muon

The muon is a fundamental particle in the Standard Model, a charged lepton of the second generation. It has the same electric charge as the electron but about 207 times greater mass (mass 105.658 MeV/c^2). It has spin 1/2 and participates in weak and electromagnetic interactions, but not in the strong interaction. As a result, muons decay via the weak interaction with a mean lifetime of about 2.196 microseconds in their rest frame, into an electron (or positron) and two neutrinos: μ^- → e^- + ν̄_e + ν_μ, μ^+ → e^+ + ν_e + ν̄_μ.

Most muons are produced when cosmic rays strike nuclei in Earth's atmosphere, creating pions that decay to muons; they are also produced in particle accelerators and in the upper atmosphere. Relativistic time dilation allows many muons to reach the Earth's surface before decaying. Muons lose energy mainly by ionization as they traverse matter and can penetrate substantial thickness of material, more than electrons do. In detectors, muons are often identified by their long tracks and minimal hadronic interactions.

Muons were discovered in 1936 in cosmic rays by Carl D. Anderson and Seth Neddermeyer, initially called the mu-meson before its lepton character was established. They remain a useful probe in both fundamental and applied contexts: muon tomography (muography) is used to image dense objects, and muon spin rotation (μSR) studies reveal material properties. Muons are also central to precision tests of the Standard Model, notably measurements of the muon anomalous magnetic moment.

The muon has a corresponding neutrino flavor, the muon neutrino ν_μ, and is the second-generation charged lepton; heavier third-generation tau lepton exists, with its own neutrino.