Secluded WIMP Dark Matter (0711.4866v1)

Abstract: We consider a generic mechanism via which thermal relic WIMP dark matter may be decoupled from the Standard Model, namely through a combination of WIMP annihilation to metastable mediators with subsequent delayed decay to Standard Model states. We illustrate this with explicit examples of WIMPs connected to the Standard Model by metastable bosons or fermions. In all models, provided the WIMP mass is greater than that of the mediator, it can be secluded from the Standard Model with an extremely small elastic scattering cross-section on nuclei and rate for direct collider production. In contrast, indirect signatures from WIMP annihilation are consistent with a weak scale cross-section and provide potentially observable \gamma-ray signals. We also point out that \gamma-ray constraints and flavor physics impose severe restrictions on MeV-scale variants of secluded models, and identify limited classes that pass all the observational constraints.

• The paper introduces a novel secluded framework where WIMPs annihilate into metastable mediators that decay into observable Standard Model particles.
• It employs models with U(1)' vector, scalar, and right-handed neutrino mediators to demonstrate suppressed direct detection alongside robust indirect signals.
• It highlights that indirect detection methods, such as gamma-ray observations, offer promising strategies for probing dark matter beyond traditional searches.

An Analysis of "Secluded WIMP Dark Matter"

The paper "Secluded WIMP Dark Matter" by Maxim Pospelov, Adam Ritz, and Mikhail Voloshin presents a detailed paper on a novel mechanism through which thermal relic Weakly Interacting Massive Particles (WIMPs) can be isolated from the Standard Model (SM), while still manifesting observable indirect effects. It is structured around the concept of secluded WIMPs, driven by the annihilation of WIMPs into metastable mediators that subsequently decay into SM states. This model introduces a more nuanced picture of WIMP interactions compared to typical WIMP paradigms.

The authors propose that WIMPs can have extremely suppressed interactions with nuclei while maintaining a standard weak-scale annihilation cross-section, primarily owing to their interaction through off-shell intermediate mediators. This is contrary to traditional models where WIMP interactions with ordinary matter are more directly correlated with their annihilation processes. The secluded approach suggests that indirect detection methods, such as observations of gamma-ray emissions, could provide more sensitive means for detection compared to direct production or nucleus scattering methods.

Key Model Structures

1. U(1)$'$ Vector Mediator Model: This construct introduces a new U(1)$'$ gauge boson, $V$, that kinetically mixes with the SM hypercharge field. The analysis reveals that certain portions of parameter space allow WIMPs to be substantially hidden from direct detection, only constrained by indirect signals like gamma rays produced from galactic center annihilations. The framework predicts differing levels of visible interactions based on the mass relations between WIMPs and their mediators, emphasizing the role of kinematic conditions.
2. Scalar Mediator Models: Here, a singlet scalar field $\phi$ mediates between WIMPs and the SM. The seclusion is achieved by allowing WIMPs to interact and annihilate predominantly into these scalar fields, which subsequently decay into SM particles. The paper discusses the implications for model constraints based on theoretical constructs like Higgs mixing, impacting observables such as the invisible decay width of the $Z$ boson.
3. Right-Handed Neutrino Mediator: Utilizing right-handed neutrinos as mediators provides a theoretically appealing vector given their presence in several extensions of the SM. This option presents a naturally small coupling strength to the SM neutrinos, avoiding several stringent experimental limits while secluding WIMPs from direct detection.
4. Strong Non-Abelian Interactions: A composite model with non-Abelian dynamics in the secluded sector presents an intriguing scenario. This approach draws analogies with QCD-like dynamics and suggests a phenomenology where WIMP units could be composite states. This model underscores potential strong self-interactions absent in other paradigms.

Implications and Constraints

The models compel a reconsideration of existing detection methods, where typical nuclear interaction cross-sections might be much smaller than anticipated. Indirect detection, such as through gamma-ray observations, remains a viable channel. This seclusion mechanism posits implications not just for collider phenomenology but also for cosmological signatures, potentially altering parameters used in dark matter searches, like freeze-out calculations.

The paper emphasizes constraints arising from cosmic and astrophysical observations. For instance, the contribution of secluded WIMPs must comply with gamma-ray spectral constraints and established dark matter mass density. This balancing act demands careful fine-tuning or consideration of additional dark matter components, particularly for models suggesting large hierarchies in mediator coupling constants.

Application to MeV-scale Dark Matter

The paper also explores the intersection between secluded models and MeV-scale dark matter motivated by the 511 keV gamma-ray line observed from the galactic center. It is intriguing that this approach can naturally suppress supernova energetics issues tied to neutrino interactions, a significant problem in other light dark matter frameworks involving SM neutrino couplings.

Summary and Future Developments

"Secluded WIMP Dark Matter" suggests a paradigm shift by supporting the concept of suppressed direct interactions without compromising on observable indirect signals. This paves the way for potentially unexplored strategies in dark matter detection, both through future terrestrial experiments and astrophysical surveys. Future developments may explore the phenomenological implications of these models at next-generation observatories or revise cosmological models taking into account the novel interaction dynamics of secluded WIMPs.