Recall that the DAMA experiment has observed a few percent annual modulation of the recoil rate registered by their sodium-iodide crystal detector. This modulation could be due to a change of dark matter flux as the Earth moves around the Sun. However, other dark matter detection experiments (maybe except for CoGeNT) do not observe any signal, which puts strong constraint on the properties of the dark matter particle that could explain all available data. Vanilla-flavor models are by far excluded, however until recently two slightly more involved yet still plausible scenarios appeared marginally allowed:
- Weak scale inelastic dark matter. In this scenario a dark matter particle with mass of order 100 GeV scatters to an excited state with order 100 keV mass splitting. The inelastic scenario favors heavy targets (such as DAMA's iodine, A = 127), and enhances the modulation rate (only dark matter particles from the tail of the velocity distribution can scatter, so that small changes of Earth velocity can significantly change the available phase space).
- Light (5-10 GeV) elastic dark matter. This scenario favors very light targets (such as DAMA's sodium, A = 23) and experiments with low detection thresholds (such as DAMA's 2 keV), as light dark matter particles cannot give a large push to heavier target nuclei.
The explanation of DAMA via a 5-10 GeV dark matter particle is also facing problems. The (marginal) consistency of this scenario with null results from other experiments hinges on the so-called channeling effect in sodium-iodide crystals. Normally, an incoming particle recoiling against the crystal nuclei deposits most of the recoil energy in the form of lattice excitations (not observed by DAMA) while only a small fraction goes into scintillation (observed by DAMA). Channeling refers to the situation when an incoming particle gets caught along the symmetry plane of the crystal undergoing a series of small-angle scatterings and losing most of its energy via scintillation. Since a fraction of less energetic recoils can be detected thanks to channeling, the detection threshold of the experiment is effectively lowered. The effect is especially important for light dark matter because in this case the recoil spectrum is very sharply peaked toward lower energies. The channeling probability reported by DAMA is very large, of order 30 percent in the interesting range of recoil energies, which would greatly increase their sensitivity to light dark matter.
Given its importance you may expect that channeling in sodium-iodide crystals has been carefully studied by the DAMA collaboration. However, DAMA would not be herself if she dwelled on such trivialities. Instead the collaboration estimated the channeling probability using monte carlo simulations based on a theoretical model not applicable for the actual problem. Recently I came across slides from the Snowpac2010 workshop describing an independent attempt to estimate the channeling fraction using more reliable theoretical assumptions. The preliminary results contradict the conclusion of the DAMA collaboration: the channeling probability in sodium-iodine is negligible. If this is right, simple models of light dark matter cannot consistently explain the DAMA oscillation results.
Assuming that both of these preliminary results are true, we are confronted with an embarrassing situation: there is no plausible theoretical interpretation of the DAMA results. What remains on the market are rather exotic models (e.g. resonant dark matter) or Frankenstein models that patch up several non-trivial effects (inelastic+form factor, inelastic+streams, and so on). So theorists need to think harder. At the same time, the need to independently verify the DAMA experimental results becomes even more acute. Maybe a socially sensitive hacker could upload DAMA's raw data on WikiLeaks ;-)