Theoretical approaches to hadrons in nuclear matter

We discuss recent developments concerning the in-medium properties of hadrons in dense and hot matter. The theoretical approaches are discussed in connection with the interpretation of experimental data from intermediate energy machines up to relativistic heavy ion collisions. Special emphasis is put on chiral restoration and its interplay with the substructure of the nucleon.Comment: Pleanary talk presented at PANIC02, Sept 30- Oct 4, Osaka, Japan. To appear in the proceedings in Nuclear Physics

Variation of hadron masses in nuclear matter in the relativistic Hartree approximation

We study the modification of hadron masses due to the vacuum polarization using the chiral sigma model, which is extended to generate the $\omega$ meson mass by the sigma condensation in the vacuum in the same way as the nucleon mass. The results obtained in the chiral sigma model are compared with those obtained in the Walecka model which includes $\sigma$ and $\omega$ mesons in a non-chiral fashion. It is shown that both the nucleon mass and the $\omega$ meson mass decrease in nuclear medium, while the $\sigma$ meson mass increases at finite density in the chiral sigma model.Comment: 10 pages, 2 figures, accepted for publication in Nucl.Phys.

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 strengh observed in $2\pi$ production experiments. While a large modification of the $\sigma$ propagator follows, due to its coupling to two pion states, we show that the NN potential remains instead unaffected.Comment: 14 pages, 4 figures, submitted to EPJ