Dispersion relations and soft pion theorems for K→ππ

We propose a new method to obtain the K→ππ amplitude from K→π which allows one to fully account for the effects of final state interactions. The method is based on a set of dispersion relations for the K→ππ amplitude in which the weak Hamiltonian carries momentum. The soft pion theorem, which relates this amplitude to the K→π amplitude, can be used to determine one of the two subtraction constants—the second constant is at present known only to leading order in chiral perturbation theory. We solve the dispersion relations numerically and express the result in terms of the unknown higher order corrections to this subtraction constant.