Despite a three-month search for the mysterious particles thought to make up 90% of the Universe's mass, dark matter has remained rooted in its shadowy world.
French high-energy physicists last week revealed that their highly sensitive detector had failed to detect a single particle of dark matter with the characteristics claimed to have been spotted by an Italian team two years ago.
The results, presented at a hastily arranged special session of the 20th International Conference on Neutrino Physics and Astrophysics in Munich, contradict findings of the four-year DAMA experiment, led by Rita Bernabei of the University of Rome.
Bernabei's team controversially claimed that it had detected WIMPs — weakly interacting massive particles — a theoretical type of dark-matter particle (see R. Bernabei et al. Eur. Phys. J. C 18, 283–292; 2000).
DAMA, based in the Gran Sasso laboratory near Rome, uses a detector containing iodine, which measures the light emitted when incoming WIMPs collide with the iodine nuclei. The change in the Earth's orientation as it orbits the Sun means that its interaction with the cloud of WIMPs that is believed to fill the Solar System should vary on a seasonal basis. Bernabei claimed to have detected this variation.
DAMA is isolated from interfering radiation by a covering of 1,400 metres of rock. But many physicists claimed that the detector was not sensitive enough to distinguish a true WIMP response from background noise. In addition, most sources of noise, such as cosmic rays, would be subject to seasonal variation, they argued.
Two years ago, the CDMS experiment, led by Bernard Sadoulet, a cosmologist at the University of California, Berkeley, failed to find conclusive evidence for the type of WIMP that Bernabei claimed to have detected. But the CDMS detector is only around 10 metres below Stanford University's campus, and background noise was too high for researchers to draw unambiguous conclusions.
The latest results were obtained by researchers working on the Edelweiss detector at the Modane Underground Laboratory, located 1,700 metres below the Alps.
Edelweiss has lower levels of background noise than the CDMS detector. And, by virtue of using germanium instead of iodine, it is more sensitive than DAMA. Germanium allows the measurement of two properties of any potential interactions — temperature change and an ionization signal — rather than just one.
“The results reported by the DAMA team could have been generated by background events or could have been statistical fluctuations,” says Gilles Gerbier of the Saclay Centre of the French Atomic Energy Commission near Paris, and a member of the Edelweiss collaboration.
Other researchers agree. “The DAMA results were hard to believe, but also hard to disprove. The Edelweiss results are very convincing,” says Yorck Ramachers, who works on another dark-matter experiment at Gran Sasso. But Bernabei is sticking to her guns, insisting that the DAMA WIMPs are real, and that experiments using different detectors are not comparable.
The search for WIMPs with other characteristics will go on. Edelweiss will be able to extend its search after two more detectors are installed in the next nine months — a total of 21 will be online by the end of 2003. In addition, the CDMS experiment will soon move to a deeper site in north Minnesota.
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