Shedding Light on Dark Matter

By MATTHEW FORDAHL AP Science Writer APRIL 02, 2000 - The invisible and so far unidentified dark matter that accounts for 90 percent of the universe could soon be brought to light as scientists develop sensitive detectors capable of sniffing out tiny particles predicted by theory but not yet proven to exist. Teams of researchers are racing to build the devices even though they might be hunting for something that occurs only in the minds of theoretical physicists. If so, a generation of theories can be tossed out. But if the weakly interacting massive particles — WIMPs — are detected, the finding could solve fundamental mysteries of the universe: how it formed after the Big Bang, the nature of its structure and whether it will all end in a Big Crunch. "It will certainly be one of the great discoveries in the history of science,'' said physicist Joel Primack of the University of California, Santa Cruz. "It will be a window on a completely different aspect of the universe.'' Astronomers have known for 70 years that visible matter is only a small part of the universe. Something that exerts a strong gravitational tug, for instance, causes the outer stars of a spiral galaxy to revolve faster than they should, given what is visible. Other dark matter possibilities have been ruled out. Dead stars, large planets and black holes, once thought to be leading candidates, are now considered unlikely. Weighty but ghostly WIMPs are currently the prime suspects. Physicists theorize that the tiny particles originated during the Big Bang, but they only interact weakly with the protons and neutrons of the visible universe. If real, 10 trillion WIMPs may be zipping through every 2 pounds of matter here on Earth every second. A dozen experiments worldwide are based on the assumption that occasionally a WIMP might smack into normal matter. But the challenge has been to differentiate them from other particles that zip through the cosmos. Scientists announced the first results from new ultracold detectors last month, ironically while all but debunking the findings of Italian researchers who claimed they possibly found the elusive particles. The Italian Dark Matter Experiment, or DAMA, used detectors that emit flashes of light whenever a particle collides with sodium iodide atoms. Researchers theorized that the number of hits would increase in June and decrease in December, as the Earth moves faster or slower through a theoretical cloud of the hypothetical particles. Sure enough, the detectors buried a mile underground registered a small increase in bombardments. Though DAMA's experiment could differentiate possible WIMPs from charged particles, it could not distinguish the elusive mystery matter from ordinary neutrons. The fact that it's a mile underground shields it from most but not all stray neutrons. "There's no way to tell what's triggering it,'' said Primack, who was a co-author of a paper first suggesting WIMPs might be cold dark matter. "That's why I call it an unsophisticated detector.'' A more discriminating detector cooled to near absolute zero and buried 30 feet beneath Stanford University registered hits like the Italian experiment, but its more detailed findings showed the events were most likely caused by ordinary neutrons. "The important aspect of the current results is that we have pushed that technology through to the forefront of the field,'' said Stanford's Blas Cabrera, a principal investigator of the Cryogenic Dark Matter Search. "In a sense, it's a bit unfortunate that there's a focus on this direct comparison with the DAMA results.'' Rather than just registering hits, the American team is able to make two specific measurements — the amount of heat released and the amount of electricity that is discharged. "For every event, getting two different kinds of information lets you see a much clearer picture of what is causing the event,'' Primack said. Ten U.S. universities worked to develop, test and run the device that soon will be moved to an abandoned iron mine in northern Minnesota, where it will be shielded by 4,300 feet of rock and earth. Sensitivity is expected to increase by a factor of 100 when the $12 million, six-year project gets under way. "This is a very difficult measurement,'' said Tony Spadafora, associate director of the Center for Particle Astrophysics at UC Berkeley. "You're looking for a new hypothetical effect and you have to eliminate known backgrounds.'' At least five other similar cryogenic experiments are being built or are planned around the world. Other researchers are focusing on creating the particles with high-speed accelerators. If found, the weight of WIMPs — estimated to be 50 times heavier than a proton — would help physicists determine the mass of the universe, a figure that could mean the difference between a cosmos that expands forever or collapses on itself. But confirmation also would validate a popular and elegant theory that predicts a yet-to-be-found partner for every known particle. WIMPs may be the lightest and most stable supersymmetric particle, said Katherine Freese of the University of Michigan. "If you discover the dark matter, you've not only discovered a major astrophysical question — what is the universe made of — but also are getting at trying to understand the nature of fundamental physics, the nature of particles,'' she said. And because WIMPs are not the ordinary particles that make up people, planets and stars, it would make everything that is known today a very small minority member of the cosmos. "What's fascinating, if we're right, is that most of the stuff of the universe is other than the protons and neutrons that we're used to,'' Spadafora said. "The implication is that most of the universe is something else. This is the ultimate Copernican revolution.''