Making Sense of the Muon's Misdemeanours
hubie writes:
The muon is often described as the electron's heavy cousin. A more appropriate description might be its rogue relation. Since its discovery triggered the words "who ordered that" (Isidor Isaac Rabi, Nobel laureate), the muon has been bamboozling scientists with its law-breaking antics. The muon's most famous misdemeanour is to wobble slightly too much in a magnetic field: its anomalous magnetic moment hit the headlines with the 2021 muon g-2 experiment at Fermilab. The muon also notably caused trouble when it was used to measure the radius of the proton - giving rise to a wildly different value to previous measurements and what became known as the proton radius puzzle. Yet rather than being chastised, the muon is cherished for its surprising behaviour, which makes it a likely candidate to reveal new physics beyond the Standard Model.
Aiming to make sense of the muon's strange behaviour, researchers from PSI and ETH Zurich turned to an exotic atom known as muonium. Formed from a positive muon orbited by an electron, muonium is similar to hydrogen but much simpler. Whereas hydrogen's proton is made up of quarks, muonium's positive muon has no substructure. And this means it provides a very clean model system from which to sort these problems out: for example, by obtaining extremely precise values of fundamental constants such as the mass of the muon.
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