Chiral nonperturbative approach to the isoscalar s - wave ππ interaction in a nuclear medium

Chiral nonperturbative approach to the isoscalar s-wave ππ interaction in a nuclear medium Abstract The s-wave isoscalar amplitude for ππ scattering in a nuclear medium is evaluated using a nonperturbative unitary coupled channels method and the standard chiral Lagrangians. The method has proved successful to describe the ππ properties in the scalar isoscalar channel up to 1.2 GeV giving rise to poles in the t matrix for the f 0 (980) and the σ. The extension of the method to the nuclear medium implies not only the renormalization of the pions in the medium, but also the introduction of interaction terms related to contact terms in the πN → ππN interaction. Off shell effects are also shown to be important leading to cancellations which reduce the coupled channel integral equations to a set of algebraic equations. For intermediate densities we find an accumulation of strength in the ππ system at energies above threshold plus narrow peaks below or close to threshold, but those peaks fade away as the nuclear density increases approaching normal nuclear matter density .