# Lepton

The elementary particles in the Standard Model: violet: quarks; green: leptons; red: exchange particles; yellow: Higgs boson

Leptons are a class of elementary particles that, together with quarks and gauge bosons, form the fundamental building blocks of matter. In the standard model of elementary particles, a conservation law applies to the number of leptons (see number of leptons ).

The name was derived from the Greek λεπτός leptós 'thin', 'small', 'fine'. It is selected to differentiate it from two other particle classes, the mesons (“medium- weight”) and the baryons (“heavy-weight”). Mesons and baryons are hadrons . As it turned out, these hadrons are composed of two quarks (mesons) or three quarks (baryons) each, so they are not elementary. Among the leptons there are also particles that are by no means “light”. For example, the τ-lepton (or tauon ) is about twice as heavy as a proton . At the time the name was given, the rope was still unknown.

There are a total of six types of leptons, which are divided into three so-called generations based on their physical properties . The following table summarizes the properties of the leptons.

generation
ration
Surname sym
bol
Electric
charge
(s)
Mass
( MeV )
( s )
1 electron ${\ displaystyle e}$ −1 0.511 ${\ displaystyle \ infty}$ (stable)
Electron neutrino ${\ displaystyle \ nu _ {e}}$   0 <2 · 10 −6 ${\ displaystyle \ infty}$ (stable)
2 Muon ${\ displaystyle \ mu}$ −1 105.66 2.197 · 10 −6
Muon neutrino ${\ displaystyle \ nu _ {\ mu}}$   0 <0.17 ${\ displaystyle \ infty}$ (stable)
3 Tauon ${\ displaystyle \ tau}$ −1 1777 2.9 · 10 −13
Tauon neutrino ${\ displaystyle \ nu _ {\ tau}}$   0 <15.5 ${\ displaystyle \ infty}$ (stable)

Leptons are subject to the weak interaction , gravity , and if they carry electrical charge, also to the electromagnetic interaction . All leptons are fermions and have a spin ½.

Electron, muon and tauon carry a negative elementary charge. The neutrinos are not charged, but differ in their flavor ( , or ). There is an antiparticle for every lepton . The anti-neutrinos also have no electrical charge. The electric charge of the antiparticles of electron, muon and tauon is a positive elementary charge. ${\ displaystyle e}$${\ displaystyle \ mu}$${\ displaystyle \ tau}$

If the flavor eigenstates do not correspond to the mass eigenstates of the neutrinos, the flavor is no longer a conserved quantity. For a neutrino that was generated in its own state , after a certain time there is a probability that it will also be detected in the state or ( neutrino oscillations ). This model can the deficit of the measured on Earth flow of solar neutrinos explain ( Solar neutrino deficit ). According to these results, neutrinos must have a mass greater than zero; Experiments such as KATRIN try to prove this . ${\ displaystyle e}$${\ displaystyle \ mu}$${\ displaystyle \ tau}$