Top-quark mass dependence of the decay Bs in the standard electroweak model

An explicit calculation for the decay bs is carried out in the standard electroweak model. We find that terms with the lowest powers of external momenta are only proportional to the dipole moment form factor responsible for the decay bs. It is shown that this result can be regarded as a non-soft-bremsstrahlung extension of Low's low-energy theorem. Without QCD corrections, the branching ratio B(Bs) is estimated to be of the order of 10-7. In addition, the photon spin orientations are found to be fairly sensitive to the top-quark mass. Its implications are also discussed.