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Scientists were wrong about this “rule-breaking” particle

A famous particle physics mystery that once hinted at a hidden fifth force has now collapsed under the most precise calculations ever performed.

Date:

May 19, 2026

Source:

Penn State

Summary:

Scientists spent decades chasing signs of a mysterious new force hidden inside the muon, one of nature’s strangest particles. But after years of supercomputer calculations, researchers discovered the apparent anomaly was likely a calculation error — and the Standard Model still reigns supreme.

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FULL STORY

Artist’s conception of the mystery of the magnetic moment of the muon—a sub-atomic particle similar to, but heavier than, an electron (represented by the Greek letter mu). More than half a century of measurements of that fundamental property of the muon did not line up with theoretical predictions, raising hopes that new physics might be behind the unexplained inconsistency. Credit: Dani Zemba / Penn State

For decades, a puzzling discrepancy involving a tiny subatomic particle called the muon fueled speculation that physicists might be on the verge of discovering an entirely new force of nature. Now, an international research team led by a Penn State physicist says the mystery appears to have been solved, and the answer supports existing physics rather than overturning it.

The researchers published their findings in the journal Nature, describing one of the most precise particle physics calculations ever completed. Their work shows that the long debated mismatch between theory and experiment was likely caused by limitations in earlier calculations rather than evidence of unknown physics.

Decades of Hopes for "New Physics"

The mystery centered on the muon, a short lived particle that resembles an electron but is about 200 times heavier. For more than 60 years, measurements of the muon's magnetic behavior appeared to disagree with predictions made by the Standard Model, the framework scientists use to describe the universe's fundamental particles and forces.

That discrepancy excited physicists because it hinted at the possibility of undiscovered particles or even a new "fifth force" beyond the four known fundamental forces.

"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 Zoltan Fodor, distinguished professor of physics at Penn State and lead author of the study. "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."

The team spent more than a decade refining the calculation. Their final result brought theoretical predictions and experimental measurements into agreement within less than half a standard deviation. According to Fodor, the new work confirms the Standard Model to 11 decimal places and significantly narrows the chances that unknown physics is hiding in this particular measurement.

"People ask me how it feels to make this discovery and, to be honest, I feel somewhat sad," Fodor said. "When we started to calculate this quantity, we thought we were going to have a good and trustworthy calculation for a new fifth force. Instead, we found there is no fifth force. We did find a very precise proof of not just the Standard Model, but also of quantum field theory, which is the foundation on which the Standard Model was built."

The Muon's Strange Magnetic Behavior

The research focused on a property known as the muon's magnetic moment, which describes how strongly the particle acts like a tiny magnet. Quantum theory predicts that the value should equal exactly two, representing the relationship between the particle's wobble and the magnetic field surrounding it.

In real experiments, however, the value shifts slightly because other particles briefly appear and disappear in empty space, subtly affecting the muon's behavior. This tiny deviation is known as the "anomalous magnetic moment," or g−2.

Because muons are much heavier than electrons, they are especially sensitive to these fleeting quantum effects. That sensitivity has made muon g−2 one of the most closely studied measurements in modern physics.

Experiments performed at CERN in the 1960s and 1970s, later at Brookhaven National Laboratory, and more recently at Fermi National Accelerator Laboratory all measured the muon's magnetic moment with remarkable precision. Those experiments recently earned the Breakthrough Prize in Fundamental Physics, one of the world's most prestigious science awards.

For years, the experimental measurements continued to appear inconsistent with Standard Model predictions, strengthening hopes that something entirely new might be influencing the muon.