The left photo shows a sequence of bubbles created by multiple scattering of a neutron. WIMPs are expected to produce a single bubble as shown in the right photo. The COUPP collaborators found no signals above background. Credit: COUPP collaboration.

When it comes to what the universe is made of, questions about mysterious "dark matter" are among the most challenging. Although physicists agree that dark matter makes up the majority of the universe, dark matter itself eludes detection.

"The ordinary stuff we know about in the universe such as people, planets, black holes, etc., is made up of atoms," explains Indiana University South Bend physicist Ilan Levine. "But this constitutes only about 15 percent of the matter in the universe. At this time, 85 percent of the matter is waiting to be discovered."

Recently, however, Levine and colleagues from the Chicagoland Observatory for Underground Particle Physics (COUPP) experiment announced a new development in the quest to observe dark matter.

Physicists theorize that dark matter particles interact with ordinary matter via mechanisms that are either dependent or independent of the nuclear spin of the atoms in the detector material. To develop their detection technique, Levine and his colleagues from the University of Chicago and the Fermi National Accelerator Laboratory turned to an "old-fashioned" technology—the bubble chamber.

Ilan Levine
Physicist, IU South Bend

Bubble chambers were used extensively in accelerator experiments until the 1970s, when new technologies made the chambers—in which superheated liquids produce bubble paths—obsolete. The COUPP experiment team dusted off the old technology, however, and found that bubble chambers offer extraordinary potential as a tool in the search for the weakly interacting massive particles (WIMPs) that are the most likely candidates for composing dark matter.

WIMPs, if they exist, rarely interact with ordinary matter. The COUPP team used a glass jar filled with about a liter of iodotrifluoromethane, a fire-extinguishing liquid known as CF3I, to detect a particle as it hits a nucleus, triggering evaporation of a small amount of CF3I. The resulting bubble initially is too small to see but it grows. Using digital cameras, COUPP scientists studied the pictures of bubbles once they reached a millimeter in size. They looked for statistical variations between photographs that signaled whether bubbles were caused by background radiation or by dark matter.

The first results were reported in the article "Improved Spin-Dependent WIMP Limits from a Bubble Chamber," which appeared in the journal Science. Those results were encouraging, according to Juan Collar, a University of Chicago professor and spokesman of the COUPP collaboration, which includes 16 scientists and students from the University of Chicago; Indiana University South Bend; and DOE's Fermilab. "We expect that COUPP will soon have a sweeping sensitivity to dark matter particles, simultaneously exploring both spin-dependent and spin-independent mechanisms for dark matter interaction," he says. "This is just one of the aspects that sets our experiment apart from the competition."

Levine, who is the 2008 recipient of the IU South Bend's Distinguished Research Award, has been collaborating with COUPP since 2005. "COUPP differs in many ways from the other dark matter search experiments," he says. "Two significant differences are the fact that we are not sensitive to many of the types of fake signals ('background') that plague other techniques, and our technique will be much less expensive than that of our competitors."

The COUPP group's results, combined with the findings of other dark matter searches, contradict claims about the observation of particles made by the Dark Matter experiment (DAMA) in Italy. Several experimental groups worldwide, including DAMA itself, had been racing to prove or disprove an earlier claim by DAMA of observing WIMPs. If the DAMA result had been due to spin-dependent WIMPs, then COUPP researchers should have found hundreds of WIMPs. They found none above background.

"The DAMA experiment has recently attributed their positive signal to be from a type of dark matter whose interactions with ordinary matter depend on the spin of ordinary nuclei," Levine explains. "If this interpretation was true, we should have seen hundreds of events above our background. So our experiment rules out this interpretation of their data."

The COUPP collaboration is funded by the National Science Foundation and the U.S. Department of Energy.