Astronomy

The Search for Galactic Dark Matter in a Minnesota Mine



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In early 2013, scientists working in a research centre installed in an old mine in Minnesota announced important progress in the ongoing search for one of the most elusive substances in the universe - dark matter. The scientists in question are with the Cryogenic Dark Matter Search (CDMS) project in Soudan Mine.

Dark matter began as the theoretical solution to an important mystery in astrophysics. During the 1930s, astronomers like Jan Oort and Fritz Zwicky realized that galaxies seemed to have much greater gravitational effects than they could explain by estimating the mass of observable stars, gas, and dust. According to Martin White, a professor of astronomy and physics at the University of California, Berkeley, they came up with a hypothetical invisible form of matter, "dark matter," to explain the problem of the "missing mass." More recently, scientists have confirmed that about one-quarter of the universe's mass-energy exists in the form of dark matter. That's more than five times as much as ordinary, visible matter. (The remaining 70 percent is another mystery entirely - something physicists call "dark energy.")

Since dark matter, whatever it is, can't be seen with telescopes, scientists have tried to come up with several ways for detecting its presence indirectly, basically building a case for its existence through circumstantial evidence. The Cryogenic Dark Matter Search (CDMS) at Soudan mine, which is being administered by the University of Minnesota, is taking that research a step further by trying to detect dark matter directly. It's a competitive field - researchers are working toward the same goal at other research sites in the U.S., Canada and Europe. The ones at CDMS are trying to be more specific about what "dark matter" really is. To that end, they've suggested that it takes the form of an exotic particle called a "weakly interacting massive particle" - "WIMP," for short.

To search for WIMPs, the CDMS team uses special germanium and silicon detectors that are cooled almost to absolute zero and can detect almost imperceptible interactions between colliding particles, as well as rule out the ones that have been caused by normal matter. The first stage of CDMS was run in a tunnel at Stanford University. For the second stage, scientists needed to go much deeper underground, where larger, heavier particles like cosmic rays can't penetrate.

For that, they chose the Soudan laboratory. Soudan used to be an iron mine, but the University of Minnesota took over the depths of the mine during the 1980s for research purposes. According to the CDMS team, their detectors, which are the latest experiment approved by the University of Minnesota at the Soudan site, are located on the 27th level of the mine, at a depth of more than 2,300 feet - almost half a mile. Over the past several years, the sensitive detectors seemed to have a few "hits" that might be WIMPs, but upon closer inspection, the scientists weren't confident that the apparent hits weren't from normal matter collisions - the equivalent of picking up white noise on the radio.

But that may have changed. In the spring of 2013, the CDMS team reported that in going over their data they had identified three new "events" which they believe were caused by dark matter WIMPs. According to coverage in Scientific American, MIT physicist Kevin McCarthy, who works on the CDMS team, says that the team is more than 99 percent confident that the hits were caused by WIMPs. The team has already built a new and even more advanced detector, called the SuperCDMS, in the Soudan mine, which it hopes will confirm their hypothesis.

This isn't the end of the mystery of dark matter, though. For one thing, Scientific American says, if the CDMS hits really were caused by WIMPs, then the dark matter particles are much lighter than physicists expected. In addition, the Daily Mail reports that other scientists, like Texas physicist Rupak Mahapatra, say the new data are still too preliminary to be accepted as solid proof that WIMPs are the real solution to the "missing mass" problem that started the search for elusive dark matter in the first place. Mahapatra says it's a "tantalizing hint" but that more research will need to be done before scientists declare the mystery solved.

The new report from the CDMS team, entitled "Dark Matter Search Results Using the Silicon Detectors of CDMS II," can be read on Arxiv.org, an online repository of scientific research papers.

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