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In 2023, Fermilab’s Muon g-2 collaboration announced their latest results, which were consistent with previous experimental results but had only half of the error and uncertainty. The ball is now in the court of theorists to determine how significant the discrepancy between the Standard Model’s predictions and the observed results are. (Credit: Ryan Postel/Fermilab)

New theoretical calculation solves the ‘muon g-2’ puzzle

A longstanding mismatch between theory and experiment motivated an exquisite muon measurement. At last, a theoretical solution has arrived.

Ethan Siegel
12 min readJul 30, 2024

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As a scientist, the most exciting moments in your professional life arise when you work hard to get a result, and — no matter how hard you try to understand it — it simply doesn’t match up with your expectations. For theorists, that moment comes when you derive a result that conflicts with what’s experimentally and observationally known to be true. For experimentalists, that moment arrives when you make a measurement that defies a theorist’s predictions. But those moments can go one of two ways: either they can be harbingers of a scientific revolution, exposing a crack in the foundations of science, or they can simply be the result of a previously undiscovered error, on either the theoretical or experimental ends.

Perhaps the greatest quest in particle physics, for perhaps half a century now, has been to find a discrepancy between theory and experiment when it comes to the Standard Model. One fascinating place to look is at the magnetic moment of the muon: a heavy, unstable relative of the electron. A Fermilab experiment known as “muon g-2″ has revealed a discrepancy between theory and

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Ethan Siegel

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.