Form factors for semileptonic, nonleptonic and rare $B\, (B_s)$ meson decays

We provide new values for the model parameters of the covariant constituent quark model (with built--in infrared confinement) in the meson sector by a fit to the leptonic decay constants and a number of electromagnetic decays. We then evaluate, in a parameter free way, the form factors of the $B(B_s)\to P(V)$-transitions in the full kinematical region of momentum transfer. As an application of our results we calculate the widths of the nonleptonic $B_s$-decays into $D_s^- D_s^+,$ $D_s^{\ast\,-} D_s^{+}+D_s^- D_s^{\ast\,+}$ and $D_s^{\ast\,-} D_s^{\ast\,+}$. These modes give the largest contribution to $\Delta\Gamma$ for the $B_s-\bar B_s$ system. We also treat the nonleptonic decay $B_{s}\to\Jpsi\phi$. Although this mode is color suppressed this decay has important implications for the search of possible CP-violating New Physics effects in $B_s-\bar B_s$ mixing.