The latest findings hint at the existence of additional -- more massive -- elementary particles that could be produced at the world's most powerful accelerators, such as the Tevatron Collider at the Fermi National Accelerator Laboratory or the Large Hadron Collider at CERN.

The muon experiment -- dubbed "g-2" (pronounced gee-minus-two) -- was performed at the U.S. Department of Energy's Brookhaven National Laboratory in Upton, NY. More than 67 scientists from 11 institutions in the United States, Russia, Japan and Germany took part in the experiment.

UI researchers designed and built the detectors and other key components used in the experiment. They also formed one of two independent teams that produced the data and one of four independent teams that performed the analysis. The scientists collected data from more than 1 billion muon decay events.

"Like its lighter weight cousin the electron, the muon possesses a property called 'spin,' which precesses in a magnetic field much like a toy top," said David Hertzog, a UI physics professor and collaborator on the g-2 experiment. "Measuring the muon's spin anomaly -- that is, the rate at which the muon spin changes direction compared to the rate at which the muon rotates -- and then comparing that with what theory predicts, provides a very sensitive test of the Standard Model."

The Standard Model ties together three of the four forces known to exist -- the strong force, the electromagnetic force and the weak force. Gravity -- the fourth force -- has not yet been incorporated into the model.

Despite the model's excellent track record, most physicists feel it is incomplete.

"Ongoing efforts at the world's atom smashers include experiments designed to search for evidence of 'new' physics that goes beyond the Standard Model," Hertzog said. "Such new physics might restore the symmetry felt to be lacking in the Standard Model and help to explain why so many arbitrary parameters are needed in the current version of the theory."

To date, those searches have come up short. But that situation may now have changed, said Paul Debevec, another UI physicist involved in the experiment.

"The discrepancy between our g-2 result and the Standard Model prediction may suggest that some kind of new physics must modify the theory."

One possibility is supersymmetry, which predicts a partner for every known particle. Another possibility is that the muon is not an elementary particle, but is constructed of smaller particles. The g-2 collaborators are now analyzing an additional body of data having four times as many muon decay events.

"The analysis of this data will further reduce the uncertainty in our measurement," Debevec said.

A paper documenting the current results has been submitted to Physical Review Letters. - By James E. Kloeppel

[Contact: James E. Kloeppel]

07-Mar-2001