Flavorviolating

Flavor violation refers to processes in which a fermion changes its flavor quantum number, such as a strange quark turning into a down quark, or a muon turning into an electron. In the Standard Model, flavor is carried by three quark generations (up, down, charm, strange, top, bottom) and three lepton generations (electron, muon, tau). Flavor-changing interactions arise from the weak force and are encoded in the CKM matrix for quarks and the PMNS matrix for leptons. Flavor-violating processes are often rare because they involve mixing between generations.

In the quark sector, charged-current transitions mediated by W bosons change quark flavor, for example a bottom quark decaying to a charm quark via W emission. However, flavor-changing neutral currents (FCNC) such as b -> s gamma are forbidden at tree level in the SM and occur only through loops (the GIM mechanism), making them highly suppressed. Meson mixing (K0-K0bar, B0-B0bar) and CP violation are natural consequences of flavor physics.

In the lepton sector, charged lepton flavor violation is extremely suppressed in the Standard Model, but neutrino oscillations demonstrate lepton flavor non-conservation in the neutral sector. Observing charged LFV at accessible rates would indicate new physics beyond the SM: examples include mu -> e gamma, muon-to-electron conversion in nuclei, and tau decays such as tau -> mu gamma.

Flavor-violating processes are a major focus of experimental particle physics. Experiments search for rare decays and mixing phenomena and place upper limits on branching ratios, guiding theories beyond the Standard Model. Models such as supersymmetry, extra dimensions, and leptoquarks often predict enhanced flavor violation, providing motivation for precision measurements.