As 2 Efforts to Find 'Dark Matter' Clash, Mystery of Universe Remains

By Faye Flam
Inquirer Staff Writer
Monday, February 26, 2000


Scientists have searched for years forevidence of "dark matter," invisible particles that are believed to make up far more of the universe - from here to there to everywhere - than what we can actually see.

Astronomers are sure it exists because they can see its effects millions of light-years away. Out there, a giant sea of dark matter appears to exert a gravitational pull strong enough to move whole galaxies, like so many leaves swept along in a current. Particle physicists have predicted that it would be found on Earth in the form of new particles that exhibit the requisite slippery properties of the dark matter. They fancifully namedthem WIMPs, for Weakly Interacting Massive Particles. Entire careers have been spent trying to find WIMPs caught in devices built for millions ofdollars in England, France, German, Italy and the United States.

Yesterday, a team of Italian scientists reported the first sighting of WIMPs, in a road tunnel deep beneath the surface of the Italian Alps. An American team, having conducted experiments using a similar device underground at Stanford, said it had found no such evidence.

And so the quest for a solution to one of the biggest questions in science - what is the universe made of? - goes on.

"Our results are totally incompatible," said Bernard Sadoulet of the University of California at Berkeley, a member of the American team who has spent 15 years developing this experiment.

Both teams reported their findings at the fourth International Symposium on Sources and Detection of Dark Matter in the Universe in Marina del Rey, Calif.

Current thinking holds that the universe has at least six times as much dark matter as the ordinary kind. The stars and galaxies we observe are just a froth riding on a WIMP sea.

Near Earth, the WIMPs would be racing so furiously through space that a million pass through your thumbnail alone every second.

But WIMPs have posed an unusual challenge because by nature they are invisible - and free from the forces that bind protons, neutrons and electrons into matter as we know it.

The Italian team, led by Rita Bernabei, cannot point to any one signal as the trail of an actual WIMP. Their detector, which relied on a 100-kilogram crystal of sodium iodide, is designed to give off a flash of light when struck by a WIMP, but it also flashes upon encountering ordinary radiation. By locating it in the Gran Sasso tunnel in the north, the crystal was shielded from much background radiation, but not all.

Knowing that, the Italians looked for signs of Earth's motion through the theoretical WIMP cloud.

The entire solar system is thought to be constantly moving through the haze of WIMPs, causing an apparent wind, like the motion of a car moving through still air. Depending on the seasonal location of the Earth around the sun, the WIMPs would be moving either with the planet or against it. If the WIMPs exist as predicted, then Earth-based detectors should encounter more particles in the wind in summer than in winter.

The Italian experiment did indeed show a small seasonal change - about a 1 percent difference - enough to be interesting but not definitive.

The American group, which includes factions from Berkeley, Stanford University and Princeton University, used as a WIMP detector a piece of exotic metal called germanium the size of a hockey puck. It registers a WIMP as a tiny temperature change. The Americans, too, were concerned about radiation causing false readings - and so located their detector 45 feet down in a lead-shielded sub-basement room.

Their goal, like the Italians', was to try to distinguish WIMP signals from those caused by garden-variety radiation.

They detected no WIMPs - only 13 hits that are most likely the result of mundane background.

That doesn't mean WIMPs don't exist, said Sadoulet. It's just that the nature of WIMPs makes them hard to see in this type of experiment.

Still, he questions the Italian result because he believes the Stanford experiment should be just as sensitive as the one in the Alps. Based on the Italian report, Sadoulet said, "we should have 20 WIMPs and we don't have them."

The U.S. group, in search of a more definitive WIMP capture, now plans to move the experiment to a deeper place - an abandoned mine called Soudan in northern Minnesota. There, they expect to increase their ability to distinguish WIMPs from radiation by a factor of 100.

Whatever the conclusion, scientists are unlikely to give up the notion that WIMPs exist. Their influence on faraway stars and galaxies is too pronounced. And that apparent gravitational pull can't be explained by ordinary matter tied up in a form we can't see - rocks or stray planets or dud stars called brown dwarfs.

But the theoretical nature of WIMPs - let alone spotting them - remains a hot field.

Favored at the moment are particles dubbed "neutralinos," which would be heavy and ghost-like, moving through matter and through each other. They would carry about 50 times as much mass as the protons and neutrons that make up the bulk of matter as we know it. The Italian findings, if confirmed, give some support to this construct.

Princeton cosmologist Paul Steinhardt has proposed something a little different: particles that can bounce off each other. Those better account for the way the dark matter appears to be distributed in the heavens, he said.

Also on the table is a particle that is less massive and more fast-moving than either of the other theories. And then there is an extra-massive WIMP theorized by Rocky Kolb, a cosmologist at Fermilab, near Chicago. He calls it a "Wimpzilla."

The answer, even in theory, will be important.

"The number-one question we have about the universe," said Kolb, "is what is dark matter?

"What is the universe made of?"


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