Measuring sin2β in B0s(t)→φKS

We show that, unlike other pure b→d penguin processes, the decay B0s(t)→φKS is dominated by a single amplitude, that of the internal t-quark. The contributions of the u- and c-quark operators each vanish due to a cancellation between the (V−A)⊗(V−A) and (V−A)⊗(V+A) matrix elements. Thus, the indirect CP asymmetry in this decay probes sin2β. Although this cancellation is complete only for certain values of the s- and b-quark masses, the theoretical uncertainty on sin2β is still less than 10% over most (∼80%) of the parameter space. By measuring the direct CP asymmetry, one can get a better idea of the probable error on sin2β.