DENVER--Dark matter has just become a shade darker. Physicists have presented the first results of a high-tech search for the invisible particles of dark matter that theorists believe make up most of the mass in the universe. Even though the new instrument is much more sensitive to dark matter than previous ones, it failed to get even a tiny glimpse of the elusive dark matter particle. The finding nails shut the coffin on the controversial claim to have spotted dark matter, but it makes the no-show particle an even more troubling hole in scientists' understanding of our universe.
Almost all astrophysicists are certain that dark matter exists. A number of lines of evidence suggest that about 85% of the universe's mass is invisible. Stranger still, these observations imply that this mass is not the ordinary sort of matter that makes up stars and planets and people. It must be made of an entirely different type of particle. The leading candidate by far is known as a WIMP: a weakly interacting, massive particle.
Despite years of trying, scientists have yet to catch WIMPs. An Italian experiment, Dark Matter (DAMA), claimed to have seen their faint signature (ScienceNOW, 29 February 2000), but nobody else has seen the slimmest hint of a passing WIMP. Yesterday, at a meeting here of the American Physical Society, scientists presented the first data from the improved Cryogenic Dark Matter Search. It is four times more sensitive than any other experiment, says Bernard Sadoulet, CDMS II team member and physicist at the University of California, Berkeley. Nevertheless, CDMS II has not spotted a WIMP in 53 days of running. Says Stanford physicist and CDMS II team member Blas Cabrera: "If there had been WIMP events in the data set, we're quite convinced we would have seen them."
Although disheartening, the results do at least belie the controversial DAMA claim. If the DAMA result were a genuine observation rather than experimental error, says Sadoulet, "we would have observed something like 150 events." Scientists still hope to find the particle responsible for dark matter. The team plans to increase the instrument's sensitivity by adding more detectors and by running it for years.