The world's largest superconducting magnet—the Muon Storage Ring at Brookhaven National Laboratory.

Quantum Electrodynamics (QED) describes how electrically charged particles and electromagnetic fields interact. First developed in the 1920s, the theory was revised and improved over the years until it seemed to agree with experimental results. But it was challenged in 2001, when a team led by Brookhaven National Laboratory conducted the most sensitive test yet of QED. The measurement of a key property (the anomalous magnetic moment) of the muon, a subatomic particle not found in ordinary matter, is a factor of 5.6 more precise than previous measurements and deviates from the value predicted by current theory. The scientists collected data from more than 1 billion muon decay events to measure the target property, which is among the most precisely known quantities in physics. The experiment showcased the latest research technology, including a very intense source of muons, the world's largest superconducting magnet, and very precise and sensitive detectors. The finding has yet to be verified; the Brookhaven data still are being analyzed, and QED was confirmed recently at Los Alamos National Laboratory, which made the most precise measurement ever of the hyperfine structure of muonium (composed of a muon and an electron), providing values for the muon mass and magnetic moment.

Scientific Impact: The Brookhaven work poses a direct challenge to the Standard Model, physicists' current overarching theory of matter and the forces of nature. The experiment may have revealed new physics, such as that described by supersymmetry theory, which predicts the existence of companion particles with unusual magnetic effects for all known particles.

Social Impact: Brookhaven's preliminary results, if supported and verified by additional analyses, would lead to new understanding of the physical world and associated improvements in science education.

Reference: H. N. Brown et al., Phys. Rev. Lett. 86, 2227 (2001).

URL: http://phyppro1.phy.bnl.gov/g2muon/index.shtml

Technical Contact: Prof. Lee Roberts, Boston University, Roberts@bu.edu

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics