Abstract
We comprehensively study experimental constraints and prospects for a class of minimal hidden sector dark matter (DM) models, highlighting how the cosmological history of these models informs the experimental signals. We study simple ‘secluded’ models, where the DM freezes out into unstable dark mediator states, and consider the minimal cosmic history of this dark sector, where coupling of the dark mediator to the SM was sufficient to keep the two sectors in thermal equilibrium at early times. In the well-motivated case where the dark mediators couple to the Standard Model (SM) via renormalizable interactions, the requirement of thermal equilibrium provides a minimal, UV-insensitive, and predictive cosmology for hidden sector dark matter. We call DM that freezes out of a dark radiation bath in thermal equilibrium with the SM a WIMP next door, and demonstrate that the parameter space for such WIMPs next door is sharply defined, bounded, and in large part potentially accessible. This parameter space, and the corresponding signals, depend on the leading interaction between the SM and the dark mediator; we establish it for both Higgs and vector portal interactions. In particular, there is a cosmological lower bound on the portal coupling strength necessary to thermalize the two sectors in the early universe. We determine this thermalization floor as a function of equilibration temperature for the first time. We demonstrate that direct detection experiments are currently probing this cosmological lower bound in some regions of parameter space, while indirect detection signals and terrestrial searches for the mediator cut further into the viable parameter space. We present regions of interest for both direct detection and dark mediator searches, including motivated parameter space for the direct detection of sub-GeV DM.
Highlights
The cosmic abundance of WIMPs is directly determined by their coupling to the Standard Model (SM), and this class of models makes sharp predictions for signals accessible to a variety of experiments
We have comprehensively assessed the current constraints on and discovery prospects for a class of minimal hidden sector freezeout models, where the dark matter (DM) relic abundance is set by the freezeout of a single DM species, χ, into a dark mediator, φ
We consider the well-motivated scenario where the leading interaction between the dark sector and the SM is renormalizable, for two simple reference models, HSFO-VP, where the mediator is a dark photon kinetically mixed with SM hypercharge, and HSFO-HP, where the mediator is a dark scalar that mixes with the SM Higgs boson
Summary
DM is part of a larger dark sector that is thermally populated in the early universe. The thermalization floor is insensitive to the mediator mass as long as 2 ↔ 2 rates dominate the scattering, a condition that holds generically (but not always) when the mediator is relativistic at the time of freezeout. Both our minimal models can be described by four independent parameters, namely the DM mass, the mediator mass, the portal coupling , and the coupling αD between DM and the mediator. For DM freezing out via annihilations to massive dark photons, the upper bound
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