Restoration of Chiral Symmetry in Nuclear medium

手征对称性的动力学破缺是强相互作用的基础理论量子色动力学（QCD）中最具特征的现象之一。由于这种现象，π介子表现为一种伪Nambu-Goldstone玻色子，它导致了核力的长程部分以及多π介子交换的核束缚。从这个意义上讲，甚至核的稳定性也与手征对称性密切相关。在过去的几年里，我们主要关注π介子和其他介子在核中的性质，并研究手征对称性在核中的作用。我们有两个主要发现：* I=J=0通道中的π介子对有一个称为σ介子的宽共振。在核介质中，由于手征对称性的部分恢复，σ介子的质量和宽度趋于减小。这导致了一个可能的长寿命的中重核的σ束缚态。我们研究了（d，t）和（d，He ~ 3）等反应，发现在2 π阈附近的反应截面中出现了一个特征结构。π-π散射中的σ介子和ρ介子都是共振态，它们之间的关系以前是未知的。我们用N/D方法结合手征微扰理论，把两种介子放在同一个基础上处理。将这种方法推广到核介质中的π介子对，我们发现σ和ρ在核介质中同时软化。此外，它们的行为是由一个单一的功能：即我们发现了一个普遍性的软化的强子谱的物质。

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.