Dark Matter Signal Refuted (6-6-02)

6-6-02

PARTICLE PHYSICS: Dark-Matter 'Sighting' Returns to Shadows Charles Seife MUNICH, GERMANY--Dark matter is, officially, still dark. Results presented at a meeting here last week* have convinced most physicists who have seen them that a controversial "discovery" of dark matter is in error.

The original claims stemmed from an experiment performed in 1998 deep underneath the Italian Alps. A sensitive detector at the heart of the DAMA (for Dark Matter) experiment at Gran Sasso National Laboratory showed a yearly increase and decrease in the number of particles it encountered. Although each individual "detection" had a high probability of being background noise in the instrument, the DAMA team concluded that the yearly cycle might be the signature of dark matter, the mysterious material that vastly outweighs the ordinary matter that makes up the visible universe. As Earth orbits the sun, the scientists proposed, it zooms toward and away from a "wind" of dark matter that blows through the solar system, and the shifts in orientation cause the number of dark-matter particles striking the detector to wax and wane (Science, 1 January 1999, p. 13; 3 March 2000, p. 1570). From this conclusion, the team calculated some properties of the dark-matter candidates, such as their energy. Although the claim was met with skepticism, the scientific community took it seriously--until now.

The challenge comes from a French experiment called EDELWEISS (for Expérience pour Detecter les Wimps en Site Souterrain). Like DAMA, EDELWEISS centers on a particle detector buried under tons of alpine rock to shield it from cosmic rays. For 3 months, EDELWEISS tried to sense DAMA's dark matter candidates. It failed.

"There is no event" that could correspond to DAMA-type dark matter, says Gilles Gerbier of the French Center for Atomic Energy (CEA) in Saclay, a member of the EDELWEISS team. During its run, EDELWEISS saw only one possible dark matter candidate--much too energetic to be DAMA-type dark matter and probably just experimental noise.

Rita Bernabei of the University of Rome, a physicist with the DAMA collaboration, says differences in the two detectors make a direct comparison between the results misleading. But other researchers say EDELWEISS has all but put the matter to rest. "For the first time, you exclude this DAMA positive evidence for dark matter," says Michel Spiro, also at CEA Saclay. "I'd prefer that it was confirmed than excluded, but this is important physics."

Yorck Ramachers of Oxford University suspects that DAMA's seasonal variation is a systematic error. The cumulative effects of annual cycles of temperature, humidity, and other factors might explain the "detection," he says. In any case, he says, several other dark-matter searches are likely to release data this year, so those who were rooting for the DAMA result might soon have fresh puzzles to console them.