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The muon is a spin-one-half charged lepton, the second-generation counterpart of the electron. A negative muon has electric charge , while the antimuon has charge . It has no quark substructure in the Standard Model and participates in electromagnetic and weak interactions.
A muon is unstable and decays weakly; many muons reaching ground level come from cosmic-ray cascades in the atmosphere. Their penetrating tracks make them useful in detector calibration. A simple lifetime measurement stops muons in a detector and measures the delay to a charged decay product, forming a distribution of decay times. Capture of negative muons by nuclei can change the observed disappearance rate, so material and muon charge matter when interpreting that distribution. Muon decay supplies a precise low-energy weak-interaction test.