The Standard Model of Elementary Particles. Fermilab


Leptons are one of the two classes of elementary particles of matter, the fermions. Leptons are not influenced by the strong interaction, unlike quarks, the other class of fermions.

Leptons are further divided into two major sub-groups:

  • Charged Leptons (or electron-like leptons)
  • Neutral Leptons (or neutrinos)

There are a total of 6 flavors of leptons, and these can be categorized in three generations. Each generation consists of a charged lepton and its associated uncharged lepton, as outlined below (in order of increasing mass):

  • Electron and Electron Neutrino
  • Muon and Muon Neutrino
  • Tau and Tau Neutrino

History of the Leptons

The first lepton, the electron, was discovered in 1897 by J.J. Thomson. It wasn't until 1936 that Carl J. Anderson discovered the muon, though it took a while to realize the particle behaved like a heavier version of an electron. (The muon was actually originally mis-classified as a meson.

At this point, physicists created a new category to classify particles like the electron, muon, and the neutrino (specifically, the electron neutrino), which did not interact through the strong interaction. In 1948, Leon Rosenfeld (building on a suggestion from Christian Moller) coined the term "lepton", which was based on Greek for "fine, small, thin," because the particles known at that time were quote small. As it turns out, this name wasn't really appropriate, because the tau particle is much more massive than a proton ... but no one knew about it at that time, as it was not detected until the 1970's.)