Nuclear scattering of dark matter coupled to a new light scalar

We consider the nuclear scattering cross section for the exciting dark matter (XDM) model. In XDM, the weakly interacting massive particles (WIMPs) couple to the standard model only via an intermediate light scalar which mixes with the Higgs: this leads to a suppression in the nuclear scattering cross section relative to models in which the WIMPs couple to the Higgs directly. We estimate this suppression factor to be of order 10^-5. The elastic nuclear scattering cross section for XDM can also be computed directly: we perform this computation for XDM coupled to the Higgs sector of the standard model and find a spin-independent cross section in the order of 410^-13 pb in the decoupling limit, which is not within the range of any near-term direct detection experiments. However, if the XDM dark sector is instead coupled to a two-Higgs-doublet model, the spin-independent nuclear scattering cross section can be enhanced by up to 4 orders of magnitude for large tan, which should be observable in the upcoming SuperCDMS and ton-scale xenon experiments.