New contribution to scattering of weakly interacting massive particles on nuclei

A weakly interacting massive particle (WIMP) is perhaps the most promising candidate for the dark matter in the Galactic halo. The WIMP detection rate in laboratory searches is fixed by the cross section for elastic WIMP-nucleus scattering. Here we calculate the contribution to this cross section from two-nucleon currents from pion exchange in the nucleus and show that it may, in some cases, be comparable to the one-nucleon current that has been considered in prior work and perhaps help resolve the discrepancies between the various direct dark-matter search experiments. We provide simple expressions that allow these new contributions to be included in current calculations.