Muon

Lore & Background

Muons were discovered by Carl D. Anderson noticed particles that curved differently from electrons and other known particles when passed through a magnetic field; they were negatively charged but curved less sharply than electrons, suggesting a mass between that of an electron and a proton. Anderson initially called the new particle a mesotron. C. Street and E. C. Stevenson's cloud chamber experiment. Because of its mass, the mu meson was initially thought to be the particle predicted by theorist Hideki Yukawa, and some scientists, including Niels Bohr, originally named it the yukon. Yukawa's predicted particle, the pi meson, was finally identified in 1947. With the development of the Standard Model in the 1970s, muons were recognized as fundamental leptons, not composed of quarks, and the term mu meson was abandoned. The muon was the first particle of the second generation of elementary particles to be discovered.

Reader's Guide

The muon's significance lies in its role as a fundamental lepton, providing a heavier counterpart to the electron and serving as a key particle in the second generation of the Standard Model. Its discovery challenged existing theories, as it was initially mistaken for Yukawa's predicted meson mediating the nuclear force, but experiments later showed it did not interact via the strong nuclear force. This led to the famous quip by Nobel laureate I. I. Rabi: 'Who ordered that?', reflecting the surprise at finding a seemingly unnecessary heavy electron. Muons are produced in high-energy interactions, such as cosmic rays hitting the atmosphere, where pions decay into muons. Because of their greater mass, muons emit less bremsstrahlung and can penetrate far deeper into matter than electrons, reaching Earth's surface and even deep mines. This property makes them useful for studying relativistic effects like time dilation, as demonstrated in the Rossi–Hall experiment. The muon's legacy includes clarifying the distinction between leptons and hadrons, and its decay properties—always producing an electron and two neutrinos—helped refine the understanding of the weak interaction. Today, muons are used in particle physics experiments and in applications such as muon tomography, though the source article does not detail the latter.

Did You Know?

• Muons were originally called mu mesons, but are no longer classified as mesons by modern particle physicists.
• The muon's mean lifetime is 2.2 μs, and its decay always produces an electron (or positron) and two types of neutrinos.