Flavour–changing Neutral Currents
The expression flavour-changing neutral current processes (FCNC) denotes semileptonic processes with a change of flavour but without net charge transfer to the leptons, or non-leptonic processes which should occur at higher order in the electroweak theory, as a consequence of the fact that the neutral current does not produce a change of flavour to lowest order; cf. equation (10.16). Well known examples of FCNC processes are the decay KL → µ+µ− and the ∆S = 2 transition K¯0-K0. In the electroweak theory, the FCNC amplitudes due to exchange of the intermediate bosons, W and Z, should be finite, as a consequence of the fact that it is not possible to add flavourchanging counterterms to the neutral current.