The Other Leptons


In 1936 Anderson made another major breakthrough in particle physics. Again, using his cloud chamber he observed a new particle with a charge equal to that of a electron but with one ninth the mass of a proton, much heavier than an electron. This particle became known as the muon. The muon, there is also an anti-muon, has the same charge, spin, magnetic field as an electron. The muon can even replace an electron in an atom to produce a muonic atom. The only other major difference is that muons break down into an electron and neutrinos. The average life is one five hundred thousandth of a second before this breakdown occurs.

A major question that pestered physicists as they examined the muon is how did it break down? Muons have a ±½ spin. Electrons and one neutrino have a combined spin of ±1 or 0 so with only one neutrino they cannot have a sum of ±½. Therefore, muons must break up into an electron and 2 antineutrinos but then the Lepton number is not conserved. With this it was found that muons must break down into an electron, a neutrino, and an antineutrino. The most important issue with these neutrinos is that it was discovered that there is an intrinsic difference between muon neutrinos and electron neutrinos. In 1961, Leon Lederman ran an experiment to see what the neutrinos of the decay of muons would produce when they interacted with other muon neutrinos. While there is no appreciable difference between electron neutrinos and muon neutrinos it was found that muon neutrinos would produce only muons and electron neutrinos would produce only electrons. For this Lederman won the Nobel Prize in 1988.

To solve this problem Physicists decided that they should assign electron and muon numbers in a similar fashion to Baryon and Lepton numbers.

Particle Electron # Muon #
Electron +1 0
Positron -1 0
Electron Neutrino +1 0
Electron Antineutrino -1 0
Muon 0 +1
Antimuon 0 -1
Muon Neutrino 0 +1
Muon Antineutrino 0 -1


There has also been the discovery of the Tauon, a Lepton that little is know about but it is thought to have similar properites to the other Leptons.(Asimov 250) It is known to have a mass 3650 times that of an electron, an average life of 3.4 x 10-13 seconds, the same charge, and an antiparticle. (University of Hertfordshire "Tauon") With these three base particles, their neutrinos, antiparticles, and antineutrinos we think we know of all twelve leptons that exist.

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