The pion, as the Goldstone boson of dynamical chiral symmetry breaking of the strong interaction, is the lightest QCD bound state. Because of its light mass, pion plays a dominant role in the long-range nucleon-nucleon interaction. Understanding the pion’s internal structure is important to investigate the low-energy, non-perturbative aspects of QCD. Nevertheless, the uncertainties of partonic density functions (PDFs) of pions are relatively huge due to lacking rest pion targets.
In this talk, we will review recent theoretical and experimental progress of extracting the pion PDFs. Furthermore, we will provide quantitative evidence within the CEM and NRQCD frameworks that the existing pion-induced fixed-target J/psi data, are sensitive to the gluon density of pions, and favor the pion PDFs with relatively large gluon contents at large x.
 C.Y. Hsieh, Y.S. Lian, W.C. Chang, J.C. Peng, S. Platchkov, and T. Sawada, Chin. J. Phys. 73, 13 (2021); arXiv:2103.11660.
 W.C. Chang, J.C. Peng, S. Platchkov, and T. Sawada, Phys. Rev. D 102, 054024 (2020).Rev. D 99, 014032 (2019); arXiv:2006.06947.