Measuring sin2 β in B0s( t)→ φKS

Abstract

We show that, unlike other pure b→ d penguin processes, the decay B 0 s ( t)→ φK S 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 sin2 β. Although this cancellation is complete only for certain values of the s- and b-quark masses, the theoretical uncertainty on sin2 β 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 sin2 β.