Executive Summary : | Quantum chromodynamics is the study of color quantum number quarks and gluons, which form colorless hadrons. In the 1970s, hadrons were thought to be made up of only valence quarks and bound by gluons. However, over time and increased probed energy, we have learned more about the structure of hadrons, including sea quarks and antiquarks. These partons, termed as partons by Feynman, are now known as partons. The DIS experiment at HERA has revealed the dominant role of gluons in nuclei. Various experimental and theoretical frameworks have been developed to extend our understanding of proton structure beyond just 1D pictures. Transverse momentum dependent distributions (TMDs) of (un)polarized partons provide a more comprehensive description of the 3D tomographic structure of the proton. Gluon TMDs are still poorly known, but they can be accessed through studying transverse momentum distributions and azimuthal asymmetries for bound or open heavy-quark pair production in lepton-proton and proton-proton collisions. The production of quarkonium states, such as the J/ψ meson, in ep and pp collisions is an excellent tool for probing gluon TMDs. At Leading Twister-2, there are eight gluon TMDs, with unpolarized and linearly polarized gluon TMDs being the only two that describe the dynamics of gluons inside an unpolarized hadron. The Electron-Ion Collider (EIC) is expected to be the real breakthrough in this field, as its experimental setup and kinematic coverage will complement and extend Q² and x coverage, allowing for unprecedented precision in mapping the 3D hadron structure. |