Effect of thallium impurities in the DAMA experiment on the allowed parameter space for inelastic dark matter.

The inelastic dark matter scenario was proposed to reconcile the DAMA annual modulation with null results from other experiments. In this scenario, weakly interacting massive particles (WIMPs) scatter into an excited state, split from the ground state by an energy δ comparable to the available kinetic energy of a galactic WIMP. We note that for large splittings δ the dominant scattering at DAMA can occur off of thallium nuclei, with A∼205, which are present as a dopant at the 10(-3) level in NaI(Tl) crystals. For a WIMP mass mχ≈100 GeV/c2 and δ≈200 keV, we find a region in δ-mχ-parameter space which is consistent with all experiments. These parameters, in particular, can be probed in experiments with thallium in their targets, such as KIMS, but are inaccessible to lighter target experiments. Depending on the tail of the WIMP velocity distribution, a highly modulated signal may or may not appear at CRESST-II.