Measurement of the K-pi+ S-wave System in D+ to K-pi+pi+ Decays from Fermilab E791

A new approach to the analysis of three body decays is presented. Measurements of the S-wave K pi amplitude are made in independent ranges of invariant mass from threshold up to the upper kinematic limit in D+ to K-pi+pi+ decays. These are compared with results obtained from a fit where the S-wave is assumed to have kappa(800) and K*0(1430) resonances. Results are also compared with measurements of K-pi+ elastic scattering. Contributions from I = 1/2 and I = 3/2 are not resolved in this study. If I = 1/2 dominates, however, the Watson theorem prediction, that the phase behavour below K-eta' threshold should match that in elastic scattering, is not well supported by these data. Production of K-pi+ from these D decays is also studied.