By Gail Gallessich

Scientists conducting the Cryogenic Dark Matter Search experiment, of which UCSB is a participant, have regained the lead in the worldwide race to find the particles that make up dark matter.

Dark matter, which has been inferred but never detected directly, is thought to comprise about one-fifth of the energy in the universe, and 85 percent of all of its matter. Neutrons, protons, and electrons make up the other 15 percent of matter in the universe.

On Feb. 22, results from the CDMS experiment were presented at a UCLA symposium on detection of dark matter and dark energy. Rupak Mahapatra, a high-energy particle physicist from UCSB who led the data analysis team, presented data from the CDMS that showed that while no signal has yet been detected, the experiment is now considered the most sensitive in the world.

The roomful of approximately 200 scientists from around the world erupted in applause during and after Mahapatra’s presentation. The CDMS experiment, conducted a half-mile underground in a mine in Soudan, Minn., increased by a factor of three the number of germanium crystals used as detectors.

Stanford University’s Blas Cabrera, co-spokesperson of the CDMS experiment, said, “With our new result we are leapfrogging the competition. We have achieved the world’s most stringent limits on how often dark matter particles interact with ordinary matter, and how heavy they are.”

Teams searching for dark matter have quadrupled in the past few years, and now number 20. UC Santa Barbara is one of 16 institutions involved in the CDMS experiment. UCSB’S David Caldwell, emeritus professor of physics, was one of the originators of the experiment.

“The Big Bang and current observations suggest that the dark matter is related to the ‘Weak Interaction,’ which governs certain radioactive decays, like the decay of potassium that is in bananas and people’s bones,” said Harry Nelson, UCSB professor of physics and one of the principal investigators in the experiment. “Dark matter is also thought to consist of a massive particle, about 100 times the mass of a proton.” Together these concepts make up the name of the particle: a Weakly Interacting Massive Particle, or WIMP.

Nelson explained that the experiment uses a “billiard ball” scattering technique to seek evidence for WIMPs. It is as though the WIMP is the cue ball, and germanium atoms are the pool balls. “We can’t see the WIMP directly, but our sensors can detect the sound, like a ring of a bell, if a germanium atom suddenly gets struck,” he said.

Added Blas, “Our experiment is now sensitive enough to hear WIMPs even if they ring the ‘bells’ of our crystal germanium detector only twice a year. So far, we have heard nothing.”

If they exist, WIMPs might interact with ordinary matter at rates similar to those of low-energy neutrinos, elusive subatomic particles first proposed in the 1930’s. The CDMS collaboration found that if WIMPs have 100 times the mass of protons, they collide with one kilogram of germanium less than a few times per year; otherwise, the CDMS experiment would have detected them.