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Subjects Study on the Nucleon Spin Structure by the Large Polarized Target
Representative
researcher

Yamagata University Takahiro IWATA
It has been believed for a long time that the origin of the nucleon spin is the quark spin. However, recent experiments have made it clear that the contribution of the quark spin is less important. The question about the origin of the nucleon spin still remains. According to QCD theory which is believed to describe correctly interactions of quarks, the gluon spin may play an important role to construct the nucleon spin.
We have been studying the contribution of the gluon spin in the international collaboration named COMPASS at CERN using a large polarized target.
We have measured the gluon polarization with the best accuracy ever obtained leading to the conclusion that the gluon spin contribution is not so large as expected by several models. In other words, the nucleon spin may not be explained only with the spins of quarks and gluons, implying that the orbital motion of quarks and gluons plays an important role.
Other our results give an interesting speculation: observed zero-Sivers-asymmetry for the single hadron production on the transversely polarized deuteron leads to that the orbital motion of gluons does not contribute to the nucleon spin, in other words, only that of quarks does.