Physicists think they've solved the muon mystery

The Muon g-2 experiment (pronounced “gee minus two”) is designed to look for tantalizing hints of physics beyond the Standard Model of particle physics. It does this by making precise measurements of the wobble that occurs when a muon is placed in a magnetic field, in response to virtual particles popping in and out of existence. If the value of the wobble disagrees with the exacting prediction of the Standard Model, that’s a strong hint that some new physics might be involved. The final result, announced in 2006, found an intriguing discrepancy with the predicted value of the Standard Model: The muon’s measured magnet moment came in at a smaller value. Even more intriguing, that result was deemed a 3.7-sigma effect. (A signal’s strength is determined by the number of standard statistical deviations, or sigmas, from the expected background in the data, producing a telltale “bump.” This metric is often compared to a coin landing on heads several times in a row. A three-sigma result is a strong hint. The gold standard for claiming discovery is a five-sigma result, comparable to tossing 21 heads in a row, for example.)