Parton distribution functions (PDFs) are important to characterize the structure of the hadrons such as protons and neutrons. The contribution to the structure from quarks has been studied in detail during the past few decades. The structure in the gluon sector is also important but less studied. For high-energy hadrons, the gluon contribution dominates at small $x$, where $x$ is the momentum fraction carried by a quark or gluon. At large $x$, the uncertainty of the gluon PDF is large, especially compared to that of the quark PDFs at large $x$. Gluon PDFs for nucleons and pions are mostly extracted from global analysis of experimental data using perturbation theory as a guide. Theoretically, lattice QCD provides an independent non-perturbative theoretical approach to calculate the gluon PDFs.We present the exploratory study of nucleon gluon PDFs from lattice QCD using the quasi-PDF approach. Using valence overlap fermions on the $2+1$-flavor domain-wall fermion gauge ensemble, the quasi-PDF matrix elements we obtain agree with the Fourier transform of the global-fit PDF within statistical uncertainty. We further study the $x$-dependent nucleon and pion gluon distributions via the pseudo-PDF approach on lattice ensembles with $2+1+1$ flavors of highly improved staggered quarks (HISQ) generated by the MILC Collaboration. Using clover fermions for the valence action, and adding momentum smearing, PDFs are found for pion boost momenta up to 2.56~GeV. Several lattice sizes and spacings ($a\approx 0.9, 0.12$ and 0.15~fm) were evaluated, resulting in three pion masses, $M_{\pi}\approx 220$, 310 and 690~MeV/$c^2$.
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