as withstood experimental challenge for some 30 years. In February 2001, the muon g-2 collaboration published a finding that deviated from the value predicted by the Standard Model.
The result of that experiment, which like the current one, was performed at Brookhaven's Alternating Gradient Synchrotron, had a one percent statistical chance of being explained by the theory as it was understood at that time.
After that announcement, perhaps because of the startling experimental result from Brookhaven, many theoretical and experimental physicists took 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 at Brookhaven in 2001 was less statistically significant.
The experimental result released in July 2002 was twice as precise as the previous measurement and was in excellent agreement with it, making that measurement a much more sensitive test of the Standard Model. Since that time, much additional work has been going on to improve scientists' understanding of and confidence in the theory prediction for the Standard Model value. While a consensus seems to be developing, it is still an active topic of investigation by a large number of scientists from around the world.
The Standard Model theory for g-2 is composed of contributions from three of the four forces in nature: the weak, the electric, and the strong force. While the contributions from the weak and electric forces can be calculated from first principles, the contribution from the strong force cannot. This latter contribution must be determined using experimental data. The direct determination uses data obtained by colliding electrons and anti-electrons and measuring the particles that are created from the strong force in the collision. The indirect method uses data from the decay of tau particles, which are heavy brothers of the muon and electron, along with some additional theoretical assumptions.
At present the two methods do not agree very well, and in light of this disagreement some physicists only use the direct method to determine the theory value. The data used in both methods were obtained at accelerators in Russia, Europe, China, and the U.S. Data from additional experiments at accelerators in the U.S., Italy, and Japan, when available, should help to further refine the Standard Model theory value for g-2, giving scientists greater confidence in the number used for comparison with the experimental result announced today.
This research was funded by the Office of High Energy Physics within the Office of Science of the U.S. Department of Energy, the U.S. National Science Foundation, the German Bundesminister fur Bildung und Forschung, and the Russian Ministry of Science, and through the U.S.-Japan Agreement in High Energy Physics.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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