Proof of new physics from the muon’s magnetic moment? Maybe not, according to a new theoretical calculation

When the results of an experiment don’t match predictions made by the best theory of the day, something is off.

Fifteen years ago, physicists at Brookhaven National Laboratory discovered something perplexing. Muons – a type of subatomic particle – were moving in unexpected ways that didn’t match theoretical predictions. Was the theory wrong? Was the experiment off? Or, tantalizingly, was this evidence of new physics?

Physicists have been trying to solve this mystery every since. One group from Fermilab tackled the experimental side and on April 7, 2021, released results confirming the original measurement. But my colleagues and I took a different approach.

I am a theoretical physicist and the spokesperson and one of two coordinators of the Budapest-Marseille-Wuppertal collaboration. This is a large–scale collaboration of physicists who have been trying to see if the older theoretical prediction was incorrect. We used a new method to calculate how muons interact with magnetic fields.

My team’s theoretical prediction is different from the original theory and matches both the old experimental evidence and the new Fermilab data. If our calculation is correct, it resolves the discrepancy between theory and experiment and would suggest that there is not an undiscovered force of nature.

Our result was published in the journal Nature on April 7, 2021, the same day as the new experimental results.

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