Exploring the Muon g−2 Anomaly in the Standard Model

Wednesday - August 23 2023, 14:34 UTC - 1 year ago

The Muon g-2 anomaly is a decades-long effort to tease out the exact values of the muon’s magnetic moment and its g-factor. The discrepancy between real and hypothetical values arises because the muon constantly interacts with other "virtual" particles. The Muon g-2 experiment at Fermilab is twice as precise as the 2021 figure and the statistical significance of the discrepancy has jumped to 5 sigma. The g-2 anomaly might be evidence of a new physics beyond the Standard Model.

The numbers shared two weeks ago in Liverpool—and today with the world—are the latest update in a decades-long effort to tease out the exact values of the muon’s magnetic moment and its g-factor. The magnetic moment of a particle—a measure of the torque exerted on it by a magnetic field—is proportional to the particle’s charge and spin via a dimensionless parameter called the g-factor. In a hypothetical world where the muon behaves as an isolated, idealized point particle, its g-factor will equal 2 .

In the real world, where the muon constantly interacts with other "virtual" particles flashing in and out of existence, its g-factor is slightly larger than 2. The difference between real and hypothetical values—the so-called g−2 anomaly—arises because the virtual particles modify the effective charge of the muon and the speed at which its spin rotates in a magnetic field. The Muon g-2 experiments measure this speed and use it to determine the muon’s magnetic moment and the g−2 anomaly .

Calculations based on the standard model of particle physics currently predict that the muon has a g-factor of 2.00233183620(86).The new experimental value is 2.00233184110(48) is twice as precise as the 2021 figure and the statistical significance of the discrepancy has jumped to 5 sigma or, if the earlier results are averaged together with the new ones, 5.2 sigma. The new uncertainty now standing at 0 .

20 [parts per million]. The muon g-2 anomaly has been a constant source of puzzlement for physicists since its discovery in the 1940s. While it had been well known that the measured value of the muon magnetic moment differed from the theoretical value derived from the Standard Model, it was not until the high-precision measurements of the Muon g-2 experiments were taken in the early 2000s that the difference between two values became statistically significant .

These results have caused much speculation and have led many to suggest that the discrepancies may be evidence of a new physics beyond the Standard Model.A great deal of work has gone into attempting to recreate the values derived from the Muon g-2 experiments and to better understand the difference between theoretical predictions and experimental results. While the magnitude of the discrepancy is still open to debate, some researchers suggest that recalculating the Standard Model g-factor might eliminate the difference between the measured and calculated values of the muon g-2 anomaly .

This could provide more insight into the physics governing the muon and underlying the Standard Model and, if successful, would allow for a greater understanding of the vast and complex universe of particle physics.Regardless of the outcome, the muon g-2 anomaly is sure to remain the object of intense study for decades to come, as its implications for physics could be far-reaching.