction cross-section requirements.

By detailing these novel interactions as a compelling alternative to traditional WIMP-based models, the paper positions the SIMP paradigm as a promising new direction for theoretical and experimental inquiry in the field of dark matter research.