Experimental research on partial restoration of chiral symmetry

本项目的目的是寻找核中手性跃迁的前兆现象。为此，利用2.4 GeV激光电子光子束在SPring-8上进行了一些实验。实验中使用了一套伽玛探测器来探测核目标产生的多重伽玛事件。数据分析主要进行了以下4个过程：1)γA→π^0π^0X2)γN‘→ωN, ω→π^0γ （N’： A中的核子）3)γN'→π^0N4)γp→π^0ηp。我们试图找到过程1)中与手性跃迁序参量有密切关系的标量σ的特征变化的信号。钨与碳靶的2π^0不变质量之比在2π阈值区域大于1，这可能表明核介质中手性对称性部分恢复。然而，存在来自3π^0通道的假2π^0事件。详细的分析正在进行中。在过程2)中，数据显示低动量ω事件的ω不变质量分布有一点偏移，而相对高动量ω事件的ω不变质量则保持在正确的位置。通过测量φ产光截面的A依赖性，得到φ- n截面。得到的σ_<φ n >大于自由空间中的σ_<φ n >，表明在核介质中φ性质可能发生变化。本项目完成并改进了一条新的GeV γ束流线，从而提供了强度为3×10^7光子/s的强稳定标记光子束。

The aim of this project was to search for precursory phenomena of the chiral transition in nuclei. Some experiments have been conducted at SPring-8, for this purpose, by utilizing a 2.4 GeV laser electron photon beam. A set of gamma detectors was employed in the experiments to detect multi-gamma events coming about with a nuclear target. The data analyses have been going on mainly on the following 4 processes :1)γA→π^0π^0X2)γN'→ωN, ω→π^0γ (N': nucleon in A)3)γN'→π^0N4)γp→π^0ηp.We tried to find any signal concerning character change of the scalar σ having a close relation to the order parameter of the chiral transition in process 1). The ratio of 2π^0 invariant mass for the tungsten to the carbon target shows a larger value than 1 at the 2π threshold region, which may indicate a sign of partial restoration of chiral symmetry in the nuclear medium. However, there exist fake 2π^0 events coming from the 3π^0 channel. And a detailed analysis is underway. In process 2), the data show a bit of shift in the ω invariant mass distribution for low momentum ω events while the ω invariant mass stays at the right place for ω`s with relatively higher momentum. The φ-N cross section was obtained by measuring the A dependence of φ photoproduction cross sections. The obtained value σ_<φN> was rather larger than one in free space, suggesting that the φ properties might change in the nuclear medium..In this project a new GeV γ beam line was completed and improved so as to provide a strong and stable tagged photon beam having an intensity of 3×10^7 photons/s.