Editing Lepton (section)

=== Spin and chirality ===
[[File:Right left helicity.svg|thumb|200px|right|Left-handed and right-handed helicities]]

Leptons are [[Spin (physics)|spin]]&nbsp;{{sfrac|1|2}} particles. The [[spin-statistics theorem]] thus implies that they are [[fermion]]s and thus that they are subject to the [[Pauli exclusion principle]]: no two leptons of the same species can be in the same state at the same time. Furthermore, it means that a lepton can have only two possible spin states, namely up or down.

A closely related property is [[chirality (physics)|chirality]], which in turn is closely related to a more easily visualized property called [[helicity (particle physics)|helicity]]. The helicity of a particle is the direction of its spin relative to its [[momentum]]; particles with spin in the same direction as their momentum are called ''right-handed'' and they are otherwise called ''left-handed''. When a particle is massless, the direction of its momentum relative to its spin is the same in every reference frame, whereas for massive particles it is possible to 'overtake' the particle by choosing a faster-moving [[Lorentz transformation|reference frame]]; in the faster frame, the helicity is reversed. Chirality is a technical property, defined through transformation behaviour under the [[Poincaré group]], that does not change with reference frame. It is contrived to agree with helicity for massless particles, and is still well defined for particles with mass.

In many [[quantum field theories]], such as [[quantum electrodynamics]] and [[quantum chromodynamics]], left- and right-handed fermions are identical. However, the Standard Model's [[weak interaction]] treats left-handed and right-handed fermions differently: only left-handed fermions (and right-handed anti-fermions) participate in the weak interaction. This is an example of [[parity violation]] explicitly written into the model. In the literature, left-handed fields are often denoted by a capital <small>L</small> subscript (e.g. the normal electron e{{su|b=L|p=−}}) and right-handed fields are denoted by a capital <small>R</small> subscript (e.g. a positron e{{su|b=R|p=+}}).

Right-handed neutrinos and left-handed anti-neutrinos have no possible interaction with other particles (see ''[[Sterile neutrino]]'') and so are not a functional part of the Standard Model, although their exclusion is not a strict requirement; they are sometimes listed in particle tables to emphasize that they would have no active role if included in the model. Even though electrically charged right-handed particles (electron, muon, or tau) do not engage in the weak interaction specifically, they can still interact electrically, and hence still participate in the [[Electroweak unification|combined electroweak force]], although with different strengths ([[Weak hypercharge|{{math|''Y''}}<sub>W</sub>]]).
{{clear}}