Spontaneous Breaking of Chiral Symmetry and an Iso-singlet Scalar Meson

本研究的目的是通过分析pi^-P-gt；pi^0pi^0n反应中pi^0-pi^0通道的数据，探索质量约为600 MeV的等单态标量介子--sigma的存在。我们认为这个项目是一个“萌芽研究”，因为我们的最终目标是捕捉手征对称性恢复或前兆现象到手征跃迁的信号。根据最新的分析结果，在pi^0-pi^0不变质量分布中观察到了一个中心在600 MeV左右的宽凸起。通过研究每个pi^0-pi^0事件在静止坐标系中的衰减角分布，发现与凸起相对应的态是标量态。然而，有必要对pi^0-pi^0通道进行更详细的分析，并进行实验来探索核物质中强子的特征变化，以揭示碰撞是否对应于标量介子起源的手征对称制动。我们在国际会议和研讨会上讨论了这项研究的初步成果，并在研究项目报告《科学研究助学金》(C)的序言中介绍了这项研究的初步结果。从那时起，我们通过电子邮件收到了许多来自国外的问题、评论和鼓励。我们计划继续进行更详细的分析。另一方面，我们正在准备一项向东京大学INS提出的实验，使用AlGeV标记的光子束捕获手征对称性部分恢复的信号。

The aim of this research was to explore the existence of an iso-singlet scalar meson, sigma, having a mass of about 600 MeV by analyzing the data of the pi^0-pi^0 channel in the pi^-P->pi^0pi^0N reaction. We regarded this project as a "budding research" since our final goal was to catch signals of chiral symmetry restoration or those of precursory phenomena to chiral transition.According to the result of the analysis up to date, a broad bump centered at around 600 MeV is observed in the pi^0-pi^0 invariant mass distribution. And the state corresponding to the bump is found to be a scalar by investigating the decay angular distribution in the rest frame of each pi^0-pi^0 event. It is mecessary, however, to carry out more detail analysis of the pi^0-pi^0 channel and to conduct an experiment to explore character change of hadrons in nuclear matter, in order to reveal whether the bump corresponds to the scalar meson originating chiral symmetry braking. Thus the first aim of this project as a "budding research" is attained.We gave talks on the preliminary result of this research at the international conferences and the workshops shown in the preface of the report of research project, grant-in-aid for scientific research (C). Since then, we have received many questions, comments and encouragement from abroad through E-mail. We are planning to continue the analysis in more detail. On the other hand, we are preparing for an experiment proposed to INS, Tokyo University, using al GeV tagged photon beam to catch a signal from partial restoration of chiral symmetry.