Chiral restoration in nuclear medium and related topics

Since QCD was established as the fundamental theory of the strong interactions, 'Physics of Hadron Many-Body System' came to be studied on the basis of QCD in place of nuclear forces. In this report, an overview of the physics of the chiral phase transition in nuclear medium is presented and relation to the physics at J-PARC is discussed briefly. Fundamental features of the QCD are summarized as the confinement of quarks and gluons, spontaneous chiral symmetry breaking, isospin conservation, ΔI=1/2 law, vector meson dominance, existence of nuclei as the stable hadron many-body system, and axial anomaly. Some of them are well interpreted as the consequence of the dynamic breaking of chiral symmetry and it is important that deconfinement phase transition and chiral symmetry restoration phase transition may possibly occur when temperature and density are increased. At the chiral symmetry restoration σ-meson is expected as the soft mode coupled with the fluctuation of the order parameter. The increase of spectral function of I=J=0 channel with increasing target nuclear mass number observed in the recent pion-production experiment can be interpreted on this model. Other experiments are also mentioned of the deeply coupled π mesic nuclei, vector channel phenomena, and strangeness related phenomena. (S. Funahashi)