Using $K^0{\pi }^{-}\to {\pi }^{-}$ transitions to compute $K\to (\pi \pi {)}_{I=0}$ decay amplitudes at NLO in the chiral expansion

It is proposed to compute matrix elements for the (unphysical) $K^0\pi^-\to \pi^-$ transition to determine the next-to-leading order low energy constants of the weak chiral Lagrangian. This allows us to evaluate $K\to (\pi \pi {)}_{I=0}$ decay amplitudes at this level of precision. This approach has several significant advantages over the use of $K\to \pi \pi$ transitions, most notably the elimination of s-channel disconnected diagrams and the use of fewer inversions.