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

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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.