The muons we’ve discussed so far are “cosmic-ray muons,” produced when cosmic rays collide with the atmosphere. Elsewhere, researchers also produce large numbers of muons artificially with accelerators, studying their properties in precise detail to probe the limits of particle physics’ fundamental laws.
Measuring a Muon’s Tiny Deviation #
Place a muon in a magnetic field and it behaves like a tiny magnet, wobbling like a spinning top. This wobbling motion is called precession, and the strength of the wobble is captured by a quantity called g-2 (the anomalous magnetic moment). g-2 can be calculated from theory with extraordinary precision, then compared directly against a measured value from experiment. Past experiments have reported a g-2 value that deviates slightly from the theoretical prediction, drawing attention as a possible hint of an as-yet-undiscovered particle or force.
The muon g-2/EDM experiment underway at J-PARC cools muons and then re-accelerates them, producing a tighter, more tightly bunched stream of muons — called a beam — than before, letting researchers measure the g-2 deviation with a smaller apparatus and greater precision. Alongside g-2, the experiment also looks for the electric dipole moment (EDM): any imbalance in how positive and negative charge are distributed inside the muon. Combining both measurements, g-2 and EDM, the experiment searches for physics the Standard Model can’t explain.
Catching a Muon Turn Into an Electron #
Electrons and muons are like siblings — closely related particles that differ mainly in mass. In the Standard Model, a particle’s “type” — technically called its lepton flavor — is thought to be conserved almost exactly from before a reaction to after. That means a muon essentially never turns directly into an electron. If that reaction ever did happen, it would be an instance of charged lepton flavor violation (CLFV) — something the Standard Model can’t explain.
The COMET experiment, underway at the high-intensity proton accelerator facility J-PARC, is an international project aiming to search for this muon-to-electron transformation with unprecedented precision. The experiment proceeds in stages: the first stage, called Phase-I, searches with greatly improved sensitivity, and its results will feed into a second stage, Phase-II, which aims to push sensitivity more than 100 times further still. If this transformation were ever observed, even once, it would be decisive evidence for new physics beyond the Standard Model.
References #
- Moritsu, M. (on behalf of the COMET Collaboration), “Search for Muon-to-Electron Conversion with the COMET Experiment,” Universe 8, 196 (2022). https://doi.org/10.3390/universe8040196
- J-PARC muon g-2/EDM Collaboration, “Overview,” https://g-2.kek.jp/overview/