GoKawiilPhysicists have spent the last 20 years pondering an apparent discrepancy between experimental results and theoretical predictions for the magnetic properties of the muon, the electron’s heavier cousin—a mismatch that hinted at a possible fifth force. But according to a new paper published in the journal Nature, the discrepancy is due to a calculation fluke, not exciting new physics, so the Standard Model of particle physics is still holding strong.
“There were many calculations in the last 60 years or so, and as they got more and more precise, they all pointed toward a discrepancy and a new interaction that would upend known laws of physics,” said co-author Zoltan Fodor, a physicist at Penn State. “We applied a new method to calculate this discrepancy quantity, and we showed that it’s not there. This new interaction we hoped for simply is not there. The old interactions can explain the value completely.”
As previously reported, the muon (a member of the lepton classification) is the heavier second-generation cousin of the electron—the tau is the third-generation cousin—and that makes muons particularly sensitive to virtual particles popping into and out of existence in the quantum vacuum, since they can briefly interact with those virtual particles. Muons are special to physicists because they are light enough to be plentiful yet heavy enough to be used experimentally to probe the accuracy of the Standard Model of particle physics.
The muon has an internal magnet and an angular momentum (spin); “g” (the “proportionality constant”) refers to the ratio between the internal magnet’s strength and the rate of gyration. The muon’s magnet would typically rotate to align along the axis of the magnetic field, much like a compass does in Earth’s magnetic field. But because of the muon’s angular momentum, this doesn’t happen; instead, the field exerts a torque on the muon’s spinning magnetic moment, causing it to precess around the axis of the field. Because the muon can interact with virtual particles, the value for g differs from the classical value of 2 by about 0.1 percent—so it’s technically known as the anomalous magnetic moment of the muon.