Final State Interactions and New Physics in B -> pi K Decays

Within the Standard Model, and if one assumes that soft rescattering effects are negligible, the CP asymmetry A^dir_CP (B^\pm -> \pi^\pm K) is predicted to be very small and the ratio R = BR(B_d -> \pi^\mp K^\pm)/BR(B^\pm -> \pi^\pm K) provides a bound on the angle \gamma of the unitarity triangle, sin^2 \gamma \leq R. We estimate the corrections from soft rescattering effects using an approach based on Regge phenomenology, and find effects of order 10% with large uncertainties. In particular, we conclude that A^dir_CP \sim 0.2 and sin^2 \gamma \sim 1.2 R could not be taken unambiguously to signal New Physics. Using SU(3) relations, we suggest experimental tests that could constrain the size of the soft rescattering effects thus reducing the related uncertainty. Finally, we study the effect of various models of New Physics on A^dir_CP and on R.