Physics of the Nucleon Sea Quark Distributions

TL;DR

This review analyzes whether the nucleon sea arises solely from perturbative QCD or includes significant nonperturbative flavor asymmetries. It first outlines perturbative sea generation and DGLAP evolution within the improved parton model, then surveys experimental findings and models for SU(2) flavor breaking, strange sea, and heavy-quark content. The Gottfried sum rule violation and Drell-Yan measurements indicate a sizable nonperturbative flavor asymmetry (notably $\overline d > \overline u$), while the strange and charm sectors reveal more nuanced behavior with ongoing debates about $s$ vs $\overline s$ and intrinsic charm. The discussion covers meson-cloud, Pauli-blocking, chiral, and CSV scenarios and highlights the need for further data to disentangle nonperturbative effects from perturbative evolution in the nucleon sea.

Abstract

Sea quark distributions in the nucleon have naively been expected to be generated perturbatively by gluon splitting. In this case, there is no reason for the light quark and anti-quark sea distributions to be different. No asymmetries in the strange or heavy quark sea distributions are predicted in the improved parton model. However, recent experiments have called these naive expectations into question. A violation of the Gottfried sum rule has been measured in several experiments, suggesting that $\bar u < \bar d$ in the proton. Additionally, other measurements, while not definitive, show that there may be an asymmetry in the strange and anti-strange quark sea distributions. These effects may require nonperturbative explanations. In this review we first discuss the perturbative aspects of the sea quark distributions. We then describe the experiments that could point to nonperturbative contributions to the nucleon sea. Current phenomenological models that could explain some of these effects are reviewed.

Figures (29)

• Figure 1: The proton structure function $F_2^{\gamma p}$ as a function of $Q^2$ for muon beams with energies 120, 200, 240, and 280 GeV measured by the European Muon Collaboration EMC. From top to bottom the values of the $x$ bins are: 0.0175, 0.03, 0.05, 0.08, 0.125, 0.175, 0.25, 0.35, 0.45, 0.55, 0.65, and 0.75. To separate the data, $F_2^{\gamma p}$ in each $x$ bin is scaled by a factor of 1.5 from the next higher $x$ bin. Therefore, only the highest $x$ data (lowest points) have the correct scale. Only the statistical uncertainty is shown.
• Figure 2: The nucleon structure functions $F_2^{W^\pm N}$ (a) and $x F_3^{W^\pm N}$ (b) as a function of $Q^2$CCFRfig from neutrino-iron deep-inelastic scattering. The $x$ bins from top to bottom are: 0.015, 0.045, 0.08, 0.125, 0.175, 0.225, 0.275, 0.35, 0.45, 0.55, and 0.65. To separate the data, the structure functions in each $x$ bin are scaled by a factor of 2 from the next higher $x$ bin. Therefore, only the highest $x$ data (lowest points) have the correct scale. Only the statistical uncertainty is shown.
• Figure 3: The proton structure function $F_2^{\gamma p}$ as a function of $Q^2$ at the HERA collider (26.7 GeV electrons on 820 GeV protons) h1. From top to bottom the values of the $x$ bins are: 0.000178, 0.000261, 0.000383, 0.000562, 0.00075, 0.000825, 0.00133, 0.00237, 0.00421, 0.0075, 0.0133, 0.0237, 0.0421, 0.075, and 0.133. To separate the data, the structure function in each $x$ bin is scaled by a factor of 2 from the next higher $x$ bin. Therefore, only the highest $x$ data (lowest points) have the correct scale. Only the statistical uncertainty is shown.
• Figure 4: The CTEQ5 NLO $\overline{\rm MS}$ scheme proton parton distributions are given at $Q^2 = 4$ and 100 GeV$^2$. The up and down valence distributions are shown in the solid and dashed lines respectively in (a) and (b). The up, down and strange sea distributions and the gluon distributions are given in the solid, dashed, dot-dashed and dotted curves respectively in (c) and (d).
• Figure 5: The MRST NLO $\overline{\rm MS}$ scheme proton parton distributions are given at $Q^2 = 4$ and 100 GeV$^2$. The up and down valence distributions are shown in the solid and dashed lines respectively in (a) and (b). The up, down and strange sea distributions and the gluon distributions are given in the solid, dashed, dot-dashed and dotted curves respectively in (c) and (d).