Two-pion production processes, chiral symmetry and NN interaction in the medium

We study the many-body effects in the propagation of the scalar meson in the nuclear medium arising from its coupling to two pion states. We identify the source of the medium effects as the influence of the individual nucleon response arising from their pion clouds. The same modification applies to the QCD scalar susceptibility. It reflects the reshaping of the scalar strength observed in $2\pi$ production experiments. While large modifications of the $\sigma $ propagator occur, we show that the NN scalar potential remains instead unaffected.