A sensitive Italian experiment has found no sign of dark matter in 100 days of searching for the invisible material that is believed to account for 80 percent of the mass of the cosmos. But even in the absence of a discovery, data collected by the XENON100 experiment may shed light on fundamental physics, team leader Elena Aprile of Columbia University and her collaborators say.
The negative result, announced online April 13, doesn’t mean that dark matter doesn’t exist. It’s just harder to detect than some researchers had imagined.XENON100 is a tank filled with 161 kilograms of chilled liquid xenon buried beneath 1,400 meters of rock in the Gran Sasso Underground Laboratory in Italy. Cosmic rays, which can mimic the action of dark matter particles, can’t easily penetrate to that depth. A dark matter particle striking a xenon nucleus causes it to recoil, prompting the emission of light and ionization. The ratio of the amount of light emitted to the amount of ionization indicates whether a particle of dark matter has been found.
The new analysis puts the experiment in direct conflict with other experiments where evidence for relatively low-mass versions of dark matter particles called WIMPs, for weakly interacting massive particles, has been found.
The contradiction with other searches “is a major outcome of the analysis,” notes XENON100 collaborator Rafael Lang of Columbia.
XENON100, has also begun to place intriguing new limits on how strongly dark matter interacts with ordinary matter. If the interaction of dark matter particles is controlled by their association with another proposed particle, the long-sought Higgs boson, XENON100 is now sensitive enough to begin to probe that relationship and the presence of the Higgs, says theorist Neal Weiner of New York University.
The XENON100 results are also likely to eliminate some versions of the particle physics theory known as supersymmetry. According to supersymmetry, every known particle has a heavier, unseen partner. “This is the beginning of people really diving into the range of supersymmetric models” to test “whether or not there is anything there,” says Weiner.
The XENON scientists viewed their experiment’s latest and most extensive results on April 4. Aprile and her young collaborators gathered in a laboratory on the 10th floor of Columbia’s Pupin Hall while other members of the team watched in Zurich. Aprile’s team crowded around a computer screen as a software program unveiled the analysis of 100.9 days of data recorded by XENON100 between January and June of 2010.
“It’s like being at a wedding waiting for the bride,” one nervous team member said. In a few minutes, first one red dot appeared on the computer screen, then another, and another, until there were six red dots in all — six possible WIMPs. Aprile hugged and kissed her colleagues — along with two reporters.
Over the next few days, however, three of the red dots proved to be electronic noise. That left three WIMP candidates. But the researchers calculated that the experiment’s radioactive background would create two events mimicking WIMPS. With only one extra WIMP beyond the number predicted from noise, a bona fide detection — the stuff of Nobel Prizes — could not be claimed. “It was like taking a cold shower,” says Aprile.
Still, the experiment provides new limits on the strength with which dark matter particles interact with ordinary matter. The upper limit of the interaction strength is about one-tenth the best previous estimate, Lang says. There could even be a link between the strength of that interaction and two recent hints from Fermilab’s Tevatron suggesting a new elementary particle that would communicate a new type of force, Weiner says. And Aprile says she’s hopeful that once a full year’s worth of data from the XENON100 experiment is analyzed, her team can claim a true detection of a WIMP. She and her collaborators are also pursuing plans to build an even larger underground xenon experiment using a ton of the liquid.
Image: Physicists lower the XENON100 experiment into position 1,400 meters beneath a mountain outside of Rome. /XENON100 collaboration
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