Selected topics on parton distribution functions

We report recent studies on structure functions of the nucleon and nuclei. First, clustering effects are investigated in the structure function $F2$ of $9Be$ for explaining an unusual nuclear correction found in a JLab experiment. We propose that high densities created by formation of clustering structure like $2α+neutron$ in $9Be$ is the origin of the unexpected JLab result by using the antisymmetrized molecular dynamics (AMD). There is an approved proposal at JLab to investigate the structure functions of light nuclei including the cluster structure, so that much details will become clear in a few years. Second, tensor‐polarized quark and antiquark distributions are obtained by analyzing HERMES measurements on the structure function $b1$ for the deuteron. The result suggests a finite tensor polarization for antiquark distributions, which is an interesting topic for further theoretical and experimental investigations. An experimental proposal exists at JLab for measuring $b1$ of the deuteron as a new tensor‐structure study in 2010's. Furthermore, the antiquark tensor polarization could be measured by polarized deuteron Drell‐Yan processes at hadron facilities such as J‐PARC and GSI‐FAIR. Third, the recent CDF dijet anomaly is investigated within the standard model by considering possible modifications of the strange‐quark distribution. We find that the shape of a dijet‐mass spectrum changes depending on the strange‐quark distribution. It indicates that the CDF excess could be partially explained as a PDF effect, particularly by the strangeness in the nucleon, within the standard model if the excess at $mjj≈140 GeV$ is not a sharp peak.