By Ron Cowen
Physicists this week duked it out over a bunch of WIMPs.
One team reported that it might have detected several of these hypothetical elementary particles, while another team presented evidence to the contrary. The new findings have generated intense interest because detection of the particles, called both WIMPs (weakly interacting massive particles) and neutralinos, could settle a decades-old mystery in cosmology and help unify the four fundamental forces of physics.
WIMPs are among several candidates for the unseen material, or dark matter, believed to make up at least 90 percent of the mass in the universe. Numerous studies have revealed that rapidly rotating galaxies and galaxy clusters need some kind of not-yet-detected matter to keep from flying apart.
The elusive particles may also hint at how gravity, electromagnetism, and the weak and strong forces can have a common origin and yet behave so differently. To solve this puzzle, some theories employ an idea called supersymmetry, which requires that each elementary particle, such as the photon and quark, has a partner as yet undetected. The lightest and easiest to find of these proposed partners would be the WIMP.
In their search, Rita Bernabei of the University of Rome and her colleagues have gone to great depths. Beneath the Apennine Mountains at the Gran Sasso National Laboratory east of Rome, the team analyzes faint flashes of light emitted by sodium iodide detectors when subatomic particles collide with them. Some of those flashes, the team says, may be signs of WIMPs.
Over a 3-year period, the researchers have documented an annual rise and fall in the number of flashes recorded at a particular energy. That’s precisely what would be expected if a cloud of WIMPs swaddles our galaxy, they say.
The galaxy’s rotation would carry the solar system through the stationary WIMP cloud. In the summer, Earth moves around the sun in roughly the same direction as the sun moves through the galaxy. Earth would thus travel through the cloud of WIMPs faster and experience a stronger wind of these subatomic particles in summer than it would in winter, when the WIMP detection rate should fall.
At its Web site (http://www.lngs.infn.it/), the team says its findings “favor the possible presence of a WIMP.” The researchers estimate the mass at 50 times that of a proton, they reported this week at a meeting on dark matter in Marina del Rey, Calif.
Other scientists worry that different effects, such as detector stability, could cause the yearly variation. David Lewin of the Rutherford Appleton Laboratory in Chilton, England, notes that the team doesn’t measure the duration of each flash. Such information could reveal whether a scintillation was triggered by a WIMP or a more mundane particle, like a neutron or cosmic ray, he says. Lewin told Science News that his group, which also works with sodium iodide detectors, sees a similar rise and fall but doesn’t believe WIMPs are responsible.
Another team, using a different method for detecting WIMPS, traces the signals it finds to neutrons. In a cave 12 meters below the Stanford campus, Bernard Sadoulet of the University of California, Berkeley and his collaborators rely on germanium crystals cooled almost to absolute zero. They presented data at the dark-matter meeting.
Two detectors recorded each of several signals, strongly pointing to neutrons as the source, says Sadoulet. It would be extremely unlikely for a WIMP to interact with more than one detector, he argues.
If WIMPs were present in the numbers reported by Bernabei’s team, the Stanford experiment would have revealed evidence of many more single collisions than the 13 it detected, Sadoulet says. “I’ve devoted 14 years to searching for WIMPs, so I’m a little sad to have to report this finding,” he adds.
No one, however, is about to give up the search. Finding a WIMP would be “Nobel prize material,” Sadoulet says.
