Particle Physics
The muon is an elementary particle similar to the electron, with a negative electric charge of −1 e and a spin of 1/2, but with a mass approximately 207 times that of the electron.1 It is classified as a lepton, along with the electron, the tau, and their associated neutrinos. The muon is unstable, decaying with a mean lifetime of about 2.2 microseconds into an electron and two neutrinos.2 Because of its relatively long lifetime and high penetrating power, muons are the most numerous charged particles at sea level, originating from cosmic-ray interactions in the upper atmosphere.3 The muon was discovered in 1936 by Carl D. Anderson and Seth Neddermeyer while studying cosmic rays, and its existence initially puzzled physicists, leading Isidor Isaac Rabi to famously quip, "Who ordered that?"4
The muon is a fundamental particle in the Standard Model, classified as a lepton with a spin of 1/2, making it a fermion. It has no known substructure and is point-like. Its mass is approximately 105.66 MeV/c², which is about 206.77 times the electron mass.1 The muon participates in electromagnetic, weak, and gravitational interactions, but not in strong interactions. It has an associated neutrino, the muon neutrino, which is distinct from the electron neutrino. The muon's magnetic moment is a key property, and its anomalous part (the deviation from the Dirac value) is predicted by the Standard Model with high precision, providing a sensitive test for new physics.5
Muons are produced abundantly in the Earth's atmosphere when cosmic rays—high-energy protons and nuclei from space—collide with air molecules, creating pions that decay into muons and neutrinos. Because of their high energy and relatively long lifetime, muons can travel several kilometers through the atmosphere and penetrate deep underground before decaying.3 At sea level, the flux of muons is about 1 per square centimeter per minute, making them the most numerous charged particles at the surface. Their penetrating ability is exploited in muon tomography, a technique used to image the interiors of large structures such as pyramids, volcanoes, and nuclear reactors, by measuring the absorption of muons.6
The muon decays via the weak interaction, predominantly into an electron, an electron antineutrino, and a muon neutrino, with a mean lifetime of about 2.2 microseconds.2 This decay is a classic example of a three-body decay and is used to study the weak force. Muons are also used in particle accelerators, such as the planned Muon Collider, where they could provide a clean environment for high-energy collisions. In addition, muons are used in muon spin rotation (μSR) spectroscopy, a technique that probes magnetic fields in materials by implanting spin-polarized muons and observing their precession.7 The muon's magnetic moment anomaly (g−2) has been measured with high precision at Fermilab, showing a significant deviation from Standard Model predictions, hinting at possible new physics.5
The muon was initially mistaken for the meson predicted by Hideki Yukawa to mediate the strong force, but it was later found to be a lepton, leading to Rabi's quip.4 Muons are also produced in the atmosphere by neutrino interactions, and they can be used to study neutrino oscillations. In addition, muons are a major background in dark matter detectors, and their decay can produce rare processes such as muon-to-electron conversion, which is a target of experiments like Mu2e. The muon's lifetime is affected by its environment: in a bound state like muonium (a muon-electron atom), the lifetime is slightly different due to the weak interaction.8 Furthermore, muons have been used to date archaeological artifacts via muon radiography, and they are a source of radiation exposure for air travelers and astronauts.6
The muon's discovery and properties continue to challenge physicists, making it a key probe for beyond-Standard-Model physics.
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