The Cryogenic Dark Matter Search

The Cryogenic Dark Matter Search (CDMS) collaboration reported their first results at the APS meeting this week. They did not find any specific evidence for weakly interacting massive particles (or WIMPs), a finding which is at odds with positive results reported a few years ago by the Dark Matter (DAMA) group in Italy.

Both teams maintain sensitive detectors far underground, the better to filter out extraneous particles from entering their apparatus which operate, in effect, as underground telescopes. As observatories, they don't form images of celestial objects. Their mission is rather more basic: they try to record the very existence of WIMPs which may well be a component of the much sought dark matter, which supposedly lurks unseen in and around and among galaxies.

In CDMS, located 2341 feet deep in the Soudan mine in Minnesota, a target of germanium and silicon is maintained at temperatures close to absolute zero. At masses as high as 100 times the mass of a proton, an intruding WIMP, if it interacted inside the target at all, would engender a characteristic pattern of crystalline vibrations and secondary particles in the semiconductor target material.

At the meeting Harry Nelson (UC-Santa Barbara) said that the CDMS null measurement could be cast in the form of a cross section, which is what particle physicists do when estimating the likelihood of detecting certain kinds of interaction. In this case the CDMS apparatus established a cross section of less than 4 x 10^-43 square centimeters for a 60-GeV-mass WIMP particle to show up in their detector. This level of sensitivity is the best yet for dark matter searches, and is about four times better than another detector group, the EDELWEISS experiment, located near Grenoble, France. (CDMS Webpage)