Clues to the Unseen Universe
Subatomic Tracking. Dark matter/dark energy? "..a vast shadow universe of normally unseen matter existing side by side with ours, [says] scientists at the Brookhaven"
Subatomic Tracking Finds Clues to the Unseen Universe
By JAMES GLANZ
An experiment that tracks subtle motions of subatomic particles called muons has found tantalizing evidence for a vast shadow universe of normally unseen matter existing side by side with ours, scientists at the Brookhaven National Laboratory said yesterday.
The significance of the findings has been thrown into doubt by a series of mathematical errors and theoretical disagreements by physicists around the world who have been weighing the evidence for what would, if correct, rank as one of the greatest discoveries in science.
The Brookhaven "g minus 2" experiment has produced extraordinarily minute observations of the gyrating muons. In a dispiriting turn for the experimenters, though, the theoretical predictions of how encounters with ordinary matter should affect the dance of the particles have come into doubt. Only through differences between the expected and observed behavior of the muons (pronounced MEW-ahnz) could the existence of new matter be inferred.
"If you could believe the theory value was stable and reliable, you'd say, `Hey, there's no question,' " said Thomas B. Kirk, associate director for high energy and nuclear physics at Brookhaven. "But the theory situation is still not under control. It's just maddening to me."
The existence of the new matter is predicted by an unconfirmed theory called supersymmetry. According to the theory, every known particle in the universe from the electron to the neutrino has a counterpart that has eluded detection. Some versions of the theory suggest that "dark matter," a substance that seems to outweigh ordinary matter in the cosmos, actually consists of tremendous swarms of supersymmetric particles that waft through space.
The test at Brookhaven, at Upton on Long Island, involved a multinational team of scientists. It works something like the high school experiment called Brownian motion, which long ago provided evidence for the atomic structure of ordinary matter. When seen through a microscope, dust motes in liquids jitter about, because they are repeatedly struck by otherwise invisible atoms and molecules.
Physicists know that seemingly empty space is populated by a kind of fizz of particles that flit into and out of existence. The muons, charged particles that are heavier cousins of electrons, gyrate like tops in a powerful magnetic field in a vacuum chamber. Like the dust motes, the muons encounter the other particles and gyrate differently as a result.
The new Brookhaven group studied four billion spinning muons with negative electrical charges. The findings seem to agree with the group's earlier examination of positive muons, suggesting to some scientists that the shadow universe of supersymmetry may have been dimly sighted.
Gordon Kane, a particle physicist at the University of Michigan, said that he believed the disagreements were close to being resolved and that the experiment should continue to collect data. If the anomalous readings continue, Dr. Kane said, "you would have the first true, absolutely firm evidence for new physics."
In another misfortune for the g minus 2 group, the experiment has not received financing from the Energy Department to continue the work. The results presented yesterday emerged from extensive figures collected in 2001. Physicists in the experiment said they would renew their proposal to continue the work.
"When you get a discrepancy at this level, the usual procedure is to keep making careful measurements and answer the unanswered questions," said Lee Roberts, a physicist at Boston University and the group spokesman. "We will write a proposal that will say what we expect we can do."
So far, financing has gone instead to machines that have a hope of seeing the new particles directly, said Robin Staffin, associate director for high-energy physics in the science office of the Energy Department. Dr. Staffin cited the American contribution to the Large Hadron Collider, a powerful particle accelerator in Geneva scheduled to begin running in 2007.
Dr. Staffin called the new results "a very interesting indication of the new physics beyond the Standard Model," the theory that physicists use to describe ordinary matter. But he was noncommittal on whether the new findings would prompt a reconsideration on financing.
The Brookhaven experiment begins when protons spit out from the Alternating Gradient Synchrotron strike a nickel target and produce a spray of particles. Some of those particles decay into muons that are polarized, ones that have their spins, like little tops, all lined up.
Those muons are shunted into a 50-foot-diameter particle racetrack consisting of superconducting magnets. Moving at nearly the speed of light, the muons circle the track about once every 149-billionths of a second. At the same time, said David Hertzog, a member of the group who is a physicist at the University of Illinois at Urbana-Champaign, the muons wobble like precessing tops in the powerful magnetic field.
Physicists have long known that if not for the background fizz of other particles, the frequency that the muons zip around the track would be the same as their wobble frequency. But a set of 24 detectors found that they were different. Much of that discrepancy could be explained by known particles in the fizz, which exists because the strange science of quantum mechanics lets the muons spend part of their lives in the guise of those other particles.
But not all of the difference could be explained that way — at least, according to the calculations Dr. Kane and some other theorists believe to be correct. Other calculations show less of a discrepancy with standard physics, and that is largely why the experimenters have not declared that they have discovered the shadow universe of new particles.
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