By studying an exotic atom called muonium, researchers are hoping misbehaving muons will spill the beans on the Standard Model of particle physics. To make muonium, they use the most intense continuous beam of low energy muons in the world at Paul Scherrer Institute PSI. The research is published in Nature Communications.

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 misdemeanor is to wobble slightly too much in a : its 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 behavior, which makes it a likely candidate to reveal new physics beyond the Standard Model.

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