A long-sought electrical property of the muon remains invisible, but physicists have narrowed even further the range where it could be hiding.
The muon is a particle similar to the electron, but about 207 times more massive and highly unstable. Researchers are studying, among other things, how its electric charge might be distributed. If this distribution were slightly asymmetrical, the muon would possess what physicists call a permanent electric dipole moment.
Fermilab’s g -2 storage ring magnet has a geometry that makes it possible to establish a very uniform magnetic field inside the ring.
Image Wikimedia
In practical terms, this property would cause the muon’s orientation to respond to an electric field in a very precise way. Within the Standard Model of particle physics, the expected effect is extremely small. A value measurable with current experiments would therefore indicate the existence of a phenomenon still absent from this theory.
The Muon g-2 collaboration searched for this effect using data recorded at Fermilab in 2019 and 2020. Positive muons circulated in a ring exposed to a magnetic field. When they decay, they produce, among other particles, positrons. Their trajectories provide information about the muon’s orientation just before it disappears.
The detectors therefore measured the average vertical angle of these positrons. A permanent electric dipole moment would have added a small characteristic oscillation to this motion. The analysis found no evidence of such a signal. The measured value remains compatible with zero given the statistical and experimental uncertainties.
The researchers now set a direct limit of 1.10 × 10-19 e·cm, with a confidence level of 95%.
The previous best direct limit, published in 2009 from the Brookhaven experiment, was 1.80 × 10-19 e·cm. The new analysis therefore narrows the range still allowed. However, it does not prove that the muon’s dipole moment is exactly zero: it only indicates that it must be below the sensitivity achieved here.
The study covers only part of the Muon g-2 data, collected between 2019 and 2020. Future analyses may use data from other operating periods and refined methods. For now, the publication is available as a preprint on arXiv.