Wolfenstein parametrization ($\lambda ,A,\rho ,\eta$)

The Wolfenstein parametrization expands the three-generation CKM matrix in its small $1$-$2$ mixing parameter $\lambda =s_{12}$, defining $s_{23}=A{\lambda }^2$ and $s_{13}{e}^{i\delta }=A{\lambda }^3(\rho +i\eta )$. To cubic order its rows are $(1-{\lambda }^2/2,\lambda ,A{\lambda }^3(\rho -i\eta ))$, $(-\lambda ,1-{\lambda }^2/2,A{\lambda }^2)$, and $(A{\lambda }^3(1-\rho -i\eta ),-A{\lambda }^2,1)$. Higher orders restore exact unitarity. Rephasing-invariant barred parameters are defined from ratios of CKM entries rather than this truncated expansion.