Thursday, 4 March 2010

Another experiment sees dark matter?

The echoes of the CDMS story have not completely faded away and already there is another one out there. This time it's not a rumor inflated by irresponsible bloggers :-) but a legitimate arXiv publication. Last Friday the CoGeNT collaboration put out a paper where they make - carefully phrased and full of caveats but still - a claim that they might be seeing a signal of dark matter in their data. The story has already been picked up by theorists (the usual speed-of-light constraints do not apply in our field :-) who argue that the CoGeNT signal is marginally consistent with all other dark matter experiments.

CoGeNT is one in the long list of direct detection experiments attempting to observe scattering of dark matter particles off ordinary atoms. The experiment bears some similarity to the more famous CDMS. (Actually, both experiments are located just meters away from each other in the Soudan mine.) It uses germanium as the target and it measures the ionization signal to find out if an interesting scattering event occurred inside the detector. Background discrimination in CDMS is much more sophisticated : any single event surviving their cuts may, with a large probability, be attributed to dark matter. CoGeNT's signal, on the other hand, is contaminated by hundreds of events from mundane backgrounds. For vanilla-flavor WIMP particles, that is ones with masses of 100 GeV or larger, CoGeNT's sensitivity is therefore much worse. However, one important advantage of CoGeNT is that its energy threshold above which a scattering can be observed is much smaller: 0.4 keV compared to 10keV in CDMS. For this reason CoGeNT is more sensitive to light, 1-10 GeV dark matter particles. A light particle, even giving away all its kinetic energy, could not give a germanium atom a kick large enough to be registered by CDMS (recall that that dark matter particles are expected to have small velocities today, $v \sim 10^{-3}$ of the speed of light).

The plot shows the CoGeNT results corresponding to 8 weeks of data-taking after applying their discrimination cuts. There is a lot of background there, including clear lines from radioactive pollution of the detector. But there is also an intriguing feature - an exponential rise at low energies near the threshold. Although this could well be due to a background, for example some other radioactive elements that they have not identified, the exciting thing is that scattering of light dark matter particles would produce a similar shape.

On the next plot, what looks like a geologic section of San Joaquin Valley is in reality a fit of dark matter parameters to CoGeNT's supposed signal. The region favored by CoGeNT is marked by red boundaries. It corresponds to the mass of 7-11 GeV and the cross section on matter of order $10^{-4}$ pb. The violet band is the region consistent with the dark matter interpretation of the 2 CDMS events and it cuts right through the CoGeNT region. The two green regions somewhat away are favored by the DAMA signal with and without including the channeling effect.

Of course, the hypothesis that the background has been underestimated is far more likely at this point. The good news is that, if this is dark matter, it will be confirmed very soon, probably this year. One more reason to bite your fingernails while waiting for the first XENON100 data; the CDMS collaboration may also dig deeper into their data below the 10 keV threshold. More excitement ahead.