Muon

Related Topics:
Muonio

In the Standard Model of particle physics, a muon (from the letter mu (μ) used to represent it) is a fundamental particle with negative electric charge and a spin of 1/2. It has a lifetime of 2.2μs, longer than any other unstable lepton, meson or baryon except for the neutron. Together with the electron, the tau and the neutrinos, it is classified as a lepton. Like all fundamental particles, the muon has an antimatter partner of opposite charge but equal mass and spin: the antimuon. Muons are denoted by μ⁻ and antimuons by μ⁺.

For historical reasons, muons are sometimes referred to as mu mesons, even though they are not classified as mesons by modern particle physicists (see History). Muons have a rest mass of 105.6 MeV/c², which is 207 times the electron mass. Since their interactions are very similar to that of the electron, a muon can often be thought of as an extremely heavy electron. However, due to their much greater mass, muons do not emit as much bremsstrahlung radiation; consequently, they are much more penetrating than electrons.

As with the case of the other charged leptons, there is a muon-neutrino which has the same flavor as the muon. Muon-neutrinos are denoted by ν_μ.

Muon sources

On earth, most naturally occuring muons are created by cosmic rays. When a cosmic ray impacts the upper atmosphere, pions are created. They decay into muons and neutrinos. These muons are moving at very high velocities, so despite their short lifetime, the time dilation effect of special relativity makes them easily detectable at the earth's surface.

This same reaction is used by particle physicists to produce muon beams such as the one which is used for the g-2 experiment.

Muon decays

Since muons are the lightest charged particle except for the electron, they must decay to an electron and other particles with a net charge of zero. Nearly all of the time, they decay into an electron, an electron-antineutrino, and a muon-neutrino. Antimuons decay to a positron, an electron-neutrino, and a muon-antineutrino:

\mu^-\to e^-\bar\nu_e\nu_\mu,~~~\mu^+\to e^+\nu_e\bar\nu_\mu

Rarely, a photon or electron-positron pair is also present in the decay products.

Muonic atoms

The muon was the first elementary particle discovered that does not appear in ordinary atoms. Negative muons can, however, form muonic atoms by replacing an electron in ordinary atoms. Muonic atoms are much smaller than typical atoms because the larger mass of the muon gives it a smaller ground-state wavefunction than the electron.

A positive muon, when stopped in ordinary matter, can also bind an electron and form the muonium (Mu) "atom," in which the muon acts as the "nucleus." Such substances do not actually fall under the formal definition of the chemical atom, though they share some properties. The reduced mass of muonium, hence its Bohr radius, is very close to that of hydrogen, hence this short lived "atom" behaves chemically − in first approximation − like its heavier isotopes, hydrogen, deuterium and tritium.

History

Muons were discovered by Carl D. Anderson in 1936 while he studied cosmic radiation. He had noticed particles that curved in a manner distinct from that of electrons and other known particles when passed through a magnetic field. In particular, these new particles curved to a smaller degree than electrons, but more sharply than protons. It was assumed that their electric charge was equal to that of the electron, and so to account for the difference in curvature, it was supposed that these particles were of intermediate mass (lying somewhere between that of an electron and that of a proton).

For this reason, Anderson initially called the new particle a mesotron, adopting the prefix meso- from the Greek word for "intermediate". Shortly thereafter, additional particles of intermediate mass were discovered, and the more general term meson was adopted to refer to any such particle. Faced with the need to differentiate between different types of mesons, the mesotron was renamed the mu meson (with the Greek letter μ (mu) used to approximate the sound of the Latin letter m).

However, it was soon found that the mu meson significantly differed from other mesons; for example, its decay products included a neutrino and an antineutrino, rather than just one or the other, as was observed in other mesons. Other mesons were eventually understood to be hadrons, that is, particles made of quarks, and thus subject to the residual strong force. In the quark model, each meson is composed of two quarks. Mu mesons, however, were found to be fundamental particles (leptons) like electrons, with no quark structure. Thus mu mesons were not mesons at all (in the new sense of the term), and so the term mu meson was abandoned, and replaced with the modern term muon.

External links

References

S.H. Neddermeyer and C.D. Anderson, "Note on the Nature of Cosmic-Ray Particles", Phys. Rev. 51, 884–886 (1937). Full text available in ^*.

Serway & Faughn, College Physics, Fourth Edition (Fort Worth TX: Saunders, 1995) page 841

Emanuel Derman, My Life As A Quant (Hoboken, NJ: Wiley, 2004) pp. 58-62.

Marc Knecht ; The Anomalous Magnetic Moments of the Electron and the Muon, Poincaré Seminar (Paris, Oct. 12, 2002), published in : Duplantier, Bertrand; Rivasseau, Vincent (Eds.) ; Poincaré Seminar 2002, Progress in Mathematical Physics 30, Birkhäuser (2003) 3-7643-0579-7. Full text available in PostScript.

Leptons