Darkest Puzzle of the Cosmos
With one team claiming success, physicists still seek proof of 'dark matter,' which is thought to comprise almost all of the material in the universe.

By USHA LEE McFARLING
LA Times Science Writer
March 20, 2000



     PALO ALTO--It's happening again. Some physicists are claiming to have solved the greatest mystery in the universe.
     They say they have found "dark matter"--impossibly elusive particles that make up nearly all of the universe, yet have never been captured, created in a lab or even detected.
     Previous claims have popped up regularly since the early 1980s. But as the charmingly named dark-matter candidates--neutrinos, monopoles, MACHOs, black holes and dwarfs of various colors--fail to stand up to experimental and theoretical proofs, they fade faster than presidential campaign promises.
     "It's like Elvis. There are sightings every so often that are never confirmed," said Rocky Kolb, who heads the cosmology department at the Fermi National Accelerator Laboratory in Batavia, Ill.
     Without dark matter, the universe does not make sense. Some unseen material appears to be exerting a powerful gravitational pull that explains the current structure of the universe and the speeds of the objects within it.
     But because no one can see it, the questions loom: What is this exotic dark matter? How can we detect it?
     A group of Italian and Chinese physicists is claiming to have detected the curious dark matter in a clever experiment nearly a mile below ground. Members announced their finding in 1997, and last month reintroduced it with additional supporting data.
     Other physicists doubt, and at times fiercely criticize, the team, accusing it of making an extraordinary claim without adequate proof.
     The controversy, and the many fits and starts of earlier searches, illustrate just how competitive, difficult and uncharted this cosmological game of hide and seek really is. Some liken it to trying to find a black cat in a dark cellar at midnight--with no flashlight.
     "It's taking a lot longer than I thought it would," said Vera Rubin, an astronomer at the Carnegie Institution of Washington who unofficially set off the search for dark matter in the 1970s with the finding that something invisible was causing galaxies to rotate at high speeds.
     Nevertheless, there is now a palpable excitement among those who created the theories that predict the nature of dark matter and those who seek the stuff. Even if the Italian-Chinese team has fallen short, the theoreticians believe somebody is on the verge of snaring their quarry--thanks to spectacular leaps in technology that can detect barely perceptible flickers of energy and motion.
     "We've ruled out a lot of suspects and now an arrest is imminent," said Mike Turner, an astrophysicist at the University of Chicago. "When you're working a big case--think JonBenet Ramsey or O.J.--you've got to check out every lead."

     The Nature and Fate of Our Universe

     The answer to the dark matter mystery literally wouldn't alter a hair on your head. But it could create an entirely new type of physics filled with mind-expanding ideas about things we take for granted, like the power of gravity. It could also help explain the nature and ultimate fate of our universe.
     "It's about the most important thing we could possibly be looking for," said David Caldwell, a professor emeritus of physics at UC Santa Barbara who founded one team now hunting for dark matter. Adds UC Santa Cruz physicist Joel Primack: "We are, after all, talking about what most of the universe is made of."
     The leading suspects are theoretical particles called WIMPs, or weakly interacting massive particles.
     The particles are thought to be heavy compared with elementary atomic building blocks like protons. But because WIMPs are believed to race right through ordinary matter without leaving a trace, they are nearly impossible to detect.
     Physicists around the world are so sure that WIMPs exist that many are staking their careers on finding them. Twenty international teams are searching for WIMPs in all manner of strange places: caves, tunnels and mine shafts, where detectors can be protected from most stray signals caused by cosmic rays and are therefore able to focus solely on WIMPs.
     They're using superheated droplets, magnetic grains, vibrating wires and huge crystals cooled to the limits of cryogenic science in hopes of detecting just a fraction of the millions of WIMPs postulated to pass through an area the size of our thumbnail each second.
     While it's possible that the Italian-Chinese team has already found WIMPs--as it claims--or that a dark-horse team could claim that prize, most bets are on a team based at Stanford that is perfecting exquisitely sensitive new WIMP detectors.
     Dubbed CDMS for cryogenic dark matter search, the project relies on ultra-pure superconductor crystals cooled to nearly 500 degrees below zero. The atomic structure of crystals at such temperatures is so still that any perturbations caused by particles like WIMPs should be measurable as slight increases in temperature.
     "Like a pool where there's no storm, when you drop a pebble in, you see the waves," said Blas Cabrera, a physicist who previously led, and abandoned, the search for a dark-matter candidate called a monopole. He now co-directs the $25-million CDMS project, funded by the Department of Energy and the National Science Foundation.
     But the team must still determine if the signals they see are caused by WIMPs or by other particles from cosmic rays. To date, it appears that the signals recorded by the detectors here have almost certainly come from ordinary particles like neutrons and not from WIMPs, Cabrera said.
     The Italian-Chinese team used an entirely different technique. For the last three years, its members have looked for signs of a seasonal difference in the number of WIMPs that would be an expected effect of the Earth's orbit around the sun. In comparing the total number of particles detected at different times of the year, the researchers inferred that any differences would be due to increases or decreases in the number of WIMPs.
     They found an increase each June--but only of 1%--compared with the level in December. Other scientists said they'd be more convinced if a larger difference had been detected, and if there had been more evidence that the variation didn't have an earthly cause, like temperature changes.
     A conclusive answer may arrive only with the next generation of experiments. The 10-university collaboration running the CDMS project at Stanford is now moving its experiment to the former Soudan iron mine in northeastern Minnesota. Placing the experiment nearly half a mile beneath the surface and adding more sophisticated detectors should make it possible to screen out nearly all background signals, including pesky neutrons.
     When those detectors go on line in Minnesota next spring--an event the scientists dub "First Dark"--team members anticipate being able to clearly differentiate any WIMPs from background signals for the first time. WIMP-hunting physicists say these experiments will be the first time they reach what some call the promised land: the place where they truly have a chance to catch their prey.
     But they also might find nothing at all.

     Carrying On Einstein's Quest

     That's a risk Bernard Sadoulet is willing to take--and has been willing to take for the 14 years he's been hunting for WIMPs. A native of Nice, France, Sadoulet started his physics career by helping run Nobel Prize-winning particle-smashing experiments that captured fundamental (but not dark) particles like the charm quark and W and Z bosons.
     Sadoulet, who now directs the Center for Particle Astrophysics at Berkeley, says he's skeptical of the many "grand ideas" that theoretical physicists cook up to explain the universe. "I'm not ready to spend 10 or 15 years of my life testing an idea some theorist came up with at breakfast," he said.
     But 15 years ago, Sadoulet became captivated by dark matter--and convinced that the theoretical arguments predicting its existence were sound. Finding dark matter, he says, would be the "ultimate Copernican revolution"--proof that not only are we not at the center of our universe, but we're not even made of the same material as most of it.
     The find would also confirm decades of theoretical work that attempt to explain inconsistencies in physical laws that govern our universe. Einstein hoped to formulate a unified model that could weave all of the universe's various forces and materials into one grand theory. He failed, but many of the physicists following his lead think dark matter may provide an all-important handhold to such a complete theory.
     But scientists are also reluctantly bending to the idea that their explanations of the universe may not be so elegant after all. "In physics we always like simple solutions," said Harry Nelson, a physicist at UC Santa Barbara and a member of the CDMS team. "It almost makes you feel sick to think dark matter is made up of different types of things, but it probably is."
     Said Frank Avignone III, a physicist at the University of South Carolina and a longtime WIMP hunter: "We may not be looking for one suspect. Maybe we're dealing with a gang."
     While WIMPs are the leading contender, there are a few others in the running.
     A search based at the Lawrence Livermore National Laboratory is looking for another theoretical particle called an axion. The axion should weigh millions and millions of times less than a single electron. Although proof of its existence could solve several fundamental problems in physics--from the dark-matter mystery to the way neutrons are held together--they have been sidelined because of the current furor over WIMPs.
     "There's a bandwagon effect that this theory [that predicts the existence of WIMPs] must be true because it's so beautiful and explains everything. People argue for it based on its aesthetics," said Leslie Rosenberg, a physicist at the Massachusetts Institute of Technology who co-directs the US Axion Search with Karl Van Bibber, a physicist at Livermore.
     On the other end of the size spectrum from the axion is an extremely heavy particle proposed by Fermilab's Kolb called a WIMPzilla. But even Kolb admits that he can't dream of a way to detect the monster.
     It's also possible that dark matter could be made of something so strange that it hasn't yet been theorized, something so elusive that humans may never detect it.
     "There's no guarantee that it will be in a form that we can ever discover," said Kolb. "That's what wakes me up at night in a cold sweat."
     
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     Searching for Dark Matter

     Dark matter makes up more than 90% of the universe but has never been detected using ordinary methods. A leading candidate for dark matter is the WIMP ,or weakly interacting massive particle. WIMPs are hundreds of times heavier than protons, but rarely interact and pass through most material, including our bodies, without leaving a trace. About 20 groups are searching for evidence of WIMPs, which exist only in theory.
     The illustration below is of an Italian experiment:

     Hunting for WIMPs

     A WIMP-hunting team based at Stanford will soon move its experiment nearly a half-mile below the earth's surface to a defunct iron mine in north-eastern Minnesota. Placing the experiment underground will protect the detectors, shown at right, from numerous stray particles that come from cosmic rays but rarely penetrate the ground. Wrapping the detectors in layers of lead will protect them from natural radioactivity from the surrounding rock.
     The detectors are hockey puck-sized superconducting crystals of germanium and silicon. These pure crystals are cooled to about 500 degrees below zero. A particle hitting a detector disturbs the molecular structure of the crystal and registers as a slight temperature increase. Because WIMPs easily pass through most matter, they can pass through the shields and register a signal. To date, the detectors at Stanford have registered a handful of signals, but an analysis suggests that these were caused by stray particles that originally came from cosmic rays and managed to penetrate the 35 feet of rock over the detectors. By improving the detectors and moving them into the deep mine, scientists hope to decrease the number of false signals hitting the detectors and be more certain that the signals they do find are WIMPs.

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