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The Cryogenic Dark Matter Search, or CDMS, is a collaboration between several institutions, including Stanford, Brown, Berkeley, CWRU, and Fermilab. Its purpose is to either conclusivly detect or eliminate the possibility of the existence of Weakly Interacting Massive Particles.

WIMPs are thought by some to be the most likely candidate to be the dark matter missing from our observations of the universe. The attributes of such particles are simple: massive enough to make up perhaps 95% of the universe's mass, which would exert the considerable gravitational effects that we have observed for almost 75 years, but tame enough that it would not interact strongly enough with baryonic matter that we would be able to notice it without extreme difficulty. Anything that is high on gravity but low on the other 3 elementary forces that we recognize in the standard model would be a good candidate for dark matter. WIMPs would fit this bill.

Unfortunately, the "weakly interacting" property makes these buggers one hell of a pain in the ass to find. Luckily, if they exist, we expect billions of them to be passing through you right now. That helps. But even so, there are plenty of other subatomic things that are also passing through you right now -- enough to make figuring out which are WIMPs very difficult, if you happen to be a particle detector.

CDMA wants to go about the searching process the hard way. Rather than checking for WIMPs by looking for characteristic variations in their appearance, like DAMA, another well-known group based in Rome (this would be like finding out of there are mosquitoes outside by counting the total number of bugs that are the right size per day, and then seeing if that number changes during mosquito season), the CDMA researchers prefer to look directly for WIMPs (looking at every bug and seeing if it is a mosquito or not).

They propose to do this by using detection materials (made of germanium and silicon) that would generate heat when a WIMP knocked directly into the nucleus of an atom in the device. This only works if the detectors are very cold, in the milli-Kelvin range. That's mere fractions of a degree above absolute zero.

The problem with this scenario is that WIMPs aren't the only things that might cause an event signature, and so all the other possibilities have to be accounted for and/or eliminated. At present, the experiment is running in an old mine in Minnesota. It is located in a clean room at a depth of 2341 feet below the surface, and is shielded by lead and polyethylene.

As difficult as this might be, some feel that CDMS is our best hope in conclusively confirming or eliminating the possibility of WIMPs, and some others, who may be the same people, feel that WIMPs are our best hope of making progress in the search for dark matter. At present, CDMS has put the most stringent limit on the attribues of WIMPs of any other experiment in the world, and the experiment currently running may increase these limits by an order of magnitude.

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