The muon resembles the electron but is 200 times heavier and lives for two millionths of a second. Its magnetic moment, how the particle responds to a magnetic field, has been measured with fantastic precision. Yet the measured value is slightly larger than theory predicts. The difference is tiny, like finding an extra penny every time you count your change at the store.
The vacuum is not empty: pairs of particles constantly pop in and out of existence, like bubbles in boiling water. These fleeting visitors slightly alter the muon's magnetic properties. To account for their contribution, physicists use lattice QCD — a method that slices space and time into a tiny grid, like millimeter paper. The calculations run on supercomputers and take months.
A team of researchers has for the first time achieved sub-percent precision for the next most important contribution. The resulting number is −101.57×10⁻¹¹, with an uncertainty of 0.6%. This is twice as precise as previous calculations.
The new result disagrees with estimates from accelerator data prior to the recent CMD-3 experiment by 4.6 standard deviations. Such a discrepancy is not just a technical detail. It could be a trace of new particles not in the Standard Model — the current catalog of everything. Or there might be an unnoticed error in the old experiments. The coming years will tell who is right.
Muons are born in the upper atmosphere from cosmic rays and reach the Earth's surface, even though by classical standards they should have decayed in flight: they are saved by time dilation from relativity.
In the series 'The Expanse', a muon catalyst is used to ignite fusion engines on ships.