flavorchanging

Flavorchanging refers to processes in which the flavor quantum number of a fermion changes. In particle physics, flavors are associated with generations of quarks (up, down, strange, charm, bottom, top) and leptons (electron, muon, tau and their neutrinos). Flavor-changing interactions arise primarily through the weak interaction. Quark flavor changes are common in charged-current processes mediated by W bosons, with rates governed by the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which encodes inter-generational mixing. Lepton flavor changes occur in the neutrino sector via neutrino mixing described by the PMNS matrix, as seen in neutrino oscillations.

In contrast, flavor-changing neutral currents (FCNCs) change flavor without changing electric charge and are highly suppressed in the Standard Model: they are forbidden at tree level and occur only through loop diagrams due to the Glashow-Iliopoulos-Maiani (GIM) mechanism. Examples of quark-level FCNC transitions include s -> d, b -> s, and c -> u, driving neutral meson mixing such as K0-K0bar, B0-B0bar, and D0-D0bar, as well as rare decays like K -> pi l+ l- and B -> K l+ l-. Lepton flavor-violating decays such as mu -> e gamma have not been observed and provide stringent limits on new physics.

Beyond the Standard Model, new heavy particles or interactions could enhance FCNC rates, making them sensitive probes of new physics such as supersymmetry, extra dimensions, or heavy Z' bosons. The study of flavor-changing processes thus tests the flavor structure of fundamental interactions.