Early Results from Large Dark Matter Detector Cast Doubt on Earlier Claims

The XENON100 detector in Italy should have been able to confirm previous dark matter signals
Xenon 100 dark matter experiment

Courtesy XENON100 Collaboration

An experiment looking for the signal of dark matter deep in an underground lab in Italy turned up no candidate signals in 11 days of early operation, the experimental collaboration reported in a paper posted online Monday. The underground detector, called XENON100, only recently began taking data but is already challenging prior claims and hints of dark matter signals, according to the team, which published its findings on the physics preprint repository

XENON100 is one of a number of cryogenic underground detectors designed to sense subtle recoil effects expected to be induced by the rare collision between passing dark matter particles and the atoms of the detector material (liquid xenon, in this case). Dark matter is the stuff that has long been invoked to explain the disparity between how things look and how they behave at the largest scales of the universe—galaxies within clusters, for instance, move as if they contain far more mass than they appear to have. A wealth of evidence supports the notion that atoms and molecules represent only a fraction of the gravitationally interacting mass in the universe.

Direct detection of dark matter particles, however, has proved elusive—and contentious. Researchers working on an experiment known as DAMA (short for dark matter), which occupies the same subterranean Italian laboratory as XENON100, have for years claimed that they have identified dark matter by spotting annual fluctuations as Earth moves through its orbit—and, presumably, through the halo of dark matter particles enveloping the Milky Way.

With its high sensitivity, XENON100 should have seen something if the DAMA interpretation were correct, the experimental collaboration contends. Brown University physicist Richard Gaitskell, who works on the larger xenon-based dark matter detector LUX (Large Underground Xenon), which he expects will be installed underground in South Dakota next year, says the DAMA results have not been borne out by other searches. "It has been very difficult to obtain any results which support the interpretation of the DAMA experiment as particle dark matter," Gaitskell says.

Massachusetts Institute of Technology physicist Peter Fisher says that procedural questions have also clouded the claimed detection. "The DAMA collaboration has been, to say the least, difficult to deal with," Fisher says. "People have asked many questions about their apparatus and data quality checks, and they are not forthcoming. This has led people, to some extent, to discount their results."*

Late last year another research group in the chase announced an interesting signal as well. Physicists working on CDMS, an underground detector in Minnesota, said in December that they had registered two potential hits from dark matter. At the time they took care to note that the "results cannot be interpreted as significant evidence" of such interactions and that either or both could have been background noise. "The fact that they saw two events is very likely a simple statistical fluctuation in that background," Gaitskell says. "This is the most likely explanation."

The dark matter hunt is still wide open—and, as Fisher predicted last month in a public lecture, "the person who finds it is going to get a quick trip to Stockholm," where the Nobel prizes are awarded.

Gaitskell says that the early XENON100 data bode well for the new generation of large detectors based on the inert liquid, including his own project that is now nearing realization. "In just 11 days of exposure they have been able to match the sensitivity of CDMS, which needed in excess of 12 months of data for the same dark matter search sensitivity," Gaitskell says. "For comparison, the larger LUX detector will reach the same sensitivity in less than 36 hours."

*Update (4:20 P.M.): This paragraph was added after initial publication.

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