Until yesterday CRESST was a sort of a legend: everybody heard of them but nobody ever saw them. That is to say, the CRESST excess has been informally discussed for a long time. Moreover, the events from one of the modules displaying an excess of events in the oxygen band have been shown at conferences since more than a year. However we were in the dark about the significance of the excess, backgrounds and systematic effects. Now CRESST has finally come out with a paper that spells out the excess and provides interesting details.
CRESST is in a way the fanciest of all dark matter experiments. Located in the Gran Sasso underground laboratory, it uses the target of CaWO4 crystals cooled down to 10 miliKelvins. When a particle scatters inside the target the deposited energy is converted into phonons and scintillation light, both of which can be detected. The light-to-phonon ratio helps discriminating the dark matter signal from backgrounds, for example electrons and photons produce mostly light. Furthermore, that ratio depends on the atom of the crystal molecule on which the scattering occurred: it is largest for oxygen, intermediate for calcium, and smallest for tungsten. This leads to characteristic bands in the light yield vs. recoil energy plane that you can see in the plot above showing events from one of the eight CRESST modules used in this analysis. These bands provide another handle on the signal, as heavy dark matter would show up mostly via scattering on tungsten, while the light one would pop up in the oxygen band.
At the same time CRESST is paying a price for their innovative technology, as they have to deal with incalculable and sometimes unexpected backgrounds. Apart from the usual neutron background and the leakage of e/γ events into the signal region they had to face α particles and Pb atoms emitted from the clamps holding the crystals, not to mention the exhaust fumes from the nearby DAMA detector. Some of these backgrounds will be reduced in future runs, but for the moment CRESST needs to estimate their contribution in the signal region using sideband analysis. Having done so, CRESST finds that a fraction (slightly less than a half) of the 67 events in the signal region cannot be understood in terms of the known backgrounds. Therefore they study the likelihood of the background plus dark matter signal hypothesis assuming vanilla elastic scattering of dark matter on the target. Here is their result for the preferred mass and cross section of the dark matter particle:
The likelihood function has 2 minima corresponding to 4.7 and 4.2 sigma rejection of the background-only hypothesis. We can safely forget about the deeper one: for these parameters Xenon100, CDMS and Edelweiss would see an elephant in their data. The shallower minimum, where the preferred dark matter mass is 9-15 GeV, also seems excluded by orders of magnitude. This one however lies in the tantalizing proximity to the CoGeNT and DAMA preferred region; actually the mass region (though not the cross section) perfectly agrees with the DAMA low-mass region. Some argue that CDMS and Xenon collaboration grossly overestimate their sensitivity near the threshold. This may be imagined in the case of 5-7 GeV dark matter, in which case combining experimental and astrophysical uncertainties with some good will and the presumption of innocence one can try to argue that the CoGeNT signal is marginally consistent with the Xenon and CDMS exclusion limits. On the other hand, 10 GeV dark matter would produce observable signals further away from the threshold of these 2 experiments, and it's unlikely it could escape their attention. Therefore, given CRESST is facing pesky backgrounds very similar to the suspected signal (both in spectral shape and the order of magnitude), the hypothesis of unknown and/or underestimated backgrounds faking the signal is currently the most probable one.
Summarizing, the new CRESST results are welcome and illuminating but they do not change significantly the landscape of dark matter searches. Clearly, experiment is closing in on IDM; what is not clear is whether that stands for Inelastic or Italian Dark Matter ;-)
See also Lubos, Matt, and again Matt.