Extracting \gamma from {\mathrm{B_{s(d)}}}-> {\mathrm J}/\psi{\mathrm{K_S}} and {\mathrm{B_{d(s)}}}->{\mathrm{D(+}_{d(s)}}) {\mathrm{D(-}_{d(s)}})

A completely general parametrization of the time-dependent decay rates of the modes ${\mathrm{B_s}}\to{\mathrm J}/\psi{\mathrm{K_S}}$ and ${\mathrm{B_d}}\to {\mathrm J}/\psi{\mathrm{K_S}}$ is given, which are related to each other through the U-spin flavour symmetry of strong interactions. Owing to the interference of current–current and penguin processes, the ${\mathrm{B_s}}\to{\mathrm J}/\psi {\mathrm{K_S}}$ observables probe the angle $\gamma$ of the unitarity triangle. Using the U-spin symmetry, the overall normalization of the ${\mathrm{B_s}}\to{\mathrm J}/\psi{\mathrm{K_S}}$ rate can be fixed with the help of the CP-averaged ${\mathrm{B_d}}\to {\mathrm J}/\psi{\mathrm{K_S}}$ rate, providing a new strategy to determine $\gamma$. This extraction of $\gamma$ is not affected by any final-state-interaction effects, and its theoretical accuracy is only limited by U-spin-breaking corrections. As a by-product, this strategy allows us to also take into account the penguin effects in the determination of $\beta$ from ${\mathrm{B_d}}\to{\mathrm J}/\psi{\mathrm{K_S}}$, which are presumably very small, and to predict the direct CP asymmetry arising in this mode. An analogous strategy is provided by the time-dependent ${\mathrm{B_d}}\to{\mathrm D}^+{\mathrm D}^-$ rate, if its overall normalization is fixed through the CP-averaged ${\mathrm{B_s}}\to{\mathrm D}^+_{\mathrm s}{\mathrm D}^-_{\mathrm s}$ rate.