![]() This issue... Magnetic Moment |
Standard Model Challenged in a Magnetic Momentby Karen McNulty WalshBrookhaven National Laboratory's precision measurement of "the anomalous magnetic moment of the muon"a type of subatomic particledeviates from the value predicted by the Standard Model. This opens experimental exploration to other physical theories that go beyond the assumptions of the current Standard Model.
February 19Scientists at the Department of Energy's Brookhaven National Laboratory and collaborators from 11 institutions worldwide announced on February 8 an experimental result that directly confronts the 30-year-old Standard Model of particle physics. "This work could open up a whole new world of exploration for physicists interested in new theories, such as supersymmetry, which extend the Standard Model," says Boston University physicist Lee Roberts, co-spokesperson for the experiment. Until late January, scientists at Brookhaven did not know whether their 1997-to-2001 experiment, dubbed "the muon g-2" (pronounced gee-minus-two), would confirm or poke holes in the prediction of the Standard Model. "We are now 99 percent sure that the present Standard Model calculations cannot describe our data," says Brookhaven physicist Gerry Bunce, project manager for the experiment. The Standard Model The g-2 values for electrons and muons are among the most precisely known quantities in physicsand have been in good agreement with the Standard Model. The g-2 value measures the effects of the strong, weak, and electromagnetic forces on a characteristic of these particles known as "spin"like the spin of a toy top or the earth on its polar axis. Using Standard Model principles, theorists have calculated with great precision how the spin of a muon (a particle similar to the electron, but heavier) would be affected as it moves through a magnetic field. Previous experimental measurements of this g-2 value agreed with the theorists' calculations, and this has been a major success of the Standard Modeluntil now. The Magnetic Moment
The scientists and engineers at Brookhaven used an intense source of muonsthe Alternating Gradient Synchrotron, to deliver a custom muon beam into the world's largest superconducting magnetthe "muon storage ring," and then used very precise and sensitive detectors to measure the muon's spin anomaly, termed g-2 or the "magnetic moment," to a much higher level of precision. The new result is numerically greater than the prediction of the Standard Model. "There appears to be a significant difference between our experimental value and the theoretical value from the Standard Model," says Yale physicist Vernon Hughes, who initiated the new measurement and is co-spokesperson for the experiment. What Does It Mean?
"Many people believe that the discovery of supersymmetry [a theory that predicts the existence of companion particles for all the known particles] may be just around the corner," Roberts says. "We may have opened the first tiny window to that world." What's Next? The team expects that analysis to come within the next year. Furthermore, Hughes adds, substantial additional data that have not yet been used in evaluating the theoretical value of g-2 are now available from accelerators in Russia, China, and at Cornell University. These data could reduce significantly the error in the theoretical value. A paper entitled "Precise Measurement of the Positive Muon Anomalous Magnetic Moment" has been submitted to Physical Review Letters for publication. It is available in pdf or postscript format from the E821 Muon (g-2) website. This research was funded by the U.S. Department of Energy's Division of High Energy Physics, the U.S. National Science Foundation, the German Bundesminister fur Bildung und Forschung, the Russian Ministry of Science, and through the U.S.-Japan Agreement in High Energy Physics. Media contacts: Research contacts: Related Information from DOE's Virtual Reference Library:Earlier papers on the g-2 experiment
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