An experiment being carried out at Fermilab to measure the magnetic property of the muon has the potential to show the way to new physics.
A HIGH-PRECISION, and very important, experiment (christened E989) to measure the magnetic property of the fundamental particle called muon got under way in February at Fermilab, the high-energy particle accelerator laboratory in Illinois, United States (Figure 1). The importance of this experiment arises from the fact that the present measured value of the muon’s magnetic strength, or its “magnetic moment”, which determines its behaviour in a magnetic field, is significantly higher than the theoretical predictions of the Standard Model (SM) of particle physics, the highly successful theoretical framework with which scientists by and large understand the universe today.
The currently best measured and accepted value is due to an experiment (E821) carried out at the turn of the century at the Brookhaven National Laboratory (BNL) in New York, U.S., to the precision level possible then. This already was an improvement by a factor of 14 over the 1970s’ measurement of the quantity at CERN (the European Council for Nuclear Research). The experiment achieved 540 ppb (parts per billion) accuracy in its measurement, while the accuracy achieved in the SM theoretical calculations was about 420 ppb. The BNL experiments were performed between 1997 and 2001, and the final corrected results were published during 2004-06, according to which the experimental value was higher at about 2.5 ppm (parts per million) level than the theoretical prediction in the SM. In statistical
terminology, this is equivalent to a “3.5 sigma” discrepancy, which, in lay language, implies that there was less than a one in 750 chance that the difference was due to a statistical fluctuation or a fluke.
The physicists perceive this variance between theory and experiment to be a pointer to new physics—involving particles as yet not seen— that lies beyond the SM (Frontline, May 25, 2001). It must be emphasised, however, that, in terms of statistical significance, the discrepancy is not yet enough for physicists to regard it as “proof” of existence of new physics but only as strong evidence. It will be proof only when the discrepancy is at a “5 sigma” level— equivalent to a one in 3.5 million chance of it being a random fluctuation—or more because in particle physics it has often been seen that many discrepancies between theory and experiments at around 3 sigma have just disappeared with improved statistics and more accurate measurements.
So, until Fermilab produces conclusive proof, muon magnetic moment data will remain consistent with the SM although the departure is significantly large (Figure 2). The BNL experiment was essentially statistics limited. Using 21 times more data, and four times more precise measurements than the BNL experiment (140 ppb accuracy compared with 540 ppb), the new experiment is expected to either con-
FIGURE 1. The g-2 storage-ring magnet at Fermilab for experiment E989. Originally designed for the Brookhaven g-2 experiment (E821), it was moved to Fermilab. The geometry allows for a very uniform magnetic field to be established in the ring.