Studies of Spin Structure of Proton and Neutron by HERMES - High Precision Hadron Indentification with a Ring Imaging Cherenkov Detector

本研究的目的是研究质子和中子的自旋1/2是如何由夸克和胶子组成的。17年前，人们发现夸克自旋对质子自旋的贡献很小。这被称为“核子自旋问题”。我们用一种新的实验技术研究了这个问题。这项实验被称为HERMES，在德国的DESY进行。用27.5 GeV电子束和极化质子、氘核靶进行了深度非弹性散射实验。HERMES收集的数据被发送到东京工业大学，我们在那里有数据分析设施。我们的新技术是环形成像切伦科夫探测器，我们可以识别与散射电子重合的强子。作为第一个成果，我们可以引用螺旋分布的夸克风味分解Δu， Δd， Δs等。实验结果清楚地表明，u夸克的极化方向与质子自旋平行，而d夸克的极化方向与质子自旋相反。海夸克的极化几乎为零。第二项成果是首次观测到深度虚康普顿散射引起的自旋不对称。这是研究广义Parton分布的有用数据。作为第三个成果，我们利用横向极化质子靶，成功地分别提取了Sivers效应和Collins效应。这为强子自旋物理学开辟了一个新的领域。

The purpose of this research is to study how spin 1/2 of proton and neutron are composed from quarks and gluons. It was discovered 17 years ago that the quark spin contribution to the proton spin is small. This is called 'Nucleon Spin Problem'. We investigated this problem with a novel experimental technique.The experiment was called HERMES and was carried out at DESY in Germany. It is a deep inelastic scattering experiment with 27.5 GeV electron beam and polarized proton and deuteron target. Data accumulated by HERMES were sent to Tokyo Tech where we have a facility of data analyses. Our new technique is Ring Imaging Cherenkov Detector with which we can identify hadrons which are detected in coincidence with the scattered electon.As the first achievement, we can cite the quark flavor decomposition of helicity distribution Δu, Δd, Δs et al. The experimental results clearly show that u quark is polarized parallel to the proton spin, while d quark is polarized in opposite direction. The polarization of sea quarks are almost zero.As the second achievement, spin asymmetry due to deeply virtual Compton scattering was observed for the first time. This was useful data as input to the study of Generalized Parton Distributions'.As the third achievement, using a transversely polarized proton target, we succeeded in extracting Sivers effect and Collins effect separately. This has opened up a new field in the hadron spin physics.