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 beyond the tree level, but they are highly suppressed (the GIM mechanism).

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.
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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.

In most cases of interest, the boson involved is not a new boson S but the Z boson itself (FCNCs involving the photon can't occur at zero momentum transfers because of the unbroken electromagnetic gauge symmetry; as such, FCNCs involving the photon at a nonzero momentum transfer are relatively suppressed compared to FCNCs involving the Z boson). This can occur if the coupling to weak neutral coupling is (slightly) nonuniversal. The dominant universal coupling to the Z boson does not change flavor, but subdominant nonuniversal contributions can.

FCNCs involving the Z boson for the down-type quarks at zero momentum transfer are usually parameterized by the effective action term

\frac{g}{2\cos \theta_W}\overline{d}_{L\alpha}U_{\alpha\beta}\gamma^\mu d_{L\beta}Z_\mu

This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay of B0 mesons in Belle and BaBar. The off-diagonal entries of U parameterizes the FCNCs and current constraints restrict them to be less than one part in a thousand for |Ubs|. The contribution coming from the one-loop SM corrections are actually dominant, but the experiments are precise enough to measure slight deviations from the SM prediction.

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 in the Standard Model proper at the tree level 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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