Research of the internal structure of nucleons is entering a new phase. More than forty years of effort to understand the structure of nucleons in the language of elementary constituents (partons, or quarks and gluons) has brought fundamental knowledge. It was obtained at the approximation of the "one-dimensional" parton structure, which corresponds to the "collinear" movement of all partons forming the nucleon. The quantities that contain relevant information are so-called parton distribution functions (PDFs). In recent years, due to significant advances in theory, this simplified picture has been expanded to 3D. Within this extension, quite new notions such as “Generalized Parton Distributions” (GPDs) and “Transverse Momentum Dependent Parton Distributions” (TMDs) are introduced. These tools can be used to formulate previously asked questions about the movement of quarks and gluons within nucleons, their orbital moment and spin, and their spatial distribution. Current research aims to a much deeper understanding of the 3D internal structure of nucleons in the language of the QCD, whose perturbative and also non-perturbative effects are the key to a detailed picture of nucleons and other hadrons.
The three-dimensional quark-gluon structure of the nucleon
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