New Bound on γ from B → πK Decays

A new bound on the angle γ of the unitarity triangle is derived using experimental information on the CP-averaged branching ratios for the rare decays B → π±K0 and B → π0K±. The theoretical description is cleaner than the Fleischer–Mannel analysis of the decays B → π±K0 and B0 → πK in that the two decay rates differ only in a single isospin amplitude, which has a simple structure in the SU(3) limit. As a consequence, electroweak penguin contributions and strong rescattering effects can be taken into account in a model-independent way. The resulting bound excludes values of cos γ around ≈ 0.6 and is thus largely complementary to indirect constraints derived from a global analysis of the unitarity triangle. (Submitted to Physics Letters B)