Factorization in B→Kπ + - Decays

We derive factorization relations for the transverse helicity amplitudes in B→Kπ + - at leading order in Λ/mb, in the kinematical region with an energetic kaon and a soft pion. We identify and compute a new contribution of leading order in Λ/mb to the B→Kπ + - amplitude which is not present in the one-body decay B→K* + -. As an application we study the forward-backward asymmetry (FBA) of the lepton momentum angular distribution in B→Kπ + - decays on and off the K* resonance. The FBA in these decays has a zero at q02=q02(MKπ), which can be used, in principle, for determining the Wilson coefficients C7,9 and testing the standard model. We point out that the slope of the q02(MKπ2) curve contains the same information about the Wilson coefficients as the location of the zero, but is less sensitive to unknown nonperturbative dynamics. We estimate the location of the zero at leading order in factorization, and using a resonant model for the B→Kπ + - nonfactorizable amplitude.