Muon (elementary particle)
A muon is a charged, spin‑1/2 lepton similar to the electron but heavier and unstable. It plays roles in particle physics, cosmic rays, muonic atoms and applications such as imaging and materials studies.
Overview: The muon is an elementary particle in the lepton family. It carries a negative electric charge (charge −1 in units of the proton charge), has intrinsic spin 1/2 (spin-½), and is represented by the symbol μ−. The muon is classified as a lepton, a type of fundamental fermion that does not experience the strong nuclear force.
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Properties and decay
Compared with the electron, the muon is much heavier—about two hundred times the electron mass and close to 105 MeV/c² in energy units—so it behaves like a heavy electron in many respects (comparison to the electron). Unlike the stable electron, an isolated muon is unstable: its average lifetime is about 2.2 microseconds (mean lifetime ≈ 2.2 μs). The dominant decay mode for the negative muon is into an electron, an electron antineutrino, and a muon neutrino (μ− → e− + ν̄e + νμ), a weak-interaction process (decay into lighter particles).
Production and detection
Muons are routinely produced in the upper atmosphere when high-energy cosmic rays strike atomic nuclei, creating showers of secondary particles. Many of these muons reach the Earth’s surface because their high speed increases their apparent lifetime through relativistic time dilation. In laboratories, muons are produced by particle accelerators and studied with detectors sensitive to charged, penetrating particles.
Uses and significance
- Scientific research: Muons are probes in particle physics experiments and precision tests of the Standard Model, including studies of magnetic moments and rare decays.
- Applied techniques: Muon tomography uses cosmic muons to image dense structures such as volcanoes, pyramids, or shipping containers.
- Material science: Muon spin rotation (μSR) techniques exploit implanted muons as local magnetic probes of materials.
Historical and theoretical context
The muon was discovered in cosmic-ray experiments in the 1930s and initially confused with the predicted particle responsible for nuclear forces; careful experiments clarified it as a distinct lepton. In theoretical extensions of the Standard Model that include supersymmetry, the hypothetical scalar partner of the muon is called the smuon, though such particles have not been observed.
Notable distinctions
Muons form short-lived bound states called muonic atoms when they replace an electron in an atom, which produces shifts in atomic energy levels useful for precision measurements. Their combination of charge, penetrating power, and well-understood weak decay makes them valuable both as natural probes from cosmic rays and as controlled tools produced in accelerators. For further foundational reading see general references on elementary particles and leptons (charge, spin, leptons) and experimental reviews (lifetime, decay, electron comparison).
Cosmic rays
Muons are a major component of secondary cosmic rays. This is produced by reactions of the actual cosmic rays (mainly protons coming from space) with atomic nuclei of the upper atmosphere. Most muons are produced in the outer atmosphere: at an altitude of about 10 km, 90 percent of all muons produced in the entire atmosphere have already been produced. The reactions of the primary radiation initially produce pions and, to a lesser extent, kaons; their decay through the weak interaction produces, among other things, muons and muon neutrinos. At sea level, the particle flux density of these "cosmic" muons is around 100 per square meter per second, and the measured ratio μ 
is about 1.27.
At the Auger Observatory in 2016, there was growing evidence of a muon excess in cosmic rays that cannot be explained by standard models of high-energy physics, pointing either to new physics (at primary energies of cosmic rays of 1019 eV in the upper atmosphere, this corresponds to centre-of-mass energies of collision with air molecules of 110 to 170 TeV, ten times the value achievable at the LHC) or to gaps in our understanding of hadronic collision processes.
Detection of muons
With their usually high kinetic energy, muons generate long ionization tracks in matter through many successive collisions, which can be used for detection. Since they usually travel at nearly the speed of light, they generate Cherenkov radiation in water, for example.
Scintillators and semiconductor detectors are also sensitive to muons. The muons from secondary cosmic rays, for example, often account for the main part of the null effect in gamma spectrometers, because they can penetrate several meters of lead due to their high energy and can therefore hardly be shielded in the laboratory.
In particle physics experiments, muons are distinguished from other particles by various techniques:
- By measuring longer tracks, the point of origin and the direction of motion of the muons can be determined.
- By measuring tracks in magnetic fields, the ratio of charge to momentum can be determined. Together with a velocity measurement, the mass of the particle can be concluded.
- The high penetration capacity for matter can also be used for identification.
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AlegsaOnline.com Muon (elementary particle) Leandro Alegsa
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