Rescattering and electroweak penguin effects in strategies to constrain and determine the CKM angle gamma from B ->pi K decays

A general parametrization of the B+ --> pi(+) K-0 and B-d(0) --> pi(-)K(+) decay amplitudes is presented. It relies only on the isospin symmetry of st rong interactions and the phase structure of the Standard Model and involve s no approximations. In particular, this parametrization takes into account both rescattering and electroweak penguin effects, which limit the theoret ical accuracy of bounds on gamma arising from the combined B+/- --> pi(+/-) K, B-d --> pi(-/+)K(+/-) branching ratios. Generalized bounds making also u se of the CP asymmetry in the latter decay are derived, and their sensitivi ty to possible rescattering and electroweak penguin effects is investigated . It is pointed out that experimental data on B+/- --> K+/-K allow us to in clude rescattering processes in these bounds completely, and an improved th eoretical treatment of electroweak penguins is presented. It is argued that rescattering effects may enhance the combined B+/- --> K+/-K branching rat io by a factor of Omicron(10) to the 10(-5) level; and that they may be res ponsible for the small present central value of the ratio of the combined B -d --> pi(-/+)K(+/-) and B+/- --> pi(+/-)K branching ratios, which has rece ntly been reported by the CLEO collaboration and, if confirmed, would exclu de values of gamma within a large region around 90 degrees.