Scientific American Is the Standard Model of Physics Now Broken? The discrepancy between the theoretical prediction and the experimentally determined value of the muon's magnetic moment has become slightly stronger with a new result from Fermilab. But what does it mean? Print The Muon g-2 magnetic storage ring, seen here during its relocation from Brookhaven National Laboratory on Long Island to the Fermi National Accelerator Laboratory outside Chicago, is a central component of the project's quest for new physics. Credit: Alamy Advertisement The so-called muon anomaly, first seen in an experiment at Brookhaven National Laboratory in 2001, hasn’t budged. For 20 years, this slight discrepancy between the calculated value of the muon’s magnetic moment and its experimentally determined one has lingered at a significance of about 3.7 sigma. That is a confidence level of 99.98 percent, or about a one-in-4,500 chance the discrepancy is a random fluctuation. With the just announced results from the Muon g-2 experiment at Fermi National Laboratory in Batavia, Ill., the significance has increased to 4.2 sigma. That is a confidence level of about 99.997 percent, or about a one-in-40,000 chance for the observed deviation to be a coincidence. By itself, the new Fermilab measurement has only 3.3 sigma significance, but because it reproduces the earlier finding from Brookhaven, the combined significance has risen to 4.2 sigma. Still, the latter falls short of particle physicists’ five-sigma discovery threshold.