Shedding Light on Dark Matter
By MATTHEW FORDAHL=AP Science Writer=
The invisible and so far unidentified dark matter that accountsfor 90 percent of the universe could soon be brought to light asscientists develop sensitive detectors capable of sniffing out tinyparticles predicted by theory but not yet proven to exist.
Teams of researchers are racing to build the devices even thoughthey might be hunting for something that occurs only in the mindsof theoretical physicists. If so, a generation of theories can betossed out.
But if the weakly interacting massive particles _ WIMPs _ aredetected, the finding could solve fundamental mysteries of theuniverse: how it formed after the Big Bang, the nature of itsstructure and whether it will all end in a Big Crunch.
“It will certainly be one of the great discoveries in thehistory of science,” said physicist Joel Primack of the Universityof California, Santa Cruz. “It will be a window on a completelydifferent aspect of the universe.”
Astronomers have known for 70 years that visible matter is onlya small part of the universe. Something that exerts a stronggravitational tug, for instance, causes the outer stars of a spiralgalaxy 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 leadingcandidates, are now considered unlikely. Weighty but ghostly WIMPsare currently the prime suspects.
Physicists theorize that the tiny particles originated duringthe Big Bang, but they only interact weakly with the protons andneutrons of the visible universe. If real, 10 trillion WIMPs may bezipping through every 2 pounds of matter here on Earth everysecond.
A dozen experiments worldwide are based on the assumption thatoccasionally a WIMP might smack into normal matter. But thechallenge has been to differentiate them from other particles thatzip through the cosmos.
Scientists announced the first results from new ultracolddetectors last month, ironically while all but debunking thefindings of Italian researchers who claimed they possibly found theelusive particles.
The Italian Dark Matter Experiment, or DAMA, used detectors thatemit flashes of light whenever a particle collides with sodiumiodide atoms. Researchers theorized that the number of hits wouldincrease in June and decrease in December, as the Earth movesfaster or slower through a theoretical cloud of the hypotheticalparticles.
Sure enough, the detectors buried a mile underground registereda small increase in bombardments.
Though DAMA’s experiment could differentiate possible WIMPs fromcharged particles, it could not distinguish the elusive mysterymatter from ordinary neutrons. The fact that it’s a mileunderground 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 colddark matter. “That’s why I call it an unsophisticated detector.”
A more discriminating detector cooled to near absolute zero andburied 30 feet beneath Stanford University registered hits like theItalian experiment, but its more detailed findings showed theevents were most likely caused by ordinary neutrons.
“The important aspect of the current results is that we havepushed that technology through to the forefront of the field,”said Stanford’s Blas Cabrera, a principal investigator of theCryogenic Dark Matter Search. “In a sense, it’s a bit unfortunatethat there’s a focus on this direct comparison with the DAMAresults.”
Rather than just registering hits, the American team is able tomake two specific measurements _ the amount of heat released andthe amount of electricity that is discharged.
“For every event, getting two different kinds of informationlets 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 devicethat soon will be moved to an abandoned iron mine in northernMinnesota, where it will be shielded by 4,300 feet of rock andearth. Sensitivity is expected to increase by a factor of 100 whenthe $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 UCBerkeley. “You’re looking for a new hypothetical effect and youhave to eliminate known backgrounds.”
At least five other similar cryogenic experiments are beingbuilt or are planned around the world. Other researchers arefocusing on creating the particles with high-speed accelerators.
If found, the weight of WIMPs _ estimated to be 50 times heavierthan a proton _ would help physicists determine the mass of theuniverse, a figure that could mean the difference between a cosmosthat expands forever or collapses on itself.
But confirmation also would validate a popular and eleganttheory that predicts a yet-to-be-found partner for every knownparticle. WIMPs may be the lightest and most stable supersymmetricparticle, said Katherine Freese of the University of Michigan.
“If you discover the dark matter, you’ve not only discovered amajor astrophysical question _ what is the universe made of _ butalso are getting at trying to understand the nature of fundamentalphysics, the nature of particles,” she said.
And because WIMPs are not the ordinary particles that make uppeople, planets and stars, it would make everything that is knowntoday a very small minority member of the cosmos.
“What’s fascinating, if we’re right, is that most of the stuffof the universe is other than the protons and neutrons that we’reused to,” Spadafora said. “The implication is that most of theuniverse is something else. This is the ultimate Copernicanrevolution.” ___= On the Net: An introduction to particle physics:http://www.particleadventure.org CDMS Home: http://cdms.berkeley.edu/ Italy’s Gran Sasso National Laboratory: http://www.lngs.infn.it/