Momentous Magnetism!, Physics Research Highlights, University of Illinois at Urbana-Champaign

New muon g-2 results--July 2002

An international collaboration of physicists from the U.S., Russia, Japan, and Germany announced on Tuesday (July 30, 2002) their latest result from the ground-breaking "g-2" experiment at Brookhaven National Laboratory' Alternating Gradient Synchrotron. The new measurement (submitted to PRL), based on data collected in 2000, has boosted scientists’ confidence in a previous measurement reported by the same group in February 2001 (based on data collected in 1999), which deviated significantly from the value predicted theoretically by the Standard Model.

The Standard Model, which has been under development since the 1960s, explains and organizes the zoo of subatomic particles discovered throughout the 1940s and 1950s. The theory integrates 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 value measures the effects of the strong, weak, and electromagnetic forces on a characteristic of these particles known as "spin"—behavior 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 new results suggest 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.

Since the February 2001 announcement, many theoretical physicists have taken a closer look at the predicted theoretical value for g-2. In October 2001, theorists reported that a mathematical error had been made in calculating the predicted value. As a result of the revised theory estimate, the measured difference from the Standard Model prediction reported by Brookhaven in 2001 was no longer as statistically significant.

The result released this week, however, is twice as precise as the previous measurement and is in excellent agreement with it, making this new measurement a much more sensitive test of the Standard Model. In addition, the Standard Model theory value for g-2 is being refined further based on experiments at accelerators in Russia and China, and, when available, these new results will provide an even stronger test of the Standard Model.

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 that will be a key component in reducing systematic errors in the future. Six Illinois graduate students and three postdocs have contributed to this effort.

Further information about g-2 is available from Professor Hertzog, Professor Debevec, or Physics graduate student Fred Gray. Professor Hertzog will be the September 19 Colloquium speaker, when he will describe the new results and the current, murky/changing status of the theory.