Momentous Magnetism!,

Momentous Magnetism!

An international collaboration of more than 60 physicists from 13 institutes in the United States, Germany, Russia, and Japan announced today (February 8, 2001) that the magnetism of the muon is about four parts per billion greater than predicted by the Standard Model of particle physics. Their experimental result, obtained in the g-2 experiment carried out at the muon storage ring at Brookhaven National Laboratory, could open up a whole new world of exploration for physicists interested in new theories, such as supersymmetry, to extend the Standard Model.

The Standard Model, which has been under development since the 1960s, explains and organizes the menagerie of subatomic particles discovered throughout the 1940s and 1950s at particle accelerators of ever-increasing power 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. Physicists have long looked for cracks in the Standard Model, hoping to find previously undiscovered particles that might help to explain gravity.

The g-2 values for electrons and muons, a particle similar to but heavier than the electron, are among the most precisely known quantities in physics, and heretofore 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 would be affected as it moves through a magnetic field. Previous experimental measurements of this g-2 value agreed with the theorists' calculations—a major success of the Standard Model. But the g-2 researchers have pushed the uncertainties down to just over one part in a million. At that level, the predicted and measured values appear to diverge.The new result suggests that the Standard Model is incomplete and that physicists may be on the verge of discovering a collection of new particles predicted by supersymmetry, a theory that postulates that every particle has an as-yet-undiscovered companion.

Physicists from the University of Illinois have taken leading roles in the g-2 collaboration. Professor of Physics David Hertzog was the creator of a suite of novel detectors that comprise the primary measuring device for g-2 and was the leader of one of four independent analysis teams to examine the experimental results. Professor of Physics Paul Debevec built the wire drift chambers that are used in this experiment and will be a key component in reducing systematic errors in the future. Six Illinois graduate students and three postdocs have contributed to this effort.

More information about the measurement of the anomalous magnetic moment of the muon at BNL and a new experiment to improve the precision of the measurement even further is available from Professor Hertzog or Professor Debevec.

Read what other people are saying about this groundbreaking news:
The New York Times
New Scientist
Physics News Update