Extracting γ from Bs(d) → J/ψ KS and Bd(s) → D + − d(s) Dd(s)

A completely general parametrization of the time-dependent decay rates of the modes B s → J/ψ K S and B d → J/ψ 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 B s → J/ψ K S observables probe the angle γ of the unitarity triangle. Using the U-spin symmetry, the overall nor-malization of the B s → J/ψ K S rate can be fixed with the help of the CP-averaged B d → J/ψ K S rate, providing a new strategy to determine γ. This extraction of γ 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 take into account also the penguin effects in the determination of β from B d → J/ψ 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 B d → D + D − rate, if its overall normalization is fixed through the CP-averaged B s → D + s D − s rate.