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February 8, 2001

UPTON, NY -- Scientists at the U.S. Department of Energy's Brookhaven National Laboratory, in collaboration with researchers from 11 institutions in the U.S., Russia, Japan, and Germany, today announced an experimental result that directly confronts the so-called 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.

muon storage ring
The g-2 muon storage ring at Brookhaven National Lab.  Hi-Res

The Standard Model is an overall theory of particle physics that has withstood rigorous experimental challenge for 30 years. The Brookhaven finding -- a precision measurement of something called the anomalous magnetic moment of the muon, a type of subatomic particle -- deviates from the value predicted by the Standard Model. This indicates that other physical theories that go beyond the assumptions of the Standard Model may now be open to experimental exploration. The results were reported today at a special colloquium at Brookhaven Lab and have been submitted to Physical Review Letters.

Bunce Scientists at Brookhaven, doing research at an experiment dubbed the muon g-2 (pronounced gee-minus-two), have been collecting data since 1997. Until late last week, they did not know whether their work would confirm 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 (left), project manager for the experiment.

The Standard Model, in development since the 1960s, explains and gives order to the menagerie of subatomic particles discovered throughout the 1940s and 1950s at particle accelerators of ever-increasing power at Brookhaven and other locations in the United States and Europe. The theory encompasses three of the four forces known to exist in the universe -- the strong force, the electromagnetic force, and the weak force -- but not the fourth force, gravity.

g-2 collaborators
Just a few of the contributors to the g-2 experiment. Larger

The g-2 values for electrons and muons are among the most precisely known quantities in physics -- and 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" -- somewhat similar to the spin of a toy top. Using Standard Model principles, theorists have calculated with great precision how the spin of a muon, a particle similar to but heavier than the electron, 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 Model.

HughesThe scientists and engineers at Brookhaven, however -- using a very intense source of muons, the world's largest superconducting magnet, and very precise and sensitive detectors -- have measured g-2 to a much higher level of precision. The new result is numerically greater than the prediction. "There appears to be a significant difference between our experimental value and the theoretical value from the Standard Model," says Yale physicist Vernon Hughes (left), who initiated the new measurement and is co-spokesperson for the experiment.  

"There are three possibilities for the interpretation of this result," he says. "Firstly, new physics beyond the Standard Model, such as supersymmetry, is being seen. Secondly, there is a small statistical probability that the experimental and theoretical values are consistent. Thirdly, although unlikely, the history of science in general has taught us that there is always the possibility of mistakes in experiments and theories."

Roberts"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 (left) says. "We may have opened the first tiny window to that world."

All the physicists agree that further study is needed. And they still have a year's worth of data to analyze. "When we analyze the data from the experiment's year 2000 run, we'll reduce the level of error by a factor of 2," says physicist William Morse, Brookhaven resident spokesperson for g-2. 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.

This research was funded by the U.S. Department of Energy, the U.S. National Science Foundation, the German Bundesminister fur Bildung und Forschung, and the Russian Ministry of Science, and through the U.S.-Japan Agreement in High Energy Physics.

The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.

More information
December 12, 2001

July 30, 2002
The Physical Review Letters paper
Full background information  
May 2000 and February 2001 stories on g-2 from the Brookhaven Bulletin
Additional pictures
What is a Muon?
Essentially, a "heavy" electron. The muon, electron, and tau particles are generically referred to as charged leptons, and they have the remarkable property that they are believed to be point particles. That is, they don't have any physical structure and they are not made out of any smaller building blocks, although the presence of electric and other fields do give them some dimension. Electrons are all around us, and some muons (and even taus) are produced by cosmic rays.

The Standard Model
The Standard Model is a model of the basic building blocks of matter (quarks, leptons) together with the particles that mediate the electromagnetic force (bosons, photons, gluons), the strong force (the powerful force which holds nuclei together), and the weak force (much weaker than either the strong or electromagnetic force, and responsible for the decay of the muon). Gravity is the fourth force, but has not yet been incorporated into the Standard Model. The Standard Model predicts virtually all known experimental results. However, we don't really know why we have the basic particles, and the model is not able to predict such things as their masses (the masses are believed to come from the so-called Higgs mechanism, the subject of study of many high-energy experiments, yet to be demonstrated).

'Adjusting' the Model
There are a number of potential theories which modify the Standard Model. For example, there is supersymmetry, which predicts a partner for every known particle. Every fermion would have a boson partner, and every boson would have a fermion partner. So far, none of these hypothesized partners have been seen. Under certain scenarios, the existence of such particles would have a slight effect on g-2. If the measured value of g-2 differs from the Standard Model prediction, then supersymmetry is one of the possible explanations. Another possibility is that the muon is not a point particle after all, but is constructed of unkown smaller particles. Or, the W gauge boson may have a g-value which differs from 2. These are usually listed as the most likely explanations for any discrepancy between the Standard Model and the measured value of g-2, but perhaps none of them is right!