A coupled channel unitary chiral approach to the meson-meson interaction   and pion-pion scattering in a nuclear medium
  A new tool which combines chiral perturbation theory and unitarity in coupled channels is applied with success to the study of the meson-meson interaction, extending the theoretical predictions up to sqrt{s} = 1.2 GeV, hence improving considerably the convergence radius of conventional chiral perturbation theory. The method is applied to obtain the meson-meson phase shifts and inelasticities as well as the isoscalar pion-pion scattering amplitude inside a nuclear medium. 
E. Oset, J.A. Oller, J.R. Pel\'aez, Chiang Huan Ching and M.J. Vicente   Vacas