A muon (/ˈm(j)uː.ɒn/ M(Y)OO-on; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 e and spin-1/2, but with a much greater mass.

Due to their greater mass, muons accelerate more slowly than electrons in electromagnetic fields, and emit less bremsstrahlung (deceleration radiation).

Thus Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for "mid-".

Because of its mass, the mu meson was initially thought to be Yukawa's particle and some scientists, including Niels Bohr, originally named it the yukon.

The fact that the mesotron (i.e. the muon) was not Yukawa's particle was established in 1946 by an experiment conducted by Marcello Conversi, Oreste Piccioni, and Ettore Pancini in Rome.

In this experiment, which Luis Walter Alvarez called the "start of modern particle physics" in his 1968 Nobel lecture,[9] they showed that the muons from cosmic rays were decaying without being captured by atomic nuclei, contrary to what was expected of the mediator of the nuclear force postulated by Yukawa.

The eventual recognition of the muon as a simple "heavy electron", with no role at all in the nuclear interaction, seemed so incongruous and surprising at the time, that Nobel laureate I. I. Rabi famously quipped, "Who ordered that?

[12] About 10,000 muons reach every square meter of the earth's surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere.

Traveling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms.

The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light.

From the viewpoint (inertial frame) of the muon, on the other hand, it is the length contraction effect of special relativity that allows this penetration, since in the muon frame its lifetime is unaffected, but the length contraction causes distances through the atmosphere and Earth to be far shorter than these distances in the Earth rest-frame.

Since muons are unusually penetrative of ordinary matter, like neutrinos, they are also detectable deep underground (700 meters at the Soudan 2 detector) and underwater, where they form a major part of the natural background ionizing radiation.

Antimuons, in mirror fashion, most often decay to the corresponding antiparticles: a positron, an electron neutrino, and a muon antineutrino.

[15] Certain neutrino-less decay modes are kinematically allowed but are, for all practical purposes, forbidden in the Standard Model, even given that neutrinos have mass and oscillate.

The values of these four parameters are predicted unambiguously in the Standard Model of particle physics, thus muon decays represent a good test of the spacetime structure of the weak interaction.

More generally in the Standard Model, all charged leptons decay via the weak interaction and likewise violate parity symmetry.

[19] The results of these measurements diverged from the then accepted value giving rise to the so called proton radius puzzle.

Therefore this bound muon-electron pair can be treated to a first approximation as a short-lived "atom" that behaves chemically like the isotopes of hydrogen (protium, deuterium and tritium).

For this reason, the muon's anomalous magnetic moment is normally used as a probe for new physics beyond the Standard Model rather than as a test of QED.

[29] In 2020 an international team of 170 physicists calculated the most accurate prediction for the theoretical value of the muon's anomalous magnetic moment.

[36] The current experimental limit on the muon electric dipole moment, |dμ| < 1.9 × 10−19 e·cm set by the E821 experiment at the Brookhaven, is orders of magnitude above the Standard Model prediction.

The observation of a non-zero muon electric dipole moment would provide an additional source of CP violation.

One example is commercial muon tomography used to image entire cargo containers to detect shielded nuclear material, as well as explosives or other contraband.

[37] The technique of muon transmission radiography based on cosmic ray sources was first used in the 1950s to measure the depth of the overburden of a tunnel in Australia[38] and in the 1960s to search for possible hidden chambers in the Pyramid of Chephren in Giza.

[40] In 2003, the scientists at Los Alamos National Laboratory developed a new imaging technique: muon scattering tomography.

With muon scattering tomography, both incoming and outgoing trajectories for each particle are reconstructed, such as with sealed aluminum drift tubes.

In August 2014, Decision Sciences International Corporation announced it had been awarded a contract by Toshiba for use of its muon tracking detectors in reclaiming the Fukushima nuclear complex.

[42] The Fukushima Daiichi Tracker was proposed to make a few months of muon measurements to show the distribution of the reactor cores.

[43] The International Research Institute for Nuclear Decommissioning IRID in Japan and the High Energy Accelerator Research Organization KEK call the method they developed for Unit 1 the "muon permeation method"; 1,200 optical fibers for wavelength conversion light up when muons come into contact with them.

[44] After a month of data collection, it is hoped to reveal the location and amount of fuel debris still inside the reactor.