Study of Spin Structure of the Nucleon with Hadron Coincidence Method

在这个项目中，我们利用实验技术研究了质子和中子的自旋，即1/2，是如何由夸克和胶子组成的。我们参加了德国DESY的爱马仕实验。高能极化电子束被极化靶核子散射。散射的电子以及在末态产生的强子（介子、核子、强子等）都是巧合这些数据被发送到世界各地的合作大学和研究所。东京工业大学的数据分析设备在数据分析中发挥了重要作用。在本项目中，研制了环形成像切伦科夫计数器（RICH），并将其安装到爱马仕实验中。π介子、K介子、质子和反质子现在都被认为是RICH，核子中极化的u、d和s夸克也可以被研究。确定了它们的动量分布。π-k介子分离在几GeV/c以上一直是个难题，爱马仕利用RICH解决了这个难题，这一技术突破影响很大，作为物理学的一个成果，最大的成就是研究了横极化夸克分布。我们通过观察电子散射平面和π介子产生方向之间的方位角来实现这一点。测量了方位角分布对靶自旋方向的依赖性。用这种方法研究了自旋垂直于核子自旋的夸克的分布。这将成为一个新的研究领域，并吸引大量的物理学家进一步研究这一课题。

In this project of Grand-in-Aid we have studied how the spin of proton and neutron, which is 1/2, is composed of quarks and gluons with use of experimental techniques. It is a research of Quantumchromodynamics which is a theory of strong interaction.We participated in HERMES experiment at DESY in Germany. The high energy polarized electron beam is scattered off the polarized target nucleon. The scattered electron as well as produced hadrons in the final state (mesons, nucleons, lambdas etc.) were detected in coincidence. The data were sent to the collaborating universities and institutes worldwide. The data-analysis equipment at Tokyo Institute of Technology played an important role in the data-analysis. The results were published in journals.During this project a Ring Imaging CHerenkov Counter (RICH) was developped and installed into HERMES experiment. Pions, kaons, protons and anti-protons are now identified with the RICH.Polarized u, d, and s quarks in the nucleon can be studied. Their momentum distributions have been determined. Pion-kaon separation had been difficult above several GeV/c up to now but HERMES solved it with the RICH.This technical breakthrough has a large impact.As an outcome of phycics, the largest achievement is that the transversely polarized quark distribution was studied. We realized this by looking at the azimuthal angle which is the angle between the electron scattering plane and the pion production direction. The dependence on the target spin direction of the azimuthal angle distribution was measured. This way the distribution of the quark which has the spin perpendicular to the uncleon spin is studied. This will become a new field of research and attract a large number of physicists to further investigate this subject.