Proton quasifree scattering at 1 GeV and medium effects in nuclear field

该项目主要作为国际合作实验在俄罗斯彼得堡核物理研究所进行，使用1 GeV同步回旋加速器。利用大阪大学核物理研究中心的回旋加速器进行400 MeV质子束实验，并与南非Schtellenbosch大学的Hillhouse教授合作进行实验，也是该计划的重要组成部分。高豪斯教授在2004年和2005年分别在福冈大学待了2 ~ 4个月。本项目的目的是研究Dirac相对论效应，这是强子水平上可能的核介质效应之一。在俄罗斯进行了两次实验，质子在160靶（p壳核）和40Ca靶（sd闭壳核）上的准自由散射。前一种情况下，由于制备探测器系统的原因，只测量了微分截面，而后一种情况下，测量了截面和诱导极化。质子准自由散射的分析能力（Ay）与在自由空间中核子-核子相互作用估计的值相比显著降低是一个长期存在的问题，但我们首先定量地比较了在宽入射能量区域的降低率。结果表明，在1 GeV和400 MeV能量下，还原速率基本一致，表明还原过程不是由一些复杂过程混合而成的。从多种靶核的实验结果还发现，这种还原反应表现为反应q值的单调函数。从理论方面来看，发现只有50%的这种减少被解释为狄拉克相对论效应。这可能表明存在一些额外的介质效应，如介子质量修饰。

This program was mainly performed as an international collaboration experiment held at Petersburg Nuclear Physics Institute in Russia by using the 1 GeV synchrocyclotron. Experiments at 400 MeV proton beam by using the cyclotron complex at Research Center for Nuclear Physics, Osaka University, and a collaboration work with Prof. Hillhouse, University of Schtellenbosch in South Africa, are also important parts of this program. Prof. Hillhouse stayed at Fukuoka for two to four months in 2004 and 2005, respectively. The purpose of this program was to investigate Dirac relativistic effects, which is one of possible nuclear medium effects in the hadronic level..The experiments in Russia were performed twice and the proton quasifree scattering off a 160 target, which is a p-shell nucleus, and a 40Ca target, an sd closed-shell nucleus. In the former case, only the differential cross section were measured for a reason in the preparation of detector system, while both of the cross section and induced polarization were measured in the latter case.It is a long staying problem that the analyzing power (Ay) for proton quasifree scattering is significantly reduced from the values estimated by using the nucleon-nucleon interaction in free space, but we firstly compare the reduction rate in a wide incident-energy region quantitatively. The result shows that the reduction rate is similar for both of 1 GeV and 400 MeV energies, from which we can conclude that the reduction is not caused by mixture of some complicated processes as two-step processes. From an experimental result with many kinds of target nuclei, it is also found that this reduction shows a monotone function of the Q-value of the reaction. From the theoretical side, it is found that only 50% of this reduction is explained as a Dirac relativistic effect. This may suggest existence of some additional medium effect, such as meson-mass modifications.