Flavor-changing neutral current

If they occur in nature (as reflected by Lagrangian interaction terms), these processes may induce phenomena that have not yet been observed in experiment.

Flavor-changing neutral currents may occur in the Standard Model beyond the tree level, but they are highly suppressed by the GIM mechanism.

[1][2][3] The Tevatron CDF experiment observed evidence of FCNC in the decay of the strange B-meson to phi mesons in 2005.

The MEG experiment[5] at the Paul Scherrer Institute near Zürich will search for a similar process, in which an antimuon decays to a photon and an antielectron (a positron).

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

Above : Highly suppressed tau decay via flavor-changing neutral current at one-loop order in the Standard Model .
Below : Beyond the Standard Model tau decay via flavor-changing neutral current mediated by a new S boson.
An example of a hypothetical (i.e., not yet observed) flavor-changing neutral current process in the Minimal Supersymmetric Standard Model . A strange quark emits a bino , turning into a sdown-type squark , which then emits a Z boson and reabsorbs the bino, turning into a down quark. If the MSSM squark masses are flavor-violating, such a process can occur.