Muon

The muon is an elementary particle belonging to the lepton family in the Standard Model of particle physics. Denoted by μ⁻, it has the same negative electric charge as the electron but approximately 207 times its mass. Its antiparticle, the positive muon or antimuon, is denoted by μ⁺. Muons are unstable: a free muon has a mean lifetime of approximately 2.197 microseconds before decaying into lighter particles. They occur naturally in cosmic-ray showers and can also be produced using accelerators. (pdg.lbl.gov)

Physical properties and classification

The muon has a rest mass of approximately 105.658 MeV/c², where MeV is one million electronvolts and c is the speed of light. It has spin 1/2, making it a fermion. Within the Standard Model, it is the charged lepton of the second generation, paired with the muon neutrino. The electron and tau are the corresponding charged leptons of the first and third generations. (pdg.lbl.gov)

Muons participate in electromagnetic and weak interactions. Unlike quarks, they carry no color charge and do not couple directly to the strong interaction. They are treated as elementary rather than as composite particles. Consequently, the historical name “mu-meson” is misleading: a meson is a strongly interacting composite particle, whereas a muon is a lepton. (psi.ch)

Discovery and historical significance

Carl D. Anderson and Seth Neddermeyer identified the muon in 1936 while investigating cosmic radiation. Their observations revealed particles whose behavior differed from that of electrons, including their curvature in magnetic fields. The discovery introduced a heavier electron-like particle into the emerging inventory of subatomic matter. It preceded the modern classification of particles into lepton families and generations. (muoncollider.web.cern.ch)

Muon research subsequently became closely associated with tests of relativistic motion. Cosmic-ray observations and accelerator experiments made it possible to compare the decay of rapidly moving muons with their lifetime at rest, providing experimental tests of special relativity. (cds.cern.ch)

Production, decay, and lifetime

Natural muons are secondary particles produced after energetic cosmic rays strike nuclei in the Earth’s atmosphere. These collisions create particle showers containing pions, whose decays produce muons. Accelerator facilities use a related process: a beam of high-energy protons strikes a target, producing pions that decay into muons. Magnetic systems then select and transport the resulting muon beam. (muoncollider.web.cern.ch)

The principal decay channels are

Here, ν represents a neutrino, a bar denotes an antineutrino, and e⁺ is a positron. The decay is mediated by the weak interaction. Because three particles share the available energy and momentum, the emitted electron or positron does not have a single fixed energy. Detailed measurements of its energy and angular distributions test the structure of the weak interaction. (neutrinos.fnal.gov)

The quoted lifetime is a mean lifetime, not a fixed expiration time for every muon. For free muons at rest, the surviving population follows approximately

with τ ≈ 2.197 μs. For a muon moving at speed v relative to the laboratory, time dilation gives a laboratory mean lifetime γτ, where

This increased lifetime allows many atmospheric muons to reach the ground. Measurements in CERN’s muon storage ring directly confirmed the relativistic lifetime increase. (psi.ch)

Interaction with matter and bound states

Energetic muons can penetrate substantial amounts of matter, making them useful for examining structures inaccessible to conventional surface observations. Muon tomography, also called muography, reconstructs information about an object from the absorption or scattering of muons passing through it. Depending on the technique, measurements constrain the material encountered along trajectories or reveal internal variations in density and composition. (arxiv.org)

A negative muon can replace an electron in an atom, forming a muonic atom. Its much greater mass produces substantially smaller orbits, bringing its wave function closer to the atomic nucleus. Transition energies therefore depend sensitively on nuclear charge distributions. Measurements of muonic hydrogen and other muonic atoms, combined with theoretical calculations, provide precise determinations of nuclear charge radii. (psi.ch)

Research applications

Muon magnetic properties provide sensitive tests of fundamental physics. Its anomalous magnetic moment is conventionally expressed as $a_\mu=(g_\mu-2)/2$. Experiments determine this quantity by measuring spin precession in a carefully controlled magnetic field. Comparison with Standard Model calculations probes the contributions of known interactions and can constrain possible additional particles or forces. Fermilab’s Muon g−2 experiment announced its final measurement on June 3, 2025. (news.fnal.gov)

In muon spin spectroscopy, polarized positive muons are implanted into a material. The angular distribution of their decay positrons records how their spin polarization evolves. This makes muons local probes of magnetic fields and magnetic fluctuations, with applications to magnetism, superconductivity, thin films, and interfaces. (psi.ch)

Muon colliders are a proposed application of muon beams. Compared with electrons, muons lose far less energy through synchrotron radiation in circular accelerators. Their short lifetime, however, imposes demanding requirements on beam production, preparation, and rapid acceleration. (home.cern)