"A heavy counterpart to the electron, a lepton with a mass 206.7683 times that of the electron, but otherwise identical to an electron. The muon is unstable, with a half life of 2.19709 microseconds, and decays into an electron, a muon neutrino and an electron antineutrino." - p. 245 of John Gribbins Enclyclopedia of Particle Physics, Q is for Quantum

The muon was discovered accidentally in 1936-1937 in the hunt for Yukawa's pion. At the time it seemed to serve no purpose in the scheme of particle physics. I.I. Rabi asked, "Who ordered that?", when he was told of it. It proved to be only the first of a soon to be huge number of anomalous particles that forced many new theories to be developed. Physicists learned that many of the particles previously considered fundamental, such as the proton and neutron, are actually composite objects. However, when the dust finally settled and the Standard Model was born, the muon retained it's status as fundamental, as the charged lepton in the second generation of elementary particles.

[Ed. note: Corrected muon discovery date 10/19/2001]
The muon is a member of the class of particles known as 'leptons' and was discovered in 1935 by Carl Anderson and Seth Neddermeyer looking at cosmic rays (the paper was published in 1936). Anderson commented on the cost of the project to find the muon at Caltech in the early days of subatomic physics - "To find the positive electron and the two muons cost about $15,000."

These objects appear to be point-like particles and appear to lack an internal structure (unlike the proton and neutron which have definite internal structures do not behave as points). The muon is a charged lepton - similar to that of the electron, however it is much more massive (the electron has a mass of 0.000511 GeV/c2 while the muon has a mass of 0.106 GeV/c2). The distinctions between leptons are known as 'flavor' and have absolutely nothing to do with how they are perceived - its just one of those words physicists use to designate an attribute.

Like quarks, leptons are a member of the class of particles known as fermions. All of these particles have fractional spins and may carry an electric charge. Leptons have whole electric charges and can be seen solitary, while quarks have fractional charges and are only seen in combination with other quarks forming hadrons.

Work has been done (especially in the area of fusion - known as muon-catalyzed cold fusion often abbreviated mu-c-f) at making atoms with a muon instead of an electron. The primary advantage here is that the muon is much closer to the nucleus than the electron and super high temperatures are not necessary to remove the electron from the atom. This results in 'screening' the repulsive charge from the two atoms and allowing them to get close enough together that the strong force will cause the two nuclei to fuse. When fusion occurs, the muon is kicked out of the atom from the resulting energy and may go on to allow other atoms to fuse - thus the muon-catalyzed fusion.

However, problems arise once the muon reaches the ground state (1s) of the atom. As it moves to the ground state (the muon atom is created in an excited state) the muon will be giving off x-rays and causing electrons to leave the atom (known as Auger electrons). Typically lasting for about 70 nanoseconds (for gold) 2 microseconds (for helium), the muon will either decay into two neutrinos and an electron, or it will be captured by the nucleus to combine with a proton and result in a neutron and neutrino. Furthermore, the muon will often "stick" to the helium atom formed and would then be 'lost' to the fusion process. Understanding the molecular resonances of the cycle researchers have been able to use a single muon for 150 fusions. While this has reached the breakeven point it is not a viable source of energy until the 'sticky' muon problem is overcome or the energy cost of muon production is greatly reduced.

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