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Flavor Structure of the Nucleon Sea

Wen-Chen Chang, Jen-Chieh Peng

TL;DR

This review surveys the flavor and spin structure of the nucleon sea, emphasizing unpolarized, polarized, and transverse-momentum dependent quark distributions. It integrates evidence from DIS, Drell-Yan, and SIDIS, highlighting the observed light-quark flavor asymmetry ($ar{d}>ar{u}$) and SU(3) flavor breaking in the strange sector, along with emerging insights into intrinsic charm and sea-quark polarization. The discussion contrasts models (meson-cloud, chiral, instanton, five-quark intrinsic sea) with lattice QCD results and global PDF analyses, noting areas of agreement and tension. It also covers the burgeoning field of TMDs (Sivers, Boer-Mulders, transversity) in the sea and outlines future experimental programs (SeaQuest, J-PARC, LHC, EIC) aimed at constraining sea-quark distributions with greater precision. Together, these efforts deepen our understanding of nonperturbative QCD and guide improvements in nucleon structure predictions for high-energy phenomenology.

Abstract

We review the current status and future prospects on the subject of flavor structure of the nucleon sea. The flavor structure of the nucleon sea provides unique information on the non-perturbative aspects of strong interactions allowing stringent tests of various models on the partonic structures of the nucleons as well as lattice QCD calculations. The scope of this review covers the unpolarized, polarized, and the transverse-momentum dependent sea-quark distributions of the nucleons. While the main focus of this review is on the physics motivation and recent progress on the subject of the nucleon sea, we also discuss future prospects of addressing some outstanding issues on the flavor structure of the nucleon sea.

Flavor Structure of the Nucleon Sea

TL;DR

This review surveys the flavor and spin structure of the nucleon sea, emphasizing unpolarized, polarized, and transverse-momentum dependent quark distributions. It integrates evidence from DIS, Drell-Yan, and SIDIS, highlighting the observed light-quark flavor asymmetry () and SU(3) flavor breaking in the strange sector, along with emerging insights into intrinsic charm and sea-quark polarization. The discussion contrasts models (meson-cloud, chiral, instanton, five-quark intrinsic sea) with lattice QCD results and global PDF analyses, noting areas of agreement and tension. It also covers the burgeoning field of TMDs (Sivers, Boer-Mulders, transversity) in the sea and outlines future experimental programs (SeaQuest, J-PARC, LHC, EIC) aimed at constraining sea-quark distributions with greater precision. Together, these efforts deepen our understanding of nonperturbative QCD and guide improvements in nucleon structure predictions for high-energy phenomenology.

Abstract

We review the current status and future prospects on the subject of flavor structure of the nucleon sea. The flavor structure of the nucleon sea provides unique information on the non-perturbative aspects of strong interactions allowing stringent tests of various models on the partonic structures of the nucleons as well as lattice QCD calculations. The scope of this review covers the unpolarized, polarized, and the transverse-momentum dependent sea-quark distributions of the nucleons. While the main focus of this review is on the physics motivation and recent progress on the subject of the nucleon sea, we also discuss future prospects of addressing some outstanding issues on the flavor structure of the nucleon sea.

Paper Structure

This paper contains 32 sections, 51 equations, 36 figures, 8 tables.

Table of Contents

  1. Introduction
  2. Probing the Nucleon Sea
  3. Deep Inelastic Scattering with Charged Lepton Beams
  4. Deep Inelastic Scattering with Neutrino or Antineutrino Beams
  5. Drell-Yan Process
  6. Semi-Inclusive Deep Inelastic Scattering
  7. Unpolarized Distributions of Sea Quarks
  8. Breaking of SU(2) Flavor Symmetry of Light Quarks Sea
  9. Strange Quark Sea
  10. Breaking of SU(3) flavor symmetry of quark sea
  11. Asymmetry between $s (x)$ and $\bar{s} (x)$
  12. Strange quarks from the global analysis
  13. Five-quark intrinsic sea model and lattice QCD
  14. Intrinsic and extrinsic sea
  15. Connected and disconnected sea
  16. ...and 17 more sections

Figures (36)

  • Figure 1: Inclusive lepton-hadron scattering via (a) $\gamma^*, Z$ (b) $W$ exchange.
  • Figure 2: Inclusive neutrino scattering via $W$ exchange.
  • Figure 3: Drell-Yan process for (a) $l^+ l^-$ and (b) $l \nu$ production in the collisions of two nucleons.
  • Figure 4: Semi-inclusive deep inelastic scattering via (a) $\gamma^*$ (b) $W$ exchange.
  • Figure 5: Unpolarized parton distribution $xf(x,Q^2)$ of valence quarks $u_v, d_v$ and sea quarks $\bar{u}, \bar{d}, \bar{s}$ from CT10 CT10, MSTW2008 MSTW08 and NNPDF2.3 NNPDF2.3 PDFs at $Q^2 = 4$ GeV$^2$.
  • ...and 31 more figures