Measurement of the K-π+ S-wave system in D+ → K-π+π+ decays from Fermilab E791

A new approach to the analysis of three body decays is presented. Measurements of the S-wave K π amplitude are made in independent ranges of invariant mass from threshold up to the upper kinematic limit in D+ → K-π+π+ decays. These are compared with results obtained from a fit where the S-wave is assumed to have κ and K0*(1430) resonances. Results are also compared with measurements of K-π+ 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 behaviour below Kη' threshold should match that in elastic scattering, is not well supported by these data. Production of K-π+ from these D decays is also studied. (author)