Standard model matrix elements for neutralB-meson mixing and associated decay constants

We present results of quenched lattice calculations of the matrix elements relevant for $B_d-B_d$ and $B_s-B_s$ mixing in the standard model. Results for the corresponding $\mathrm{SU}\left(3\right)\mathrm{}$-breaking ratios, which can be used to constrain or determine $|V_{\mathrm{td}}|,$ are also given. The calculations are performed at two values of the lattice spacing, corresponding to $\beta =6.0\mathrm{}$ and $\beta =6.2,$ with quarks described by a mean-field-improved Sheikholeslami-Wohlert action. As a by-product, we obtain the leptonic decay constants of B and D mesons. We also present matrix elements relevant for $D^0-D^0$ mixing. Our results are summarized in the Introduction.