Physicists Solve a Muon Mystery. Now, Old Results Don’t Add Up. | Quanta Magazine
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Physicists Solve a Big Quantum Mystery. Now, Old Results Don’t Add Up.
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particle physics
Physicists Solve a Big Quantum Mystery. Now, Old Results Don’t Add Up.
By
Matt von Hippel
July 29, 2026
New calculations seem to have put a 25-year-old particle physics puzzle to rest. But they’ve also created a clash with other experimental results.
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Years after physicists at Fermi National Accelerator Laboratory in Illinois used a giant magnetic ring to measure precisely how much the muon wobbles, researchers are still puzzling over the result.
Reidar Hahn/Fermilab
Introduction
By Matt von Hippel
Contributing Writer
July 29, 2026
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experimental physics
particle physics
physics
quantum chromodynamics
quantum physics
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For 25 years, physicists have been puzzled by an apparent one-part-in-a-million problem. Their expectations of the way that certain particles should wobble in a magnetic field were clashing with what they saw in experiments. The discrepancy was an electrifying hint that they might be seeing evidence of unknown particles.
Then in 2021, that hint seemed to evaporate. When researchers updated the way they did their theoretical calculations, they found that their predictions matched the experimental results much more precisely than before, to one part in 100 billion.
But that, in turn, has created another puzzle: The old calculations seem perfectly valid. So why don’t they match the new calculations? Those older predictions were not purely based on theory; they were also inferred from other experiments. If the older calculations conflicted with newer results, and the older calculations were based on experimental data, was something strange going on in those old experiments?
One promising clue comes from a particle collider in Siberia, which has recently started seeing its experiments dramatically diverge from what it and other colliders saw in the past. Its results have sparked a flurry of activity as physicists try to determine whether the conflicting measurements are a side effect of different experimental procedures, or a sign that new particles are popping up after all.
Weird Wobbles
The particle at the center of the mystery is the muon, a heavier cousin of the electron. A muon behaves a bit like a tiny bar magnet. Spin one in a circle inside a magnetic field and the magnetism will make it wobble, tracing out its own, smaller circles. The sizes of these circles are determined by a number called a “g-factor.”
If the muon sat isolated from other particles, its g-factor would be exactly 2. But quantum theory requires that all other particles influence the g-factor. As the muon wobbles, it releases particles such as photons, which are too short-lived to show up in detectors. These can release other particles, which can release still more particles. The muon quickly reabsorbs all these fleeting particles, and the only trace they leave behind is that the muon wobbles a little bit more. Through these intricate chains of emission and reabsorption, every particle in existence has some small effect on the movement of the muon.
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Samuel Velasco, Mark Belan/Quanta Magazine
That makes the precise size of the excess wobble, the muon’s “g–2,” invaluable as a window into the quantum world. “The measurement of muon g–2 is a proxy for saying how many particles exist in the universe,” said Alex Keshavarzi, a senior research fellow at University College London.
So when an experiment at Brookhaven National Laboratory on Long Island measured the muon’s g-factor in 2001, physicists were thrilled that it came out larger than expected. To some, it hinted that new particles — perhaps even particles that could account for dark matter — were at work.
Physicists set out to check the result with an even more precise measurement. In 2013, Brookhaven’s 50-foot-wide magnetic ring was moved via an elaborate series of barges and trucks to Fermi National Accelerator Laboratory (Fermilab) in Illinois, where an upgraded version of the experiment would take even more data.
To prepare for that new experiment, physicists also made a huge effort to understand the theoretical prediction that...