Flavor changing neutral current

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In theoretical physics, flavor changing neutral currents (FCNCs) are expressions that change the flavor of a fermion current without altering its electric charge. If they occur in the Lagrangian, they may induce processes that have not been observed in experiment. Flavor changing neutral currents may occur in the Standard Model, but they are highly suppressed.

Above: Highly suppressed tau lepton decay via flavor-changing neutral current at one-loop order in the Standard Model.

Below: Beyond-the-Standard Model tau lepton decay via flavor-changing neutral current mediated by a new S boson.

Consider a toy theory in which a new boson S may couple both to the electron as well as the tau lepton via the term

[S\bar\psi_e\psi_\tau]

The electric charge of S clearly must vanish, since the electron and tau have equal charge. A Feynman diagram with S as the intermediate particle is able to convert a tau lepton into an electron (plus some neutral decay products of the S). In the Standard Model, such a process proceeds only by emission and re-absorption of a charged W boson, which changes the tau into a neutrino and then an electron, emitting a photon to conserve energy and momentum.

FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of supersymmetry or technicolor. Their suppression is necessary for an agreement with observations, making FCNCs important in model-building.

Experiments tend to focus on flavor changing neutral currents as opposed to flavor changing charged currents, because the weak neutral current (Z boson) does not change flavor whereas the weak charged currents (W bosons) do. New physics in charged current events would be swamped by more numerous W boson interactions; new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics.

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