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Concept
Flavor-changing neutral current
Summary
In particle physics, flavor-changing neutral currents or flavour-changing neutral currents (FCNCs) are hypothetical interactions that change the flavor of a fermion without altering its electric charge.
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. Several collaborations have searched for FCNC. The Tevatron CDF experiment observed evidence of FCNC in the decay of the strange B-meson to phi mesons in 2005.
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 constraints on model-building.
Consider a toy model in which an undiscovered boson S may couple both to the electron as well as the tau (_tau) via the term
Since the electron and the tau have equal charges, the electric charge of S clearly must vanish to respect the conservation of electric charge. A Feynman diagram with S as the intermediate particle is able to convert a tau into an electron (plus some neutral decay products of the S).
The MEG experiment at the Paul Scherrer Institute near Zurich will search for a similar process, in which an antimuon decays to a photon and an antielectron (a positron). In the Standard Model, such a process proceeds only by emission and re-absorption of a charged _W boson+, which changes the _muon+ into a neutrino on emission and then a positron on re-absorption, and finally emits a photon that carries away any difference in energy, spin, and momentum.
In most cases of interest, the boson involved is not a new boson S but the conventional _Z boson0 boson itself. This can occur if the coupling to weak neutral currents is (slightly) non-universal.
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