| muon | |
|---|---|
| Name | Muon |
| Type | Lepton |
| Generation | Second |
| Status | Stable in Standard Model gauge interactions (unstable particle) |
| Mass | 105.6583745 MeV/c² |
| Charge | −1 e (μ−), +1 e (μ+) |
| Spin | 1/2 |
| Discovered | 1936 |
| Discovered by | Carl D. Anderson; Seth Neddermeyer |
muon
The muon is an elementary lepton of the second generation, denoted μ− (muon) and μ+ (antimuon). It behaves like a heavier electron with a spin of 1/2 and couples to the electromagnetic interaction and the weak interaction, making it a crucial probe in studies of Quantum field theory and tests of the Standard Model. Precision measurements of muon properties constrain theories of physics beyond the Standard Model and inform models of astrophysics and cosmic ray interactions.
The muon is a charged elementary particle with rest mass approximately 207 times that of the electron and a mean lifetime of about 2.2 microseconds in its rest frame. As a second-generation lepton, it does not participate in the strong interaction but interacts via the electromagnetic interaction and the weak interaction, and couples to the Higgs boson through its mass. The muon's quantum numbers include lepton family number (muon number) and electrical charge; its intrinsic spin is 1/2, making it a fermion described by the Dirac equation in relativistic quantum mechanics. The particle's behavior is encoded in quantum electrodynamics (QED) and more generally in the electroweak theory.
The muon was first observed in cosmic-ray experiments by Carl D. Anderson and Seth Neddermeyer in 1936, initially confusing early researchers who expected Yukawa's predicted meson. Subsequent work by Patrick M. S. Blackett and others clarified its nature. The muon's identification as a lepton distinct from the pion and its role in weak interactions were established during mid-20th century experiments at facilities such as CERN, Brookhaven National Laboratory, and Fermilab. Key milestones include precision lifetime measurements, early muon capture studies by Eugene Feenberg and collaborators, and the first measurements of the muon magnetic moment by I. I. Rabi-era techniques, which evolved into modern storage-ring experiments like the Muon g−2.
In Quantum field theory, the muon is described by a Dirac field and contributes to loop corrections in quantum electrodynamics and quantum chromodynamics via virtual processes. As part of the Standard Model, muons serve as a clean probe of electroweak symmetry breaking, flavor physics, and lepton universality tests that compare muon behavior with that of electrons and taus. Muon-related observables—such as decay rates, cross sections, and anomalous magnetic moment—are calculated using perturbative techniques and renormalization in QED and the electroweak interaction. Discrepancies between theory and experiment can indicate effects from supersymmetry, leptoquarks, or other beyond the Standard Model scenarios explored at experiments like LHCb and searches at the Large Hadron Collider.
Muon decay proceeds primarily via the charged-current weak interaction: μ− → e− + ν̄_e + ν_μ (and charge-conjugate for μ+). The decay is described by the Fermi theory at low energies and by the electroweak Lagrangian at higher precision; lifetimes and spectra test the V−A structure of the weak force. Lepton flavor conservation (muon number conservation) is an approximate symmetry in the Standard Model; searches for charged lepton flavor violation (CLFV), e.g., μ → eγ or μ → e conversion in nuclei, are sensitive probes of new physics and are pursued by experiments such as MEG, Mu2e at Fermilab, and COMET at J-PARC. Radiative corrections and phase-space factors determine the precise lifetime used in determinations of the Fermi constant, G_F, which underpins electroweak precision tests.
The muon's electromagnetic interactions are summarized by its magnetic moment μ = g(qħ/2m)S, with the anomalous magnetic moment a_μ = (g−2)/2 receiving contributions from QED, electroweak loops, and hadronic vacuum polarization. Precise theoretical calculations involve contributions computed by teams at institutions like Budker Institute of Nuclear Physics, Institute for Advanced Study collaborators, and lattice-QCD groups at CERN and Fermilab. The experimental determination by the Muon g−2 and earlier Brookhaven measurements show a tension with the Standard Model prediction, motivating theoretical work on hadronic light-by-light scattering and inputs from e+e− → hadrons data measured at colliders such as BEPCII and VEPP-2000.
Muons are produced in high-energy collisions, cosmic-ray showers, and in decays of pions and kaons. Accelerator-based sources include proton beam facilities at CERN, Fermilab, J-PARC, and Paul Scherrer Institute. Detection methods exploit ionization energy loss in tracking detectors (wire chamber, silicon detector), Cherenkov radiation, scintillation counters, and muon-specific systems like iron toroidal spectrometers used at collider detectors (ATLAS, CMS). Precision experiments use storage rings, magnetic fields, and timing electronics to measure precession frequencies and lifetimes; neutrino beamlines and muon tomography employ dedicated muon channels and cryogenic targets in some setups.
Muons enable a variety of applications: as probes in precision tests of the Standard Model, in searches for charged lepton flavor violation, and in investigations of nuclear structure via muonic atoms. In astrophysics, atmospheric muons trace cosmic ray interactions and are used in underground observatories like Super-Kamiokande for background estimation. Technological uses include muon tomography for imaging volcanos, pyramids, and industrial structures, pioneered by groups at Los Alamos National Laboratory and University of Tokyo teams. Proposed future applications include muon colliders and neutrino factories, topics studied in accelerator proposals at CERN and by the Muon Accelerator Program.
Category:Leptons Category:Subatomic particles Category:Particle physics