On Testing Unitarity of the Quark Mixing Matrix

Unitarity triangles and characteristic measurables of the $3\times 3$ Cabibbo-Kobayashi-Maskawa (CKM) matrix are discussed. Beyond the $3\times 3$ CKM scheme we make a rephasing-invariant generalization of the Gronau-Wyler-Dunietz approach to determine the weak phase shift in $B^{\pm}_u\rightarrow D K^{\pm}$ or $B^0_d\rightarrow D K^{*0}$ vs $\bar{B}^0_d\rightarrow D \bar{K}^{*0}$, which is only sensitive to the underlying new physics in $D^0-\bar{D}^0$ mixing. We also show that the weak angle $\gamma\equiv \arg (-V^*_{ub}V^*_{cd}V_{ud}V_{cb})$ is possible to be determined from the $CP$ asymmetries of some $B_d$ decays, even in a non-standard model with an extended quark sector. Brief comments are given on tests of unitarity of the $3\times 3$ CKM matrix.