Lepton

Leptons are a class of elementary particles which, together with quarks and gauge bosons, form the fundamental building blocks of matter. For the number of leptons a conservation law is valid in the standard model of elementary particles (see lepton number).

The name was formed after the Greek λεπτός leptós 'thin', 'small', 'fine'. It is chosen in distinction to two other particle classes, the mesons ('medium-weight') and the baryons ('heavy-weight'). Mesons and baryons belong to the hadrons. As it turned out, these hadrons are composed of two quarks each (mesons) and three quarks each (baryons), so they are not elementary. Among the leptons there are also particles which are by no means "light". For example, the τ-lepton (or tauon) is about twice as heavy as a proton. At the time of naming, however, the tauon was still unknown.

In total, there are 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.

Leptons are subject to the weak interaction, gravitation, and if they carry electric 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 by their flavor ( , μ or τ ). For every lepton there exists an antiparticle. The anti-neutrinos also have no electric charge. The electric charge of the antiparticles of electron, muon and tauon is a positive elementary charge.

If the flavor eigenstates do not correspond to the mass eigenstates of the neutrinos, the flavor is no longer a conservation quantity. For a neutrino produced in the eigenstate , after a certain time there is a probability to be detected also in the state μ or τ (neutrino oscillations). This model can explain the deficit of the flux of solar neutrinos measured on Earth (solar neutrino deficit). According to these results, neutrinos must have a mass greater than zero; experiments such as KATRIN are trying to detect them.

See also

• Leptoquark
• Dilepton