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

We study the two-pion propagator in the nuclear medium. This quantity appears in the pi pi T-matrix and we show that it also enters the QCD scalar susceptibility. The medium effects on this propagator are due to the influence of the individual nucleon response to a scalar field through their pion clouds. This response is appreciably increased by the nuclear environment. It produces an important convergence effect between the scalar and pseudoscalar susceptibilities, reflecting the reshaping of the scalar strength observed in 2 pi production experiments. While a large modifications of the sigma propagator follows, due to its coupling to two pion states, we show that the NN potential remains instead unaffected.