Published online 14 April 2011 | Nature | doi:10.1038/news.2011.235

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The XENON100 experiment has placed the tightest limits yet on the properties of dark matter.

The XENON100 experiment did not see any dark matter particles, but it did set new limits on supersymmetry.

After months of battling radioactive contamination that threatened to swamp its detector, the XENON100 collaboration has managed to place the strongest limits yet on the detection of dark matter. The no-show, announced today at a seminar in Gran Sasso National Laboratory in Italy, places constraints on supersymmetry, the leading alternative to the standard model of particle physics.

Dark matter is thought to make up 83% of the matter in the Universe, and a number of experiments are competing to be the first to detect its rare interactions with ordinary matter. XENON100 looks for proposed dark-matter particles called weakly interacting massive particles (WIMPs) as they pass through the central portion of 161 kilograms of liquid xenon beneath 1.4 kilometres of rock at Gran Sasso. The particles should produce electric charge and light signals as they collide with xenon nuclei.

In a paper published online last night, the XENON100 researchers report three events detected during a 100-day run of the experiment last year that might have been due to dark matter1. However, as they expected to see between 1.2 and 2.4 background events — interactions mostly caused by a radioactive contaminant in the xenon — their result is statistically negative and therefore rules out the existence of many of the more strongly interacting and heavier WIMPs.

"We were of course hoping to see evidence of dark matter and we didn't, but we did have a very high sensitivity," says particle physicist Laura Baudis of the University of Zurich in Switzerland, and a group leader in the XENON100 collaboration. Baudis notes that contamination was higher than hoped, and higher than in a previous experimental run2, because of a leak in the experiment. However, she says, it is still much lower than that in many other experiments, and low enough that the group can be confident in its findings.

The new findings significantly constrain supersymmetry, a theory that predicts the existence of a host of particles that physicists are hoping to detect through collisions in the Large Hadron Collider (LHC) at CERN, Europe's particle-physics lab in Geneva, Switzerland. The lightest of the particles predicted by supersymmetry may also be a WIMP.

In their paper, the XENON100 researchers report that their result is the first to cut into the region of heavier WIMPS that is also accessible by the LHC. Baudis says that it also contradicts reports of the lighter WIMPs that the Dark Matter experiment (DAMA) in Gran Sasso, and Coherent Germanium Neutrino Technology (CoGeNT) experiment in the Soudan mine in Minnesota claimed to detect.

WIMP limits

Dan Hooper, a theoretical physicist at Fermilab in Batavia, Illinois, says that he is enthusiastic about the limits the XENON100 collaboration have placed on heavier WIMPs. But he questions whether the detector is sensitive enough to lighter ones to challenge the DAMA and CoGeNT findings. "I'm a little more sceptical about that," he says.

Juan Collar, a cosmologist at the University of Chicago in Illinois, who works on CoGeNT, agrees. He says that a lot rests on the calibration of the XENON100 detector, which he will be looking to study in detail. "Previous attempts by the XENON collaboration to calibrate the response of their detector contained traceable mistakes in methodology," he says3. Only if his analysis confirms the calibration would he be sure that the experiment is in conflict with the DAMA and CoGeNT results.

The previous record ruled out WIMPS that were five times more likely to interact with ordinary matter as the new results, and was set in 2010 by XENON100 and the Cryogenic Dark Matter Search (CDMS), another experiment in the Soudan mine that is looking for WIMPs hitting the nuclei of atoms in germanium crystals.

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CDMS member Sunil Golwala of California Institute of Technology in Pasadena, says that XENON100's latest result is "impressive". He adds that the sudden rate of progress — improving constraints by a factor of five in just a year — is typical for this type of experiment, which is able to progress fast until its level of background becomes close to the level of the signal it is looking for. That point seems to have been reached now, so, he says, improving greatly on the current results will require the construction of larger detectors, to give WIMPS a greater mass of detector to collide with and produce a signal. The XENON100 collaboration hopes eventually to scale up to use as much as one tonne of xenon, which will be 100 times more sensitive to potential WIMPS.

Another large-scale xenon experiment expected to come online by the end of this year is the Large Underground Xenon (LUX) detector, which will search for dark matter using 350 kilograms of liquid xenon placed underground in the Sanford Underground Laboratory at Homestake, South Dakota. LUX physicist Rick Gaitskell of Brown University in Rhode Island, is upbeat about XENON100's result. "From our point of view this is very exciting because we're seeing a result from the new generation of xenon detectors that is a significant improvement over any other competing technology," he says, adding that the sensitivity of XENON100 bodes well for LUX's chances of finding dark matter — or ruling it out.