2 days ago
This Particle Isn't Following the Rules of Physics. Maybe the Rules Are Wrong.
Here's what you'll learn when you read this story:
For nearly a century, the magnetic anomaly of the fundamental particle known as a muon has served as a means to test theories against experimental reality.
Recently, an international collaboration powered by the U.S.-based Fermilab has released its most accurate data on this anomalous magnetic dipole moment, known as g-2 ('gee minus two').
These new results align closely with recent theoretical predictions, and will serve as a benchmark moving forward.
The Standard Model of Particle Physics is a remarkable scientific achievement spanning nearly a century, and its predictive power has proven incredibly consistent. However, any scientific model worth its salt also needs to withstand experimental scrutiny, and one of the places those tests are employed is Fermilab.
Starting in 2017, an international collaboration of scientists have used data from Fermilab's 50-foot-diameter magnetic ring to measure the wobble of a fundamental particle known as a muon in what is referred to as the lab's 'muon g-2 experiment.' More than 200 times heavier than electrons, muons only survive for a few microseconds, but they have spins that makes them act like tiny magnets. This wobble, or precession, is due to an external magnetic field is called a g-factor, and a century ago, this factor was found to be 2 (hence the name 'muon g-2 experiment'). However, the introduction of quantum field theory complicates this number by bringing strong, weak, and Higgs fields interactions into the equation. This slight deviation from the '2' prediction is known as the muon's anomalous magnetic dipole moment.
To better understand this anomaly, Fermilab has consistently released results from its run of experiments that stretches from 2017 to 2023. On June 3, 2025, the muon g-2 experiment finally released its full results, with a precision of roughly 127 parts-per-billion—the most sensitive and accurate measurement of the muon's magnetic anomaly to date. The results of the study were submitted to the journal Physical Review D.
'The anomalous magnetic moment, or g–2, of the muon is important because it provides a sensitive test of the Standard Model of particle physics,' Regina Rameika, the U.S. Department of Energy's Associate Director for the Office of High Energy Physics, said in a press statement. 'This is an exciting result and it is great to see an experiment come to a definitive end with a precision measurement.'
Understanding this precise measurement of muon g-2 can help scientists discover new physics, as any deviation between experimental results and theoretical predictions using the Standard Model could point toward unknowns in our understanding of the subatomic world. While experimental physicists work to perfect ways of measuring the magnetic anomaly, theoretical physicists—especially those participating in the Muon Theory Initiative, which released its own update in late May—have largely sorted themselves into two 'camps' when calculating this theoretical prediction, according to Ethan Siegel at Big Think. One camp takes a data-driven approach to Hadronic vacuum polarization and the other uses a computational-based Lattice quantum chromodynamics (QCD) technique.
In 2021, it appeared that Fermilab's initial results were much closer to the Lattice QCD computational calculations, dampening (but not eliminating) the possibility of new physics orbiting the muon. Now, with this new calculation in hand, scientists can move forward with renewed confidence in an experimental result that's been a popular test of the Standard Model of Physics for a century.
'As it has been for decades, the magnetic moment of the muon continues to be a stringent benchmark of the Standard Model,' Simon Corrodi, assistant physicist at Argonne National Laboratory and analysis co-coordinator, said in a press statement. 'The new experimental result sheds new light on this fundamental theory and will set the benchmark for any new theoretical calculation to come.'
This isn't the end for measuring the muon magnetic anomaly—the Japan Proton Accelerator Research Complex aims to make its own g-2 measurements in the 2030s (though Fermilab says that its initial precision will be worse than their own latest results).
Today, this muon g-2 result is a testament of the incredible engineering and multidisciplinary scientific effort required to uncover just a little bit more about our ever-mysterious universe.
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