  Dark matter candidates comprising several sub-states separated by a small
mass gap, and coupled to the Standard Model by (sub-)GeV force carriers, can
exhibit non-trivial scattering interactions in direct detection experiments. We
analyze the secluded U(1)-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 splittings 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
mass splittings less than 200 keV due to exothermic de-excitation events in
detectors.
