Tree-unitary sigma models and their application to strong WW scattering
  Sigma models are exhibited which have tree amplitudes for Goldstone boson scattering that satisfy elastic unitarity exactly. The models have imaginary coupling constants and the scalar propagators have poles on the imaginary axis in the complex p^2 plane. They are equivalent to K-matrix models, which are ad hoc unitarizations of low energy theorems for Goldstone boson scattering that have been used recently to describe strong WW scattering. The sigma model formulation of the K-matrix models may be used to estimate directly the effect of strong WW scattering on low energy radiative corrections. 
Michael S. Chanowitz