Photograph courtesy Reidar Hahn, Fermilab
for National Geographic News
December 18, 2009
Dark matter may have been "felt" for the first time deep in a Minnesota mine, physicists say.
Detectors in the mine, part of the Cryogenic Dark Matter Search experiment, were tripped recently by what might be weakly interacting massive particles, or WIMPs.
WIMPs are among the most popular candidates for dark matter, the invisible material that scientists think makes up more than 80 percent of the mass in the universe.
Recently detectors in the mine recorded two hits with "characteristics consistent with those expected from WIMPs," according to a statement posted on the Cryogenic Dark Matter Search Web site.
There is a one-in-four chance, however, that the particles detected are not dark matter but ordinary subatomic particles such as neutrons, the team cautions. (Related: "Dark Matter Proof Found Over Antarctica?")
Mike Shull, an astrophysicist at the University of Colorado at Boulder, also urged restraint in interpreting the results.
"I regard this as interesting but very much an interim 'progress report' on a promising technique," said Shull, who did not participate in the research.
"I hope they've detected [WIMPs]," he added, "It's exciting if it's true."
WIMPS: Best Dark Matter Candidate?
Scientists have predicted that WIMPs can interact with normal atoms but only weakly and very rarely—hence the name.
When such an interaction happens, a WIMP careens like a billiard ball off an atom, the theory goes. But the collision leaves behind a unique signature in the form of a small amount of heat, which can be detected.
The smashup also creates charged atoms, or ions, that are detectable.
The Cryogenic Dark Matter Search experiment uses 30 detectors made of germanium and silicon crystals.
The detectors were placed a half-mile (0.8 kilometer) underground at the Soudan mine, a defunct iron mine in northern Minnesota. The deep location helps block "background noise" from other particles, such as solar and cosmic rays.
Aside from the online statement, details on the new detections have yet to be published, and members of the Cryogenic Dark Matter Search team are declining comment.
But University of Chicago theorist Craig Hogan said that while the new detections are exciting, "it's not time for the champagne bottles yet."
The real significance of the apparent find would be that it could be used to help shape future dark matter detectors, which will be more sensitive and can better rule out false hits, Hogan said.
"It's not a discovery yet, but if these detections are real, we can turn it into a discovery."
Dark Matter Origins
If WIMP detections are confirmed by other experiments, then scientists will likely want to know where the particles are coming from, the University of Colorado's Shull added.
That's because the origins of dark matter particles passing Earth could help solve other cosmic mysteries.
Some theories of galaxy formation, for instance, say that our Milky Way and other "adult" galaxies are enveloped by halos of dark matter that are densest in the galactic centers.
If this is correct, Shull said, then dark matter particles would be expected to originate from the center of the Milky Way more often than from other regions of space.
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