magnetic moment

2 Articles

Muon Magnetic Moment Matches Model, Making Major Malaise

April 24, 2026 by No comments

Sometimes, a major discovery is exactly what you were hoping not to find. That’s the case with a team at Penn State who seem to have recently closed the door on any new physics stemming from a longstanding discrepency in the magnetic moment of the muon. It turns out, the model was fine, and we just needed better calculations.

The Muon is a heavier cousin to the electron. Like the electron, it has an intrinsic magnetic moment, but the traditional methods to calculate it did not quite match experiments, which was very exciting because it made us hope our models could be improved. Rather than try the traditional approximation methods for the unsolvable equations, the group at Penn State set up what you can think of as the Quantum Chromodynamic equivalent of a Finite Element Model (FEM) simulation–a grid of discrete steps in space and time. Tiny ones, of course, because the muon, like the electron, is a point-like particle with no lower size limit. In any case, according to their paper in Nature, after a decade of refinement and increasingly expensive supercomputer runs, the mystery can be put to bed. Instead of the discrepancy that so exited physicists 25 years ago when it was first found, theory and experiment now match to 11 digits, or a 0.5 sigma discrepancy, if you prefer.

Statistically, the Standard Model works– and that kind of sucks. It sucks, because it’s the gaps in the model where new physics are possible, and everyone has been pushing at those few gaps for the last 50 years to try and find what might be behind the standard model. Even [Zoltan Fodor], the principle investigator behind this project, is sad to see it work out. Sure, it’s a feather in his cap to get the calculations right at last–but ask anybody in the field, and they’d rather keep the door open to new physics than be right. We were certainly hoping it was something novel, last time the topic came up.

You might think muons are the last thing a hacker would ever encounter, but since there’s a steady rain of them from the sky in the form of cosmic rays, it’s not only easy to interact with them, you can actually put them to practical use– like muon tomography, or navigation indoors and underground.

Header Image Credit: Dani Zemba / Penn State

The Mysterious Wobble Of Muons

May 11, 2021 by 8 Comments

You might think that particle physicists would be sad when an experiment comes up with different results than their theory would predict, but nothing brightens up a field like unexplained phenomena.  Indeed, particle physicists have been feverishly looking for deviations from the Standard Model. This year, there have been tantalizing signs that a long unresolved discrepancy between theory and experiment will be confirmed by new experimental results.

In particular, the quest to measure the magnetic moment of muons started more than 60 years ago, and this has been measured ever more precisely since. From an experiment in 1959 at CERN in Switzerland, to the turn of the century at Brookhaven, to this year’s result at Fermilab, the magnetic moment of the muon seems to be at odds with theoretical predictions.

Although a statistical fluke is basically excluded, this value also relies on complex theoretical calculations that are not all in agreement. Instead of heralding a new era of physics, it might just be another headline too good to be true. But some physicists are mumbling “new particle” in hushed tones. Let’s see what all the fuss is about.

Continue reading “The Mysterious Wobble Of Muons” →