Rescattering effects, isospin relations and electroweak penguins in B→πK decays

It is argued that, in the presence of soft final-state interactions, the diagrammatic amplitude approach adopted in many analyses of hadronic B decays into light mesons can be misleading when used to deduce the unimportance of certain decay topologies. With the example of B→πK decays, it is shown that the neglect of so-called annihilation and colour-suppressed amplitudes (including electroweak penguins), as well as penguin contributions involving an up-quark loop, is not justified. The implications for the Fleischer–Mannel bound on the angle γ of the unitarity triangle, and for the CP asymmetry in the decays B±→π±K0, are pointed out.