Internal Structure of Hadrons

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1. Flavor asymmetry in antiquark distributionsWe summarized experimental and theoretical papers on Gottfried-sum-rule violation and antiquark flavor asymmetry u - d, and it was published in Physics Reports. Then, a typical meson-cloud model was used for investigating polarized antiquark flavor asymmetry. The results show Δd excess over Δu, and it could be tested experimentally in the near future.2. Polarized proton-deuteron Drell-Yan process and spin-dependent structure functions in the deuteronPolarized proton-deuteron (pd) Drell-Yan processes were investigated for studying new spin structure of a spin-1 hadron. Our formalism indicates that there exist many additional structure functions in comparison with the proton-proton (pp) process. A naive parton model suggests that at least one of them should be related to the tensor polarized distributions, which have not been measured at all. Using this formalism, we found that the difference between the pp and pd cross sections is valuable for finding not only the flavor asymmetry in longitudinally polarized antiquark distributions but also the one in transversity distributions.3. Optimum parton distributions in the nucleon and nucleiPolarized parton distribution functions were determined by using world data from the longitudinally polarized deep inelastic scattering experiments. From our analysis, we proposed one set of LO distributions and two sets of NLO ones as the longitudinally-polarized parton distribution functions. The polarized gluon distribution is difficult to be determined by the analysis, Next, optimum nuclear parton distributions were determined by an analysis of muon and electron deep inelastic scattering data on F^A_2/F^D_2. Although valence-quark distributions are relatively well determined except for the small-x region, antiquark distributions cannot be fixed at medium and large x. It is also difficult to fix gluon distributions.