Understanding novel phenomena induced by QCD higher-twist effects and study of hadron structure

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Based on perturbative QCD, we have studied the single spin asymmetries (SSA) caused by multi-parton correlations (higher twist effect), and obtained the following results: (1) The quark-gluon correlation functions in the nucleon can describe the main features of the SSAs observed in the nucleon-nucleon collion at RHIC. (2) We have derived the formulas for the contribution of the multigluon correlations in the nucleon to SSAs in semi-inclusive deep inelastic scattering and the nucleon-nucleon scatterings, and clarified the characteristic features of its contribution. (3) We have established the formalism to calculate the contribution of the twist-3 fragmentation function to SSAs. We have also shown a possibility that the combination of the twist-3 fragmentation function and the quark-gluon correlation function in the mnucleon can consistently describe the SSAs in semi-inclusive deep inelastic scattering and the nucleon-nucleon collision.