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According to the standard model of particle physics, leptons are one of the two fundamental building blocks of matter, the other being quarks. A lepton is a spin 1/2 elementary particle that is not subject to the strong force (also called the chromodynamic force or color force). In other words, leptons are colorless.

Flavors and families

There are six leptons (apart from their antiparticles), sometimes referred to as flavors of lepton:[1] the electron (e), muon), tau), and their associated neutrinos: the electron neutrinoe), muon neutrinoμ), and tau neutrinoτ). The leptons are divided into families, the e, μ, τ families.[2]

Electric charge

Leptons can possess electric charge as in the case of the electron, muon, and tau (all negatively charged), and the corresponding antileptons (all positively charged), or can be electrically charge-neutral like the three flavors of neutrino and the corresponding antineutrinos.[3][4]

Lepton number

Each family of leptons is assigned its own lepton number which is one for that family and zero for the others, and for all particles that are not leptons. For example, the electron and its neutrino are given an electron lepton number Le = 1, while all other particles have Le = 0. The antiparticles have the opposite sign of lepton number, i.e. either -1 or 0.

An over-all lepton number L is assigned to assemblies of leptons that is the sum of all the constituent lepton numbers,

Lepton number L is conserved in particle reactions, as in:

which conserves both L and individual family lepton numbers. Violations of this conservation law, called LFV for lepton flavor violation, have yet to be observed.


Having spin 1/2, all leptons are fermions,[5] and as such are subject to Fermi-Dirac statistics and the Pauli exclusion principle.

See also

The standard model page


  1. Mark Allen Srednicki (2007). “Table 88.1: The six flavors of lepton.”, Quantum Field Theory. Cambridge University Press, p. 549. ISBN 0521864496. 
  2. WN Cottingham, DA Greenwood (2007). “Chapter 9: The weak interaction: low energy phenomenology”, An introduction to the Standard Model of particle physics, 2nd ed. Cambridge University Press, p. 91. ISBN 978-0-521-85249-4. 
  3. WN Cottingham, DA Greenwood (2007). “Table 1.2: Leptons”, An introduction to the Standard Model of particle physics, 2nd ed. Cambridge University Press, p. 3. ISBN 978-0-521-85249-4. 
  4. Helen R. Quinn, Yossi Nir (2010). The Mystery of the Missing Antimatter. Princeton University Press, p. 98. ISBN 1400835712. 
  5. Don Lincoln (2004). Understanding the universe: from quarks to the cosmos. World Scientific, p. 143. ISBN 9812387056.