Restoration of Chiral Symmetry in Nuclear medium

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Dynamical breaking of chiral symmetry is one of the most characteristic pheno mena of quantum chromodynamics (QCD) which is the fundamental theory of strong interactions. Because of this phenomenon, the pion behaves as a pseudo Nambu-Goldstone boson and it leads to the long range part of the nuclear force as well as the nuclear bindings from multi-pion exchange. In this sense, even the nuclear stability is deeply related to chiral symmetry.For past few years, we have focused on the properties of the pion and other mesons in nuclei and studied the role of chiral symmetry in nuclei.We had two major findings ;* The pion pair in I=J=0 channel has a broad resonance called the sigma-meson. Due to the partial restoration of chiral symmetry, the mass and the width of the sigma meson tend to decrease in nuclear medium. This leads to a possible long-lived bound state of the sigma with medium-heavy nuclei. We have studied the reactions such as (d,t) and (d,He3) and have shown that there arises a characteristic structure in the reaction cross section near 2-pion threshold.* The relation between the sigma-meson and the rho-meson, which are both resonances in the pi-pi scattering, has not been known before. We have treated both mesons in a same footing using the N/D method combined with the chiral perturbation theory. By generalizing this approach to the pion pair in the nuclear medium, we found that the sigma and the rho have simultaneous softening in nuclear medium. Furthermore, their behavior is dictated by a single function : namely we found a universality of the softening of hadronic spectra in matter.