Nav Button - Home

This issue...

  News in Brief

  View from the Inside

  Human Genome Sequencing

  Magnetic Moment

  Scintimammography

  People

  About

  Subscribe Free






















































This issue...

  News in Brief

  View from the Inside

  Human Genome Sequencing

  Magnetic Moment

  Scintimammography

  People

  About

  Subscribe Free

Standard Model Challenged in a Magnetic Moment

by Karen McNulty Walsh

Brookhaven National Laboratory's precision measurement of "the anomalous magnetic moment of the muon"—a type of subatomic particle—deviates from the value predicted by the Standard Model.  This opens experimental exploration to other physical theories that go beyond the assumptions of the current Standard Model.

muon ring with researchers
Some of the research collaborators standing inside the "muon ring."

February 19—Scientists at the Department of Energy's Brookhaven National Laboratory and collaborators from 11 institutions worldwide announced on February 8 an experimental result that directly confronts the 30-year-old 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.

Until late January, scientists at Brookhaven did not know whether their 1997-to-2001 experiment, dubbed "the muon g-2" (pronounced gee-minus-two), would confirm or poke holes in 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, project manager for the experiment.

The Standard Model
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 U.S. 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.

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"—like the spin of a toy top or the earth on its polar axis. Using Standard Model principles, theorists have calculated with great precision how the spin of a muon (a particle similar to the electron, but heavier) 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—until now.

The Magnetic Moment
The muon has a magnetic moment, which is equivalent to saying it has a north and south pole just like a bar magnet or a compass. The north and south poles of the muon magnet are aligned along the direction of the spin. The strength of the magnet is indicated by the magnitude of the magnetic moment. Its value is sensitive to detailed properties of the muon, and its measurement is an excellent test of models that predict these properties.

muon storage ring
The Muon Storage Ring















The scientists and engineers at Brookhaven used an intense source of muons—the Alternating Gradient Synchrotron, to deliver a custom muon beam into the world's largest superconducting magnet—the "muon storage ring," and then used very precise and sensitive detectors to measure the muon's spin anomaly, termed g-2 or the "magnetic moment," to a much higher level of precision. The new result is numerically greater than the prediction of the Standard Model.

"There appears to be a significant difference between our experimental value and the theoretical value from the Standard Model," says Yale physicist Vernon Hughes, who initiated the new measurement and is co-spokesperson for the experiment.

What Does It Mean?
"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."

"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 says. "We may have opened the first tiny window to that world."

What's Next?
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 the g-2 experiment.

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.

A paper entitled "Precise Measurement of the Positive Muon Anomalous Magnetic Moment" has been submitted to Physical Review Letters for publication. It is available in pdf or postscript format from the E821 Muon (g-2) website.

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

Media contacts:
Karen McNulty Walsh, BNL, (631) 344-8350, kmcnulty@bnl.gov
Mona S. Rowe, BNL, (631) 344-5056, mrowe@bnl.gov

Research contacts:
Bill Morse, BNL resident spokesperson, (631) 344-3859, morse@bnl.gov
Vernon Hughes, BNL co-spokesperson, (631) 344-6382, Yale (203) 432-3820, Vernon.Hughes@Yale.edu
B. Lee Roberts, Boston University co-spokesperson, Boston University (617) 353-2187, BNL (631) 344-2287, roberts@bu.edu


Related Information from DOE's Virtual Reference Library:

Earlier papers on the g-2 experiment
"The Brookhaven muon g-2 experiment," Conference report for the 8th International Symposium on Polarized Phenomena in Nuclear Physics (SPIN 94), Bloomington, Indiana (September 15-22, 1995). (547K PDF)

"Improved measurement of the positive muon anomalous magnetic moment," Nov 1 2000 Physical Review D.

"New Measurement of the Anomalous Magnetic Moment of the Positive Muon," Feb 22 1999 Physical Review Letters

Basic muon data
From the 2000 Review of Particle Physics (D.E. Groom et al., The European Physical Journal C15 (2000), p. 1). Section includes theoretical value of the anomalous magnetic moment g-2. (208K PDF)

Introduction to the Standard Model
The Particle Adventure: Elementary introduction to the known basic interactions and types of subatomic particles, including muons.

Standard Model of Fundamental Particles and Interactions - Interactive

Supersymmetry
"A Supersymmetry Primer," by Stephen P. Martin (For readers who are familiar with the Standard Model and quantum field theory, but who have little or no prior exposure to supersymmetry.

"An Introduction to Supersymmetry," by Manual Drees (Lectures given at Seoul summer symposium on field theory, August 1996) Discusses why there is such interest in supersymmetry despite the former lack of direct experimental evidence.

MIT's "The Net Advance of Physics: Supersymmetry and String Theory"

Contents
Search
Comments
previous    next






















"We are now 99 percent sure that the present Standard Model calculations cannot describe our data." —Gerry Bunce
   previous    next


• Energy Science News • Energy Science News • Energy Science News • Energy Science News •
www.pnl.gov/energyscience/