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University physicists at the Soudan Underground Mine in Tower, Minnesota, examine the WIMP catching apparatus.
searching for the dark matter of the universe
By Deane Morrison
Published on December 10, 2004
A big chunk of the universe has gone missing, and physicists are looking for it in northern Minnesota. The missing part is "dark matter," the unseen material that makes up 23 percent of the universe. The total mass of ordinary matter like stars and planets is only four percent. (The other 73 percent or so of the universe is dark energy--the baffling force causing the universe to expand at an accelerating rate, a topic to be covered in a forthcoming UMNnews article.)
The existence of dark matter came to light, so to speak, when astrophysicists observed that there aren't enough stars and other visible matter in galaxies to generate the amount of gravity needed to hold the galaxies together. Something else must be hiding among the stars, invisibly reining them in as they revolve around the centers of their respective galaxies. But what?
Physicists have a quarry in mind: WIMPs, or weakly interacting massive particles. None have yet been detected, but WIMPs produced in the young universe are a major suspect as the component of dark matter.
The search for WIMPs is a multi-institution experiment called CDMS (cryogenic dark matter search), and it's under way in the Soudan Underground Mine near Tower, Minneesota, U of M physics professor Priscilla Cushman, along with postdoc Long Duong and graduate student Angela Reisetter, helped move the search from its previous home in California to a laboratory half a mile underground in the mine. CDMS involves dozens of scientists from 13 institutions and is considered the world's leading experiment in the race to unmask the constituents of dark matter.
It takes a lot of effort to find WIMPs because, true to their proper name, they hardly ever interact with normal matter. Physicists expect them to have about 100 times the mass of a proton (one of the two major particles in the atomic nucleus), but they pass through the densest matter like ghosts moving through walls. In fact, a hundred billion WIMPs may have streamed through your body since you started reading this article. To catch one bumping into a piece of ordinary matter, the CDMS apparatus uses ultralow temperatures to eliminate the "noise" of molecules constantly vibrating all over the place due to heat. The experiment takes place in an apparatus chilled to 0.05 degrees Kelvin, or just five hundredths of a degree above absolute zero.
If WIMPs don't turn up in the coldest place in Minnesota, it's going to be hard to find them anywhere.
To catch a WIMP, the physicists have turned to a quintessential element of northern Minnesota culture: the hockey puck. All right, they're not real hockey pucks; they're just puck-shaped detectors made of either germanium or silicon. If a WIMP should bump the nucleus of a germanium or silicon atom, it will generate a tiny electrical signal, which will be recorded. The pucks are kept in the ultracold center of the CDMS apparatus in stacks of six. So far, five stacks have been placed, and there's room for two more.
The main challenge is to keep extraneous particles from penetrating the apparatus and hitting a nucleus, producing background "noise." The cold keeps ordinary molecular motion in check, and the half mile of rock above (plus 25,000 miles of rock below) keeps out most cosmic rays. The biggest concern is rogue neutrons, which can be generated in the mine. Neutrons interact equally with silicon and germanium, but germanium, with its bigger nucleus, is more sensitive to WIMPs. By comparing signals coming from the silicon and germanium pucks, the physicists can home in on those that may have come from WIMPs.
"To believe we have WIMPs, we would need to see about 10 events [of the particles entering and being detected] over the background [noise] from neutrons..." says Cushman. "Once we have a signal [from the WIMPS], we can begin to understand what the particle's mass is from the energy distribution of its signals."
The experiment has been running for a year, with no WIMPs in sight yet. But that's fine with the physicists. No stray neutrons have sneaked through, so the team is confident that enough background noise is being shut out to let them detect the real McCoy. Also, because physicists want to know how often a WIMP might interact with a given amount of matter, even a long run without any interaction at all tells them something.
U of M physics professor Keith Olive and his colleagues are lending support to the CDMS experiment by applying data from other experiments and from observations of the cosmos. He's excited about the experiment because even if CDMS doesn't discover the particles that make up the dark matter of our universe, it will allow physicists to improve their understanding of a theory called supersymmetry--the notion that every particle of ordinary matter has a larger twin particle. Either way, the experiment will contribute substantially to our knowledge of the universe.