Isoscalar S-wave pi-pi interaction in the nuclear medium

The s-wave isoscalar pi pi scattering amplitude in a nuclear medium has bee n evaluated using a non-perturbative unitary coupled channels method and th e standard chiral Lagrangians. The method is successful describing the pi p i properties in vacuum, in the scalar-isoscalar channel up to 1.2 GeV givin g rise to poles in the T matrix for the f(o)(980) and the sigma. The extens ion of the method to the nuclear medium implies the renormalization of the pions in the medium and also the introduction of interaction terms related to contact terms in the pi N --> pi pi N reaction.