Theoretical perspective for the future experiments on parton densities

Abstract

I explain the current status of parton-distribution-function (PDF) studies and future experimental prospects on their determinations. First, unpolarized PDFs of the nucleon are introduced as a field of precision QCD physics including higher-order α s corrections. Second, nuclear PDFs and polarized nucleonic PDFs are discussed. Third, the determination of fragmentation functions is explained. Forth, the three-dimensional (3D) structure functions are discussed in connection with the origin of the nucleon spin and gravitational form factors of hadrons. By the 3D structure functions, gravitational sources and the origin of nucleon mass could be clarified in the microscopic level of quarks and gluons. Furthermore, 3D structure studies of hadrons could be used for clarifying internal structure of exotic hadron candidates, nuclear composition of ultra-high-energy cosmic rays, and color-entanglement phenomena. The PDF field will be developed further along with progress in other fields of science.

Conference Summary of QNP2018

“…The project on unpolarized structure functions of the nucleon is becoming one of precisionphysics fields because of accurate determination of the PDFs and theoretical efforts on higher-order perturbative-QCD corrections [5]. It is the basics for finding a new hadron-physics phenomenon and a signature beyond the standard model in any high-energy hadron reactions, especially at the Large Hadron Collider (LHC).…”

“…There are three types in the 3D structure functions: generalized parton distributions (GPDs), generalized distribution amplitudes (GDAs), and transverse-momentum-dependent parton distributions (TMDs) [5,20]. The second moments of the GPDs are related to the total angular momentum carried by partons, so that they are key quantities to solve the nucleon spin puzzle.…”

“…One of major fundamental nuclear-physics topics is to understand how hadron masses are generated because up-and down-quark masses are very small as illustrated in Fig. 3 [4,5]. It is a similar topic to investigate the origin of nucleon spin.…”

“…The nucleon spin is given by the matrix element of total angular momentum expressed by the quark-gluon energy-momentum tensor, whereas the hadron mass is expressed by the quark-gluon energy-momentum tensor. The scalar component of the matrix element of the energy-momentum tensor is the source of the gravity, namely the hadron mass, and the vector component indicates pressure and shear-forces inside the hadron [4,5]. This hadron tomography is a fast developing field, and time has come to understand "how an apple falls in terms of quarks and gluons".…”

Gluon transversity in polarized proton-deuteron Drell-Yan process

“…For probing the orbitalangular-momentum part, we need to investigate threedimensional structure functions [7], namely, generalized parton distributions (GPDs) [8], generalized distribution amplitudes (GDAs) [9], and transverse-momentumdependent parton distributions (TMDs) [10]. Such experimental studies are under investigations at experimental facilities in the world [11].In spite of much progress on longitudinal spin physics, the transversely-polarized structure functions are not known well [12][13][14][15][16][17], although there are some recent studies on quark transversity distributions [18]. Such studies provide an important and alternative information in solving the nucleon spin puzzle.…”