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Muon Attack on the Standard Model

For the past 30 years, the Standard Model--a theory that describes all of particle physics--has stood up to steady stream of challenges. But a new result from an ongoing experiment at Brookhaven National Laboratory may well end that success. Last week, scientists announced that measurements of the so-called anomalous magnetic moment of the muon, a subatomic particle similar to but heavier than an electron, differed considerably from what the Standard Model predicted it to be. "We are now 99 percent sure that the present Standard Model calculations cannot describe our data," project manager and physicist Gerry Bunce says. A paper has been submitted to Physical Review Letters.

The team at Brookhaven, in collaboration with scientists from 11 other institutions around the world, has been collecting data since 1997, measuring what is known as the g-2 value for muons. In short, it is an estimate of how the strong, weak and electromagnetic forces affect the muon's spin. Earlier experiments elsewhere were in agreement with theory. But the Brookhaven set-up includes a very intense source of muons, the world's largest superconducting magnet and more precise detectors--all of which measured a larger g-2 value than predicted.

"There appears to be a significant difference between our experimental value and the theoretical value from the Standard Model," says co-spokesman for the experiment and Yale physicist Vernon Hughes. "There are three possibilities for the interpretation of this result. First, new physics beyond the Standard Model, such as supersymmetry, is being seen. Second, there is a small statistical probability that the experimental and theoretical values are consistent. Third, although unlikely, the history of science in general has taught us that there is always the possibility of mistakes in experiments and in theories." Further study is clearly needed.

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