Lepton - History

History

See also: Electron#Discovery, Muon#History, and Tau (particle)#History

Lepton nomenclature

Particle name	Antiparticle name
Electron	Antielectron Positron
Electron neutrino	Electron antineutrino
Muon Mu lepton Mu	Antimuon Antimu lepton Antimu
Muon neutrino Muonic neutrino Mu neutrino	Muon antineutrino Muonic antineutrino Mu antineutrino
Tau Tau lepton Tauon	Antitau Antitau lepton Antitau
Tau neutrino Tauonic neutrino Tau neutrino	Tau antineutrino Tauonic antineutrino Tau antineutrino

The first lepton identified was the electron, discovered by J.J. Thomson and his team of British physicists in 1897. Then in 1930 Wolfgang Pauli postulated the electron neutrino to preserve conservation of energy, conservation of momentum, and conservation of angular momentum in beta decay. Pauli theorized that an undetected particle was carrying away the difference between the energy, momentum, and angular momentum of the initial and observed final particles. The electron neutrino was simply called the neutrino, as it was not yet known that neutrinos came in different flavours (or different "generations").

Nearly 40 years after the discovery of the electron, the muon was discovered by Carl D. Anderson in 1936. Due to its mass, it was initially categorized as a meson rather than a lepton. It later became clear that the muon was much more similar to the electron than to mesons, as muons do not undergo the strong interaction, and thus the muon was reclassified: electrons, muons, and the (electron) neutrino were grouped into a new group of particles – the leptons. In 1962 Leon M. Lederman, Melvin Schwartz and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of the muon neutrino, which earned them the 1988 Nobel Prize, although by then the different flavours of neutrino had already been theorized.

The tau was first detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC LBL group. Like the electron and the muon, it too was expected to have an associated neutrino. The first evidence for tau neutrinos came from the observation of "missing" energy and momentum in tau decay, analogous to the "missing" energy and momentum in beta decay leading to the discovery of the electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed, apart of the Higgs boson, which probably has been discovered in 2012.

Although all present data is consistent with three generations of leptons, some particle physicists are searching for a fourth generation. The current lower limit on the mass of such a fourth charged lepton is 100.8 GeV/c2, while its associated neutrino would have a mass of at least 45.0 GeV/c2.