Muon
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Image:Moons shodow in muons.gif In the Standard Model of particle physics, a muon (from Greek letter mu (μ) used to represent it) is a semistable fundamental particle with negative electric charge and a spin of 1/2. Together with the electron, the tau lepton and the neutrinos, it is classified as part of the lepton family of fermions. Like all fundamental particles, the muon has an antimatter partner of opposite charge but equal mass and spin: the antimuon.
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 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 great mass, muons do not emit as much bremsstrahlung radiation; consequently, they are much more penetrating than electrons. Muons are denoted by μ− and antimuons by μ+.
On earth, muons are created when a charged pion decays. The pions are created in the upper atmosphere by cosmic radiation and have a very short decay time — a few nanoseconds. The muons created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, muons in the atmosphere are moving at very high velocities, so that the time dilation effect of special relativity make them easily detectable at the earth's surface.
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 νμ. Muons naturally decay into an electron, an electron-antineutrino, and a muon-neutrino.
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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. 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 one or the other as was observed in other mesons. Thus mu mesons were not mesons at all, and so the term mu meson was abandoned and replaced with the modern term muon.
In the mid 1970s, experimental physicists devised experiments firing neutrinos at a proton target. According to what was then known about the weak interaction, they expected the collision to turn the neutrino into a muon, and the proton into debris. They were surprised to discover that two muons, one negatively and one positively charged, result from such collision.
This generated a good deal of theoretical discussion, until a consensus emerged on how that positive muon comes about. The neutrino/proton collision produces not only proton debris and a negative muon, but a charm quark, and the quark soon decays into a strange quark, a muon neutrino, and a positive muon.
See also
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 PDF.
- 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) [ISBN 3-7643-0579-7]. Full text available in PostScript.
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| Fermions: Quarks | Leptons | |
| Quarks: Up | Down | Strange | Charm | Bottom | Top | |
| Leptons: Electron | Muon | Tau | Neutrinos | |
| Gauge bosons: Photon | W and Z bosons | Gluons | |
| Not yet observed: Higgs boson | Graviton | Other hypothetical particles | |
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