ππ S-wave phase shifts and non-perturbative chiral approach

We extend a recent non-perturbative chiral approach to ππ S-wave scattering of Oller and Oset by including the couplings to the ηη-channel. We find that the isospin-zero and isospin-two ππ S-wave phase shifts of the model deviate considerably from a recent solution of the Roy-equations. Including the couplings with the ηη-channel does not improve the situation. In particular, no σ-meson like enhancement structure shows up in the Roy-equation solution. We also consider the ππ-scattering lengths in this approach. Received: 17 August 1998 / Revised version: 21 September 1998