Direct Detection of Multi-component Secluded WIMPs

Dark matter candidates comprising several sub-states separated by a small mass gap ∆m, and coupled to the Standard Model by (sub-)GeV force carriers, can exhibit nontrivial scattering interactions in direct detection experiments. We analyze the secluded U(1)S-mediated WIMP scenario, and calculate the elastic and inelastic cross sections for multi-component WIMP scattering off nuclei. We find that second-order elastic scattering, mediated by virtual excited states, provides strong sensitivity to the parameters of the model for a wide range of mass splittings, while for small ∆m theWIMP excited states have lifetimes exceeding the age of the universe, and generically have a fractional relative abundance above 0.1%. This generates even stronger constraints for ∆m <∼ 200 keV due to exothermic deexcitation events in detectors.