Direct detection of multicomponent secluded WIMPs

Dark matter candidates comprising several substates separated by a small mass gap Delta 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 weakly interacting massive particle (WIMP) scenario, and calculate the elastic and inelastic cross sections for multicomponent 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 Delta m the WIMP 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 Delta m less than or similar to 200 keV due to exothermic deexcitation events in detectors.