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Dark Matter Discovered? Don't Bet on It
9 December 2009 (All day)
Rumors are swirling around the blogosphere that a team of physicists may have finally detected particles of dark matter, the mysterious stuff whose gravity appears to hold galaxies together. If those rumors are true, the discovery would surely be one of the most important of all time. But don't book tickets to Stockholm just yet. Given the same team's previously published negative results and the relatively modest increase in the size of their data set since then, experts say, it's all but certain that the new find is of marginal statistical significance.
The rumors center around the Cryogenic Dark Matter Search (CDMS), a small array of particle detectors lurking in the Soudan mine in Minnesota. The detectors consist of wafers of the semiconductors germanium and silicon and are designed to detect so-called weakly interacting massive particles or WIMPs, which are hypothetical particles thought to make up dark matter. According to posts on physics-related blogs, the CDMS collaboration will report on 18 December that it has detected such particles bumping into the nuclei in the wafers to create distinctive electrical and heat signals. To detect a signal, the physicists must also weed out extraneous "background" events from ordinary particles such as neutrons that occasionally hit the detector.
In January, however, the CDMS collaboration also published results from data taken with the same array from October 2006 to July 2007 and saw no evidence for WIMPS, says Richard Gaitskell, a physicist at Brown University. Since then, he estimates, CDMS has taken about enough data to double its sensitivity. If so, the researchers probably haven't seen a signal so strong that it couldn't have resulted from a few background events. "You'd be lucky to see one event," Gaitskell says. "It's just not enough of an increase in sensitivity to have one of these 'Oh my God!' moments." The researchers might have a shot at incontrovertible sighting of dark matter had they taken enough data to increase their sensitivity by a factor of 10, he says.
Still, researchers say that searching for dark matter in this way makes perfect sense--Gaitskell himself is working on such an experiment--and many hope such efforts will pay off within years. Whatever it might be, the rumored CDMS signal could be a hint of more decisive observations to come. But in all likelihood, it won't by itself be enough convince physicists that dark matter has been spotted.