Flavour mixing (${\psi }_{\alpha }={\sum }_i{U}_{\alpha i}{\psi }_i$)

The interaction-labelled fields of several particle species can be linear combinations of the propagation or mass eigenfields. A unitary matrix relates orthonormal bases with canonical kinetic normalization. Quark mixing is described by the CKM matrix; the lepton mixing matrix describes the corresponding mismatch for leptons. A mixing angle parametrizes part of this matrix once ordering and phase conventions are fixed.

An angle parametrizing a rotation between interaction and propagation bases. Examples include the Cabibbo angle and the three angles in the lepton mixing matrix. The value must be associated with a specified matrix parametrization and basis ordering.