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Physicists say they have evidence of dark matter

April 19, 2008|Thomas H. Maugh II | Times Staff Writer

An Italian team on Wednesday renewed its claim to have discovered evidence for the existence of dark matter, the invisible material that makes up the bulk of the universe.

Critics say the University of Rome team has answered some of the objections to their earlier findings but not all of them, leaving their claims still a subject of great controversy.

"This is a Nobel Prize-winning result if it is proved," said physicist Richard Gaitskell of Brown University, who was not involved in the research. "But it needs to be confirmed, and the experiment really has to demonstrate a total mastery of the data. Neither of those criteria have been achieved, and therefore you have to bring a healthy skepticism to the result as it stands."

The dark matter in question is called weakly interacting massive particles, or WIMPs. Even though these dark-matter analogs of conventional particles are thought to be much larger than their visible-matter counterparts, they rarely interact with the visible world -- making their detection extremely difficult.

For The Record
Los Angeles Times Tuesday, May 06, 2008 Home Edition Main News Part A Page 2 National Desk 1 inches; 45 words Type of Material: Correction
Dark matter: An article in Section A on April 19 about the existence of dark matter said the invisible material makes up the bulk of the universe. Dark energy is the predominant component of the universe, and dark matter makes up most of the mass.

At least two international projects based in the United States have been attempting to find evidence for dark matter, so far without success.

The Italian experiment led by physicist Rita Bernabei of the University of Rome uses large crystals of sodium iodide that flash light when struck by a WIMP. Of course, they also flash when struck by a variety of other normal-matter particles, so the detector is buried nearly a mile beneath Gran Sasso mountain outside Rome, which reduces interference but does not totally eliminate it.

There is no easy way to distinguish between light from a WIMP and light produced by a conventional particle, but Bernabei's team has devised a technique to solve this.

Earth's motion through space causes it to constantly sweep through a background of WIMPs, the team reasoned. But about every June 2, the Earth should receive a larger flux when its rotational velocity is added to that of the solar system with respect to the galaxy. Six months later, around Dec. 2, it should receive an unusually small flux as those velocities are subtracted.

Eight years ago, the team reported observing this. On June 2, the number of light flashes was about 2% higher than the average, while on Dec. 2 it was 2% lower.

Few believed them. There were too many uncontrolled variables in their experiment, researchers said, and other experiments did not see the WIMPs.

The Rome team went back and installed a bigger detector: more than 550 pounds of sodium iodide, compared with 220 pounds in the original experiment. They also controlled for all of the variables they could think of, Bernabei said.

On Wednesday, she reported results from five years of operations with the new detector at a physics conference in Venice and in a paper posted on the lab's website. They found exactly the same results, fortifying their initial claim.

"Anyone looking at the data would have to agree. There is definitely something changing over time," Gaitskell said. "The question is, is it astrophysical in nature or a more mundane, man-made source?"


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