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 $\mathrm{U}(1{)}_S$-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 $\Delta m\lesssim 200\mathrm{keV}$ due to exothermic deexcitation events in detectors.

• Received 27 March 2009

DOI:https://doi.org/10.1103/PhysRevD.79.115019