S-wave $K^-\pi+$ System in $D^+\to K^-\pi^+\pi^+$ Decays from Fermilab E791

A new approach to the analysis of three body decays is presented. Model-independent results are obtained for the S -wave Kπ amplitude as a function of Kπ invariant mass. These are compared with results from K − π + elastic scattering, and the prediction of the Watson theorem, that the phase behavour be the same below Kη ′ threshold, is tested. Contributions from I = 12 and I = 32 are not resolved in this study. If I = 12 dominates, however, the Watson theorem does not describe these data well.