Secluded WIMP Dark Matter

TL;DR

The paper proposes a generic mechanism in which thermal relic WIMPs decouple from the SM by annihilating to metastable mediators that decay back to SM states with delayed lifetimes, allowing the WIMP sector to be highly secluded from direct detection while preserving potentially observable indirect γ-ray signals. It develops explicit realizations with a $U(1)'$ vector mediator, a Higgs-portal singlet scalar, a right-handed neutrino mediator, and a strong hidden-sector, showing how seclusion is achieved when $m_{ m WIMP} > m_{ m mediator}$ and how relic abundance fixes mediator couplings; it also analyzes MeV-scale variants, highlighting tension with gamma-ray and flavor constraints but identifying viable sub-classes, particularly with kinetic mixing and Higgs-portal mediators. The work demonstrates the complementarity of direct and indirect searches: secluded WIMPs evade underground detectors and collider missing-energy signals, yet can yield observable γ-ray signatures from annihilation, making indirect searches and flavor/astrophysical constraints crucial for testing these models. It also motivates MeV-scale scenarios as potential explanations for the 511 keV line, while noting challenges from SN1987A and related constraints. Overall, the framework broadens the landscape of WIMP dark matter and clarifies the roles of different experimental probes in constraining or discovering secluded dark matter.

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 γ-ray signals. We also point out that γ-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.

Figures (3)

• Figure 1: The secluded WIMP dark matter scenario.
• Figure 2: WIMP annihilation for: (A) $m_{\psi} < m_V$ on the left; and (B) $m_{\psi} > m_V$ on the right -- the secluded regime in which the annihilation may proceed via two metastable on-shell $V$'s, which ultimately decay to SM states.
• Figure 3: (A) On the left, the parameter space of the $U(1)'$-mediated model. The elastic cross section constraint excludes a large portion of the $m_\psi-\beta$ parameter space. The upper-left corner is also excluded due to the invisible decay of $Z$ into WIMPs. The two horizontal lines correspond to the collider limits on four-fermion interactions with $\alpha'=1$ and $\alpha'=\alpha$. (B) On the right, the parameter space of the singlet-Higgs mediated model. There are no collider constraints, and the $\sigma_{el}$ constraint is much weaker.