The Quark/Antiquark Asymmetry of the Nucleon Sea

TL;DR

This paper argues that intrinsic quark–antiquark pairs tied to the nucleon's bound-state wavefunction can exhibit significant asymmetries between quarks and antiquarks, unlike the CP-symmetric extrinsic sea from perturbative gluon splitting. Using a light-cone meson-baryon fluctuation framework with two-body light-cone wavefunctions, it predicts negative polarization for intrinsic d and s quarks while corresponding antiquarks contribute negligibly to the proton spin, and it foresees structured momentum distributions (e.g., s vs s̄) and socialized asymmetries such as dd̄ > u ū. The model provides qualitative alignment with observations related to the proton spin problem and Gottfried sum rule violation and offers explanations for discrepancies in strange-quark determinations, with testable predictions in Lambda polarization and neutrino-induced processes. Altogether, it presents a bound-state–driven perspective on nucleon structure that sets boundary conditions for QCD evolution and helps interpret several longstanding puzzles in hadron physics.

Abstract

Although the distributions of sea quarks and antiquarks generated by leading-twist QCD evolution through gluon splitting $g \rightarrow \bar q q$ are necessarily CP symmetric, the distributions of nonvalence quarks and antiquarks which are intrinsic to the nucleon's bound state wavefunction need not be identical. In this paper we investigate the sea quark/antiquark asymmetries in the nucleon wavefunction which are generated by a light-cone model of energetically-favored meson-baryon fluctuations. The model predicts striking quark/antiquark asymmetries in the momentum and helicity distributions for the down and strange contributions to the proton structure function: the intrinsic $d$ and $s$ quarks in the proton sea are predicted to be negatively polarized, whereas the intrinsic $\bar d$ and $\bar s$ antiquarks give zero contributions to the proton spin. Such a picture is supported by experimental phenomena related to the proton spin problem and the violation of the Ellis-Jaffe sum rule. The light-cone meson-baryon fluctuation model also suggests a structured momentum distribution asymmetry for strange quarks and antiquarks which could be relevant to an outstanding conflict between two different determinations of the strange quark sea in the nucleon. The model predicts an excess of intrinsic $d \bar d$ pairs over $u \bar u$ pairs, as supported by the Gottfried sum rule violation. We also predict that the intrinsic charm and anticharm helicity and momentum distributions are not identical.

Figures (4)

• Figure 1: The momentum distributions for the strange quarks and antiquarks in the light-cone meson-baryon fluctuation model of intrinsic $q \overline{q}$ pairs, with the fluctuation wavefunction of $K^+\Lambda$ normalized to 1. The curves in (a) are the calculated results of $s(x)$ (solid curves) and $\overline{s}(x)$ (broken curves) with the Gaussian type (thick curves) and power-law type (thin curves) wavefunctions and the curves in (b) are the corresponding $\delta_s(x)=s(x)-\overline{s}(x)$. The parameters are $m_q=330$ MeV for the light-flavor quark mass, $m_s=480$ MeV for the strange quark mass, $m_D=600$ MeV for the spectator mass, the universal momentum scale $\alpha=330$ MeV, and the power constant $p=3.5$, with realistic meson and baryon masses.
• Figure 2: The momentum distributions for the down and charm quarks and antiquarks in the light-cone meson-baryon fluctuation model of intrinsic $q \overline{q}$ pairs, with the fluctuation wavefunctions normalized to 1. The curves are the calculated results for $d(x)$ (thick solid curve), $\overline{d}(x)$ (thick broken curve), $c(x)$ (thin solid curve), and $\overline{c}(x)$ (thin broken curve) with the Gaussian type wavefunction. The parameters are $m_d=330$ MeV for the down quark mass and $m_c=1500$ MeV for the charm quark mass, with other parameters as those in Fig. 1.
• Figure 3: Results for the strange quark distributions $x s(x)$ and $x \overline{s}(x)$ as a function of the Bjorken scaling variable $x$. The open squares shows the CTEQ determination of $\frac{1}{2}\,x[s(x)+\overline{s}(x)]$ obtained from $\frac{5}{12}\,(F_{2}^{\nu{\cal N}}+F_{2}^{\overline{\nu}{\cal N}})(x)({\rm CCFR})- 3F_{2}^{\mu {\cal N}}(x)({\rm NMC}).$ The circles show the CCFR determinations for $x s(x)$ from dimuon events in neutrino scattering using a leading-order QCD analysis at $Q^{2} \approx 5 ({\rm GeV/c})^{2}$ (closed circles) and a higher-order QCD analysis at $Q ^2 =20 ({\rm GeV/c})^2$ (open circles). The thick curves are the unevolved predictions of the light-cone fluctuation model for $x s(x)$ (solid curve labeled a) and $\frac{1}{2}x[s(x)+\overline{s}(x)]$ (broken curve labeled b) for the Gaussian type wavefunction in the light-cone meson-baryon fluctuation model of intrinsic $q \overline{q}$ pairs assuming a probability of 10% for the $K^+ \Lambda$ state. The thin solid and broken curves (labeled a' and b') are the corresponding evolved predictions multiplied by the factor $d_v(x)|_{fit}/d_v(x)|_{model}$ assuming a probability of 4% for the $K^+ \Lambda$ state.
• Figure 4: Results for the strange asymmetry $s(x)/\overline{s}(x)$ as a function of the Bjorken scaling variable $x$. The open squares are the combined CTEQ-CCFR "data" and the closed circles are the CCFR measurement of $s(x)/\overline{s}(x)$. The thick (thin) solid curve is the calculated result of $s(x)/\overline{s}(x)$ in the light-cone meson-baryon fluctuation model for the Gaussian type wavefunction with $\alpha=330$ ($530$) MeV. The thick broken curve is the result with a larger $\alpha=800$ MeV and the thin broken curve is the above result with of about 30% extrinsic strange quarks ( i.e., $x s_{extrinsic}=0.07(1-x)^5$) included for comparison with the CCFR result at small $x$.