What is the structure and spin decomposition of the proton?

Abstract

The structure of the is the forefront of studies on the deep structure of matter. With developments of high energy experiments in particular deeply inelastic lepton nucleon scattering (DIS), a dynamical picture of the structure of the has been developed, i.e., the quantum chromodynamics (QCD) based parton model. According to this picture, a moving close to the speed of light can be viewed as a bunch of free partons, and the collision between two high energy protons can be viewed as the collision between partons. Partons in the are described by parton distribution functions (PDFs). This picture is very successful in describing data from different experiments. It has been widely accepted in high energy physics community and is now taken as the standard picture for the dynamical structure of hadrons. The picture has changed the concepts of structures of matter in the sense that the number of partons in a depends on the space and time resolution of the probe. The studies of the structure provide not only the necessary initial condition of high energy collision but also the most active and fruitful frontier for studying properties of QCD where new QCD application techniques have been developed such as the collinear expansion and factorization theorem. With developments of experimental techniques, the studies have been extended to include spin degree of freedom and to use different probes such as electrons and muons. These studies have lead to many new discoveries, initiated the well known proton spin crisis and the proton radius puzzle, and attracted much more attention in the field of particle and nuclear physics. The experimental data from polarized DIS has revealed that quark spin contributes to only a small fraction of the spin and initiated the question where the spin comes from. The electromagnetic radius of the measured from the electron-proton interaction does not coincide with that measured from the muon-proton interaction and initiated the radius puzzle. The experimental data on single-spin asymmetries in inclusive hadron production in proton-proton collisions have lead the studies of the QCD parton model from the simplest one dimensional longitudinal distribution case to the three dimensional case including spin and transverse momentum dependences. The physics in the three-dimensional case is definitely much more abundant and interesting. The transverse momentum dependent (TMD) PDFs include the simple number densities, the helicity distributions, the transversity, and many spin-momentum correlation functions as well, such as the Sivers function, the Boer-Mulders function, the Collins function, worm-gear, pretzelosity and also many so-called higher twist distributions describing quantum interference effects. Both experimental and theoretical efforts have been made in last years and many progresses have been made. The planned new instrumentations such as the Electron-Ion Collider (EIC) in the United States will definitely bring the field to a new era of rapid developments. Several new equipment discussed in China, including Super-Z Factory, Circular Electron Positron Collider (CEPC) and EIC of China (EicC) at High Intensity heavy ion Accelerator Facility (HIAF) and so on can also bring very significant contributions even play dominant role in some of the aspects in this exciting field.