Lepton

In physics, a lepton is a sub-atomic particle with spin of 1/2 that does not experience the strong interaction (that is, the strong nuclear force). The leptons form a family of fermions that are distinct from the other known family of fermions, the quarks.

Properties of leptons
There are three known flavors of lepton: the electron, the muon, and the tau lepton or tau (or sometimes tauon). Each flavor is represented by a pair of particles called a weak doublet. One is a massive charged particle that bears the same name as its flavor (like the electron). The other is a nearly massless neutral particle called a neutrino (such as the electron neutrino). All six of these particles have corresponding antiparticles (such as the positron or the electron antineutrino). All known charged leptons have a single unit of negative or positive electric charge (depending on whether they are particles or antiparticles) and all of the neutrinos and antineutrinos have zero electric charge. The charged leptons have two possible spin states, while only one helicity is observed for the neutrinos (all the neutrinos are left-handed, and all the antineutrinos are right-handed).

The masses of the leptons also obey a simple relation, known as the Koide formula, but at present this relationship cannot be explained.

When particles interact, generally the number of leptons of the same type (electrons and electron neutrinos, muons and muon neutrinos, tau leptons and tau neutrinos) remains the same. This principle is known as conservation of lepton number. Conservation of the number of leptons of different flavors (for example, electron number or muon number) may sometimes be violated (as in neutrino oscillation). A much stronger conservation law is the total number of leptons of all flavors, which is violated by a tiny amount in the Standard Model by the so-called chiral anomaly.

The couplings of the leptons to gauge bosons are flavor-independent. This property is called lepton universality and has been tested in measurements of the tau and muon lifetimes and of Z-boson partial decay widths, particularly at the Stanford Linear Collider and Large Electron-Positron Collider(LEP) experiments.

Table of the leptons

 * {| class="wikitable"

! Name ! Symbol ! Electric charge (e) ! Mass (MeV/c2) ! ! Name ! Symbol ! Electric charge (e) ! Mass (MeV/c2)
 * colspan=4 align=center | Charged lepton / antiparticle
 * colspan=4 align=center | Neutrino / antineutrino
 * colspan=4 align=center | Neutrino / antineutrino
 * colspan=4 align=center | Neutrino / antineutrino
 * Electron / Positron
 * −1 / +1
 * 0.511
 * Electron neutrino / Electron antineutrino
 * 0
 * < 0.0000022
 * Muon
 * −1 / +1
 * 105.7
 * Muon neutrino / Muon antineutrino
 * 0
 * < 0.17
 * Tau lepton
 * −1 / +1
 * 1777
 * Tau neutrino / Tau antineutrino
 * 0
 * < 15.5
 * }
 * < 0.17
 * Tau lepton
 * −1 / +1
 * 1777
 * Tau neutrino / Tau antineutrino
 * 0
 * < 15.5
 * }
 * 0
 * < 15.5
 * }
 * }
 * }

Note that the neutrino masses are known to be non-zero because of neutrino oscillation, but their masses are sufficiently light that they have not been measured directly as of 2008. However there have been measured (indirectly based on the oscillation periods) the differences of the mass squares between the neutrinos, which have been estimated $$\Delta m^2_{12} = 80{meV}^2$$ and $$\Delta m^2_{23} \approx \Delta m^2_{13} = 2400{meV}^2$$. This leads to the following conclusions: The names "mu" and "tau" seem to have been selected due to their places in the Greek alphabet; &mu; is seven letters after &epsilon;, whereas &tau; is seven letters after &mu;.
 * and are lighter than 2.2 eV (as  is and the mass differences between the neutrinos are of order of millielectronvolts)
 * one (or more) of the neutrinos is heavier than 0.040 eV
 * two (or three) of the neutrinos are heavier than 0.008 eV

Etymology
According to the Oxford English Dictionary, the name "lepton" (from Greek leptos meaning 'thin') was first used by physicist Léon Rosenfeld in 1948:
 * Following a suggestion of Prof. C. Møller, I adopt — as a pendant to "nucleon" — the denomination "lepton" (from λεπτός, small, thin, delicate) to denote a particle of small mass.

The name originates from before the discovery in the 1970s of the heavy tau lepton, which is nearly twice the mass of a proton.