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See Inside July 2011

International Particle of Mystery

Dark matter escapes its dragnet once again—or does it?



X-ray: NASA/CXC/M. Markevitch et al.; Optical: NASA/STScI, Magellan/U. Arizona/D. Clowe et al.; Lensing Map: NASA/STScI, ESO WFI, Magellan/U. Arizona/D. Clowe et al.

The ­generic line on dark matter is that nobody knows what it is because nobody has seen it. The former claim remains unassailable—any number of hypothetical particles could be dark matter. As to whether or not anybody has seen it, scientists are as divided as ever, and the discourse among rival dark matter hunters is getting chippy.

The controversy centers on an Italy-based research group that runs DAMA, a particle detector that the researchers have claimed for years is picking up dark matter particles. But the group has been secretive about its data, critics say, and physicists have by and large remained skeptical. Indeed, in April a top experimental collab­oration known as XENON100 reported findings that appeared to rule out the possibility that DAMA’s signal came from dark matter.

At issue is not the data so much as what they mean. If dark matter rings the galaxy as theory predicts, Earth should be orbiting through a sea of dark particles, and DAMA should detect this as the yearlong ebb and flow in the “ambient particle environment.” For more than 10 years now, DAMA has been registering blips that fit this pattern. “I think everyone would agree at this point that they see a signal,” astronomer Mario Livio of the Space Telescope Science Institute in Baltimore said in May at a dark matter symposium. “The question is, What is it?”

DAMA researchers have now found, at last, some preliminary validation of their claim to have seen signs of dark matter. A Minnesota detector called CoGeNT has registered seasonal blips akin to what DAMA has seen, physicist Juan I. Collar of the University of Chicago said at the symposium. He cautioned that the data are preliminary but charged that competitors—including one whose results he derided as “pure, weapons-grade balonium”—have been too quick to dismiss DAMA.

CoGeNT may turn out to be the ally DAMA has long lacked, but Collar maintains that he is not taking sides. “Maybe DAMA’s wrong, maybe they’re right, but we have to remain neutral,” he said. “I find myself caught be­­tween the believers and heathens.” The upshot: the field of dark matter research remains as murky as ever. 

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