Editing Lepton (section)

=== Electromagnetic interaction ===
[[File:Lepton-interaction-vertex-eeg.svg|thumb|right|Lepton–photon interaction]]
One of the most prominent properties of leptons is their [[electric charge]], {{mvar|Q}}. The electric charge determines the strength of their [[electromagnetic interaction]]s. It determines the strength of the [[electric field]] generated by the particle (see [[Coulomb's law]]) and how strongly the particle reacts to an external electric or magnetic field (see [[Lorentz force]]). Each generation contains one lepton with {{nowrap|1={{math|''Q''}} = −1 [[elementary charge|''e'']]}} and one lepton with zero electric charge. The lepton with electric charge is commonly simply referred to as a ''charged lepton'' while a neutral lepton is called a ''neutrino''. For example, the first generation consists of the electron {{SubatomicParticle|electron}} with a negative electric charge and the electrically neutral electron neutrino {{SubatomicParticle|electron neutrino}}.

In the language of quantum field theory, the electromagnetic interaction of the charged leptons is expressed by the fact that the particles interact with the quantum of the electromagnetic field, the [[photon]]. The [[Feynman diagram]] of the electron–photon interaction is shown on the right.

Because leptons possess an intrinsic rotation in the form of their spin, charged leptons generate a magnetic field. The size of their [[magnetic dipole moment]] {{mvar|μ}} is given by
: <math>\mu = g\, \frac{\; Q \hbar \;}{4 m} \ ,</math>
where {{mvar|m}} is the mass of the lepton and {{mvar|g}} is the so-called [[g-factor (physics)|"{{mvar|g}}&nbsp;factor"]] for the lepton. First-order quantum mechanical approximation predicts that the {{mvar|g}}&nbsp;factor is 2 for all leptons. However, higher-order quantum effects caused by loops in Feynman diagrams introduce corrections to this value. These corrections, referred to as the ''[[anomalous magnetic dipole moment]]'', are very sensitive to the details of a quantum field theory model, and thus provide the opportunity for precision tests of the Standard Model. The theoretical and measured values for the ''electron'' anomalous magnetic dipole moment are within agreement within eight significant figures.<ref>{{harvnb|Peskin|Schroeder|1995|p=197}}</ref> The results for the ''muon'', however, [[Muon g-2|are problematic]], hinting at a small, persistent discrepancy between the Standard Model and experiment.