S-Wave Pion-Pion Correlations in Dense Nuclear Matter

The s-wave correlated $\pi$-$\pi$ strength distribution in cold nuclear matter is investigated. The in-medium single pions are renormalized by means of the usual p-wave couplings to nucleons and $\Delta$'s. Two groups of models for the $\pi$-$\pi$ interaction are studied. The first one uses phenomenological models which reproduce the phase shifts but do not respect the soft-pion theorems. With increasing density a strong accumulation of strength in the subhreshold region is observed. Slightly above saturation density there even occurs an instability with respect to s-wave $\pi$-$\pi$ pair condensation. These effects are strongly moderated or disappear altogether if constraints from chiral symmetry are built into the $\pi$-$\pi$ interaction.