Muon is an elementary charged lepton with negative electric charge and spin one half. It is substantially heavier than the electron while having the same magnitude of electric charge and the same fermionic spin. The muon participates in electromagnetic and weak interactions and does not participate directly in the strong interaction because it carries no colour charge. It is unstable and normally decays into an electron or positron together with neutrinos. The muon is therefore a heavier counterpart of the electron within the charged lepton family.
The muon was discovered in cosmic ray experiments in the twentieth century and initially caused surprise because its properties did not fit the expectations of the particles then known. Its mass is approximately 207 times the electron mass. The muon has an antiparticle called the antimuon with opposite electric charge. The existence of the muon helped establish the pattern of multiple lepton generations and contributed to the later development of the Standard Model.
A free negative muon primarily decays through the weak interaction into an electron, an electron antineutrino and a muon neutrino. The corresponding positive antimuon decays into a positron, an electron neutrino and a muon antineutrino. The muons lifetime is approximately 2 point 2 microseconds in its rest frame. Relativistic time dilation allows high energy muons created in the atmosphere to travel substantial distances before decaying, which is why muons produced by cosmic rays can be detected at the surface of Earth.
Muons are important probes of particle physics because their properties can be calculated with high precision within the Standard Model. The anomalous magnetic moment of the muon has been measured experimentally with great precision and compared with theoretical calculations. Differences between experimental and theoretical results have attracted extensive scientific attention because potential discrepancies could indicate contributions from particles or interactions not included in the Standard Model. The interpretation depends on the precision and consistency of both experimental measurements and theoretical calculations.
Muons are produced naturally when cosmic rays interact with the upper atmosphere. They are also produced artificially in accelerator facilities through decays of unstable hadrons. Because they are charged, they can be steered and detected using electromagnetic fields. Their relatively long lifetime compared with many other unstable particles makes them useful for experiments requiring a transported beam. Their high mass also allows them to probe certain physical effects differently from electrons while retaining the advantages of a pointlike elementary lepton.
Muons have practical uses beyond fundamental particle physics. Muon tomography uses the natural flux of cosmic ray muons to investigate the internal structure of large dense objects. Because muons can penetrate substantial amounts of material, their attenuation can reveal information about density and internal structures in volcanoes, geological formations and large engineered objects. Muon based imaging does not require an artificial radiation source in the same way as some conventional imaging techniques, although measurement times and detector requirements depend strongly on the application.
Muons are also relevant to studies of atomic systems and precision measurement. Muonic atoms can form when a muon replaces an electron in an atom. Because the muon is much heavier than the electron, its characteristic orbit is much closer to the nucleus, making muonic atoms sensitive to nuclear properties such as charge distribution. Measurements involving muonic hydrogen have provided important information about proton structure and have also contributed to precision tests of quantum electrodynamics.
Muon should not be confused with an electron simply because both are negatively charged leptons. The muon is much heavier and unstable, while the electron is stable according to established observations. A muon also should not be thought of as an excited electron in the ordinary atomic sense. It is a distinct elementary particle with its own field and quantum numbers. Its associated neutrino is the muon neutrino, which is also distinct from the electron neutrino and tau neutrino even though neutrino oscillations connect the flavour descriptions quantum mechanically.
