What Is a Muon?

An electron’s heavier cousin

The muon (μ) is an elementary particle: as far as experiments can determine, it has no internal structure. It resembles the electron in nearly every respect, with electric charge −1, spin ½, and the same electromagnetic and weak interactions, but is about 207 times more massive. Unlike the stable electron, a muon survives on average only 2.2 microseconds at rest. It usually decays into an electron, a muon neutrino, and an electron antineutrino:

μ⁻ → e⁻ + ν_μ + ν̄_e

Its positively charged antiparticle, the antimuon (μ⁺), decays into the corresponding antiparticles.

Discovery: a surprise in cosmic rays

Carl Anderson and Seth Neddermeyer identified the muon in cosmic-ray observations in 1936; their findings were reported in 1937. Initially, physicists wondered whether this new particle was the carrier of the nuclear force proposed by Hideki Yukawa. But the muon does not participate in the strong interaction. It was instead a previously unsuspected heavier relative of the electron, an early clue that elementary matter comes in generations.

Its place in the Standard Model

The muon is a second-generation charged lepton and a spin-½ fermion. The Standard Model contains three charged leptons (electron, muon, and tau), each paired with a corresponding neutrino, and they fall into first, second, and third generations. The muon experiences electromagnetic and weak interactions (and gravity), but not the strong nuclear interaction. Its mass arises through its coupling to the Higgs field, although the Standard Model does not explain why that coupling has its particular value.

Why does the muon exist?

The Standard Model successfully describes how muons behave, how they are produced, and how they decay. It does not explain why nature has three generations of leptons, why the muon is approximately 207 times heavier than the electron, or why the masses of the generations follow their observed pattern. “Why does the muon exist?” is therefore a genuine question about the deeper structure of particle physics, not one that the current theory answers.

The remaining big questions

• Why three generations? What underlying principle produces the electron, muon, and tau, and their strikingly different masses?
• Is the muon truly identical to the electron apart from mass? Precision tests of lepton universality ask whether their fundamental interactions are exactly as the Standard Model predicts.
• Can a muon change into an electron without neutrinos? Processes such as μ → eγ and coherent μ-to-e conversion in nuclei would reveal charged-lepton flavor violation beyond the extraordinarily tiny Standard Model expectation.
• Does its magnetic moment reveal new physics? The muon’s anomalous magnetic moment, “g−2,” has been measured extraordinarily precisely. The final 2025 Fermilab measurement agrees with newer lattice-QCD-based Standard Model predictions; disagreement among theoretical calculation methods remains an important issue. It is not presently established evidence of new physics.
• Does the muon possess an electric dipole moment? A nonzero intrinsic electric dipole moment at accessible sensitivity could reveal new sources of CP violation.

Muons are continually produced when cosmic rays strike Earth’s atmosphere. Their remarkable penetrating ability also makes them useful for imaging large structures and for probing matter. Above all, the muon offers a precise experimental window into the possibility that the Standard Model is not the final theory of nature.

Selected sources

• CERN, History of the Muon
• CERN, Anderson and Neddermeyer discover the muon
• Fermilab, Muon g−2 final measurement (2025)
• Fermilab, Muon g−2 experiment and publications