Experimental exploration of the 3D nucleon structure

Abstract

Extensive experimental and theoretical explorations over the last decades showed that the nucleon (proton/neutron) is not just a simple system of 3 quarks bound by gluons, but a complex system of valence and sea quarks as well as gluons (summarized as partons) which are all interacting with each other and moving relative to each other, following the rules of quantum chromo dynamics (QCD). To understand how the properties of these colored building blocks are related to the basic properties of the nucleon like its mass, its spin or its charge, a full understanding of the relevant effective degrees of freedom and of the effective interactions at large distances is required. In the classical picture of parton dynamics in high energy interactions the description is often simplified into two cases. On the one side the classical form factors, providing a 2D picture of the transverse position distribution and on the other side, the one-dimensional picture of a fast moving nucleon as a collection of co-linearly moving quarks and gluons, described in terms of the longitudinal momentum fraction in parton distribution functions. However, recent experimental and theoretical advances during the last two decades showed, that such a simple picture is not adequate for a full description, especially if transverse spin dependent observables are involved. It turned out, that the intrinsic transverse motion of partons and also the correlation between momentum and position information have to be considered, requiring a full 3-dimensional understanding of the nucleon structure. This review will give an overview on the main experimental data for 3D nucleon structure studies, available from lepton and hadron scattering and its interpretation in terms of generalized parton distributions (GPDs) and transverse momentum dependent parton distributions (TMDs). Recent global fits of both types of distribution functions based on experimental data and their physics content will be presented and discussed on the way to a full 3D imaging of the nucleon. Furthermore, an overview of current and future trends and new perspectives in the field will be provided.