Astrophysical Signatures of Secluded Dark Matter

Abstract

We analyze the indirect astrophysical signatures of secluded models of WIMP dark matter, characterized by a weak-scale rate for annihilation into light MeV-scale mediators which are metastable to decay into Standard Model states. Such scenarios allow a significant enhancement of the annihilation cross section in the galactic halo relative to its value at freeze-out, particularly when the mediator is light enough for this process to proceed through radiative capture to a metastable `WIMP-onium' bound state. For MeV-scale vector mediators charged under a hidden U(1)' gauge group, the enhanced annihilation rate leads predominantly to a sizable excess positron flux, even in the absence of astrophysical boost factors.

Figures (2)

• Figure 1: WIMP annihilation: on the left, the direct decay to two metastable on-shell $V$'s, which ultimately decay to SM states; and on the right, the decay through the formation of a metastable (para) WIMP-onium state, which occurs with an enhanced rate for non-relativistic WIMPs in the galactic halo.
• Figure 2: The $m_{\psi(\phi)}$-$\alpha'$ parameter space for fermionic (left) and bosonic (right) secluded WIMPs, showing the cosmological abundance constraint (solid line), the region where radiative recombination into WIMP-onium is allowed (to the right of the dashed line), and a grey band where, limited by two extreme models of $e^+$ propagation, the positron excess would be above background for energies above 10 GeV.