More Trouble with DAMA

I haven't blogged about dark matter for almost 2 months, and already there is a pile of long overdue dark news. This post is about a couple of recent unpublished results that mean trouble for theorists trying interpret the DAMA signal.

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:

1. 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).
2. 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.

A few weeks ago, the former possibility was blasted by CRESST - yet another dark matter experiment under Gran Sasso mountain. CRESST uses CaWO4 crystals as the target, and detects scintillation and phonons to discriminate nuclear recoils (expected from dark matter particle) from alpha, beta, and gamma recoils (induced by ubiquitous backgrounds). The presence of tungsten (A = 184) in their crystal makes it very sensitive to the inelastic scenario. But the latest preliminary results presented at the Wonder2010 conference do not show a clear signal. Although CRESST has a handful of (most likely) background events in the signal band, the number of hits is much smaller than that predicted by the inelastic scenario consistent with the DAMA signal. The collaboration claims that the DAMA region is excluded by more than 3 sigma. This should be treated with a grain of sodium chloride as the CRESST data are not yet public and the assumptions that enter the derivation of the limits are not clearly spelled out in the slides. But most likely, the inelastic window is getting closed.

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 ;-)