  We propose a new method to obtain the K -> pi pi amplitude from K -> pi 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 -> pi pi amplitude
in which the weak Hamiltonian carries momentum. The soft pion theorem, which
relates this amplitude to the K -> pi 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.

High Energy Physics - Phenomenology