Update on the g-2 Experiment

December 12, 2001

Brookhaven National Laboratory's February 2001 announcement that the muon g-2 experiment had detected a possible "hole" in the Standard Model of particle physics was based on comparing an experimentally derived measurement for "g-2" with a value predicted by the Standard Model theory. At the time the announcement was made, the Brookhaven experimentalists had used the most up-to-date predicted value as the basis for their comparison. Their finding showed with 99 percent certainty that the measured value did not agree with the predicted value, therefore indicating the possibility that new, previously unexplored physics might be necessary to explain the experimental result.

Since that time, however, and perhaps because of the startling experimental result at Brookhaven, several theoretical physicists in France began to take a closer look at the predicted theoretical value. By carefully reexamining the calculations used to derive this value, theoretical physicists now agree that theorists made a mathematical error in calculating the predicted value for g-2. While the Brookhaven team's measured value has not changed, the comparison of this value with the corrected theoretical prediction is no longer significant in terms of indicating the possibility of new physics. That is, there is not as much difference between the measurement and the theory's prediction, and there is a 13 percent chance that the difference observed is merely a statistical fluke.

The Brookhaven experimentalists are still in the process of analyzing additional data from the muon g-2 experiment, which will be released sometime this winter or early spring. They expect their measurements from these later data runs to be even more precise than the ones used in the earlier comparison, and are looking forward to comparing their latest results with the new predicted value for g-2.

As one of the experimental physicists put it, "The challenge is not to agree or disagree with theory, but to get it right."

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