Modification of the nucleon-nucleon scattering amplitude by nuclear medium effects

用核准自由散射的方法研究了核介质效应。准自由散射的独占测量为研究核介质中核子-核子相互作用提供了一条直接途径，有望提取核场中核子和介子性质的信息。为此，利用大阪大学核物理研究中心的环形回旋加速器测量了几个目标原子核在s_<1/2>轨道上发生的质子敲除反应的微分截面和分析功率（Ay）。在自由质子-质子散射的基础上降低了Ay值，并且发现降低率与目标有关。我们发现，通过这个反应看到的平均密度可以在一种特殊情况下估计出来，在这种情况下，与本研究中采用的实验设置相对应的反冲动量为零，并且在某些情况下几乎达到核饱和密度的一半。Ay的还原表现为平均密度的单调递减函数，这强烈表明存在核介质对核子-核子相互作用的影响。这一结果与狄拉克方法的预测结果进行了比较。在这种方法中，核子旋量在核场中的修饰被描述为有效核子质量的减少。我们使用上述定义的平均密度估计有效核子质量，并按照印第安纳大学理论小组提出的程序计算核子-核子观测值的修正。计算结果与实验结果定性一致。

Nuclear medium effect has been investigated experimentally through nucleon quasifree scattering. Exclusive measurements of the quasifree scattering give a direct way to study the nucleon-nucleon interaction in nuclear medium and is is expected to extract information on the properties of nucleons and mesons in nuclear field. For this puroose, defferential cross sections and analyzing powers (Ay) have been measured for the proton knockout reaction from s_<1/2> orbits of several target nuclei by using the ring cyclotron at Research Center for Nuclear Physics, Osaka University. The Ay values are reduce from those of free proton-proton scattering and the reduction rate is found to depend on target. We found that an averaged density seen through this reaction can be estimated in a special case where the recoil momentum, which corresponds to the experimental setting employed in this study, is zero and it reaches almost half of the nuclear saturation density in some cases. The reduction of Ay shows a monotomically decreasing function of the averaged density, which strongly suggests the existence of a nuclear medium effect on the nucleon-nucleon interaction. This result has been compared with the prediction by the Dirac approach. In this approach, , modification of the nucleon spinor in nuclear field is described as a reduction of the effective nucleon mass. We estimate the effective nucleon mass by using the averaged density defined above and calculate the modefication of the nucleon-nucleon observables by following the procedure proposed by a theoretical group in Indiana university. The calculation gives a qualitatively consistent result with our experimental result.