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Title: The spin structure of the nucleon

Abstract

We review the present understanding of the spin structure of protons and neutrons, the fundamental building blocks of nuclei collectively known as nucleons. The field of nucleon spin provides a critical window for testing Quantum Chromodynamics (QCD), the gauge theory of the strong interactions since it involves fundamental aspects of hadron structure, and it can be probed in detail in experiments, particularly deep inelastic lepton scattering on polarized targets. QCD was initially probed in high energy deep inelastic lepton scattering with unpolarized beams and targets. With time, interest shifted from testing perturbative QCD to illuminating the nucleon structure itself. In fact, the spin degrees of freedom of hadrons provides an essential and detailed verification of both perturbative and nonperturbative QCD dynamics. Nucleon spin was initially thought of coming mostly from the spin of its quark constituents, based on intuition from the parton model. However, the first experiments showed that this expectation was incorrect. It is now clear that nucleon physics is much more complex, involving quark orbital angular momenta as well as gluonic and sea quark contributions. Thus, the nucleon spin structure remains a most active aspect of QCD research, involving important advances such as the developments of generalized partonmore » distributions (GPD) and transverse momentum distributions (TMD). Elastic and inelastic lepton-proton scattering, as well as photoabsorption experiments provide various ways to investigate non-perturbative QCD. Fundamental sum rules --such as the Bjorken sum rule for polarized photoabsorption on polarized nucleons-- are also in the non-perturbative domain. This realization triggered a vigorous program to link the low energy effective hadronic description of the strong interactions to fundamental quarks and gluon degrees of freedom of QCD. This has also led to the development of holographic QCD ideas based on the AdS/CFT or gauge/gravity correspondence, a novel approach providing a well-founded semiclassical approximation to QCD. Any QCD-based model of the nucleon's spin and dynamics must also successfully account for the observed spectroscopy of hadrons. Analytic calculations of the hadron spectrum, a long sought goal of QCD research, has now being realized using light-front holography and superconformal quantum mechanics, a formalism consistent with the results from nucleon spin studies. We begin this review with a phenomenological description of nucleon structure in general and of its spin structure in particular, aimed to engage non-specialist readers. A fundamental tool is Dirac's front form and light-front quantization which provides a frame-independent, relativistic description of hadron structure and dynamics, the derivation of spin-sum rules, and a direct connection to the QCD Lagrangian. Next, we discuss the nucleon spin structure at high energy, including topics such as GPDs and TMDs, as well as nucleon spin observables sensitive to final-state interactions such as the Sivers effect. We then discuss experimental and theoretical advances in the nonperturbative domain --in particular the development of light-front holographic QCD and superconformal quantum mechanics, its predictions for the spin content of nucleons, the computation of PDFs and of hadron masses.« less

Authors:
ORCiD logo [1];  [2];  [3]
  1. Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  3. Univ. of Costa Rica, San Jose (Costa Rica)
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)
OSTI Identifier:
1530179
Alternate Identifier(s):
OSTI ID: 1532411
Report Number(s):
JLAB-PHY-18-2760; DOE/OR/23177-4611; arXiv:1807.05250
Journal ID: ISSN 0034-4885
Grant/Contract Number:  
AC05-06OR23177; AC02-76SF00515
Resource Type:
Accepted Manuscript
Journal Name:
Reports on Progress in Physics
Additional Journal Information:
Journal Volume: 82; Journal Issue: 7; Journal ID: ISSN 0034-4885
Publisher:
IOP Publishing
Country of Publication:
United States
Language:
English
Subject:
74 ATOMIC AND MOLECULAR PHYSICS; QCD; non-perturbative; proton; neutron; nucleon spin structure

Citation Formats

Deur, Alexandre, Brodsky, Stanley J., and de Téramond, Guy F. The spin structure of the nucleon. United States: N. p., 2019. Web. doi:10.1088/1361-6633/ab0b8f.
Deur, Alexandre, Brodsky, Stanley J., & de Téramond, Guy F. The spin structure of the nucleon. United States. doi:10.1088/1361-6633/ab0b8f.
Deur, Alexandre, Brodsky, Stanley J., and de Téramond, Guy F. Tue . "The spin structure of the nucleon". United States. doi:10.1088/1361-6633/ab0b8f.
@article{osti_1530179,
title = {The spin structure of the nucleon},
author = {Deur, Alexandre and Brodsky, Stanley J. and de Téramond, Guy F.},
abstractNote = {We review the present understanding of the spin structure of protons and neutrons, the fundamental building blocks of nuclei collectively known as nucleons. The field of nucleon spin provides a critical window for testing Quantum Chromodynamics (QCD), the gauge theory of the strong interactions since it involves fundamental aspects of hadron structure, and it can be probed in detail in experiments, particularly deep inelastic lepton scattering on polarized targets. QCD was initially probed in high energy deep inelastic lepton scattering with unpolarized beams and targets. With time, interest shifted from testing perturbative QCD to illuminating the nucleon structure itself. In fact, the spin degrees of freedom of hadrons provides an essential and detailed verification of both perturbative and nonperturbative QCD dynamics. Nucleon spin was initially thought of coming mostly from the spin of its quark constituents, based on intuition from the parton model. However, the first experiments showed that this expectation was incorrect. It is now clear that nucleon physics is much more complex, involving quark orbital angular momenta as well as gluonic and sea quark contributions. Thus, the nucleon spin structure remains a most active aspect of QCD research, involving important advances such as the developments of generalized parton distributions (GPD) and transverse momentum distributions (TMD). Elastic and inelastic lepton-proton scattering, as well as photoabsorption experiments provide various ways to investigate non-perturbative QCD. Fundamental sum rules --such as the Bjorken sum rule for polarized photoabsorption on polarized nucleons-- are also in the non-perturbative domain. This realization triggered a vigorous program to link the low energy effective hadronic description of the strong interactions to fundamental quarks and gluon degrees of freedom of QCD. This has also led to the development of holographic QCD ideas based on the AdS/CFT or gauge/gravity correspondence, a novel approach providing a well-founded semiclassical approximation to QCD. Any QCD-based model of the nucleon's spin and dynamics must also successfully account for the observed spectroscopy of hadrons. Analytic calculations of the hadron spectrum, a long sought goal of QCD research, has now being realized using light-front holography and superconformal quantum mechanics, a formalism consistent with the results from nucleon spin studies. We begin this review with a phenomenological description of nucleon structure in general and of its spin structure in particular, aimed to engage non-specialist readers. A fundamental tool is Dirac's front form and light-front quantization which provides a frame-independent, relativistic description of hadron structure and dynamics, the derivation of spin-sum rules, and a direct connection to the QCD Lagrangian. Next, we discuss the nucleon spin structure at high energy, including topics such as GPDs and TMDs, as well as nucleon spin observables sensitive to final-state interactions such as the Sivers effect. We then discuss experimental and theoretical advances in the nonperturbative domain --in particular the development of light-front holographic QCD and superconformal quantum mechanics, its predictions for the spin content of nucleons, the computation of PDFs and of hadron masses.},
doi = {10.1088/1361-6633/ab0b8f},
journal = {Reports on Progress in Physics},
number = 7,
volume = 82,
place = {United States},
year = {2019},
month = {6}
}

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