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A dark debate

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Physicists are embroiled in a verbal slugfest over a few measly WIMPs.

WIMPs, or weakly interacting massive particles, are hypothetical subatomic particles that, if shown to exist, might account for some of the invisible dark matter that astronomers say makes up some 85 percent of the mass of the universe.

Astronomers are eager to find dark matter, because it would help them understand the unseen gravitational glue that keeps galaxies and galaxy clusters from flying apart. And a WIMP version of dark matter in particular would thrill many physicists, because it would validate a theory called supersymmetry that plugs a number of holes in present-day physics by pairing every known elementary particle with an as-yet-undetected, heavier partner.

Two recent announcements suggest the WIMPs may be at hand. In March, after analyzing an experiment that began operating in December deep in a northern Minnesota mine, physicists reported that they may have found WIMPs streaming in from space, slightly jostling a hockey-puck-sized germanium detector. The candidate WIMPs recorded by the experiment, called COGENT, are consistent with signals reported last year by a much larger germanium experiment housed in the same mine. Researchers on that experiment, known as CDMSII, reported tentative hints last year that two of the signals recorded by their detectors might be due to WIMPs.

But not so fast. A third team, which has used a whopping 40 kilograms of liquid xenon to search for WIMPs at another underground location, Italy's Gran Sasso National Laboratory near Rome, has provided a rude wake-up call. Analyzing the first 11 days of data from the experiment, known as XENON100, the team found no telltale scintillations in the xenon that they could attribute to dark-matter particles. Elena Aprile of Columbia University in New York City and her colleagues say the initial data from XENON100 cast doubt on the results reported by the COGENT team.

Those have turned out to be fighting words. In a sharply worded rebuttal posted online May 5, two members of the COGENT team, Juan Collar of the University of Chicago and Dan McKinsey of Yale University, called the XENON100 analysis flawed and “untenable,” and suggested that their colleagues ought to retract their paper. Collar and McKinsey assert that liquid xenon is not sensitive enough to record signals of low-mass WIMPs like the ones their team reported.

“To put it bluntly, this is the equivalent of expecting something out of nothing,” Collar and McKinsey noted in regard to the XENON100 findings.

Those remarks sound like sour grapes, and some of the criticism “could not be more wrong,” Aprile says. “Xenon remains a very unique target for particle dark matter searches,” she asserts. “I think these people, like most of the community, should be just very pleased to see the progress … to elucidate such an important question as that of the dark matter nature.” 

Aprile’s team already has already collected ten times the amount of data analyzed in their initial report. The new data, which the researchers plan to unveil in a month or two, may be “the best answer to their criticism,” Aprile says. But first, she notes, her team will post its own response to the COGENT team’s harsh rebuttal.

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