Study for nuclear media effect on chiral symmetry restoration

人们认为，在我们的世界中自发破坏的手性对称，应该在有限温度和/或有限密度的系统中恢复。从理论上讲，有人认为手性对称性恢复的一个特征可以看作是核物质中矢量介子的可能的质量位移。本研究项目旨在通过确定衰变后的e^+e^-对或K^+K^-对产生和衰变的ρ， ω和φ介子的质量。实验成功地在e^+e^-不变质谱中观测到ω和φ介子，并报道了介子中质量修饰的特征。这是在正常核物质密度下对矢量介子的轻子介质衰变的首次观察。实验于2002年3月完成数据采集，统计结果比前人提高了100倍，初步数据已在国际会议上发表。我们预计再过半年就能得出最终结果。

It is believed that chiral symmetry, which is spontaneously broken in our world, should be restored in a system with a finite temperature and/or a finite density. Theoretically, it has been suggested that a signature of the chiral symmetry restoration could be seen as a possible mass shift of vector mesons in nuclear matter.This research project has aimed the mass of ρ, ω and φ mesons produced, and decays in nuclei, through a determination of the invariant mass from the e^+e^- pairs, or the K^+K^- pairs following their decays. The experiment was successful to observe ω and φ mesons in the e^+e^- invariant mass spectra, and reported the signature of an in-medium mass modification of the mesons. This was the first observation in the leptonic in-medium decay of vector mesons at a normal nuclear matter density. The experiment finished the data taking in March 2002, and the statistics is improved by factor 100 compared to the previous publication and the preliminary data have been reported in international conferences. In another half year, we expect to reach the final results.