Bo-Qiang Ma

The quark-antiquark asymmetry of the nucleon sea☆

Abstract Although the distributions of sea quarks and antiquarks generated by leading-twist QCD evolution through gluon splitting g → q q are necessarily CP symmetric, the distributions of nonvalence quarks and antiquarks which are intrinsic to the nucleons 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 d and 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 d pairs over u 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.

The x-dependent helicity distributions for valence quarks in nucleons

We derive simple relations between the polarized and unpolarized valence quark distributions in a light-cone SU(6) quark-spectator model for nucleons. The explicit x-dependent Wigner rotation effect for the light-flavor quarks is calculated, It is shown that the mass difference between the scalar and vector spectators could reproduce the up and down valence quark asymmetry that accounts for the observed ratio F-2(n)/F-2(p). The proton, neutron, and deuteron polarization asymmetries, A(1)(p), A(1)(n), and A(1)(d), are in agreement with the available data by taking into account the Wigner rotation effect. The calculated x-dependent helicity distributions of the up and down valence quarks are compared with the recent results from semi-inclusive hadron asymmetries in polarized deep inelastic scattering by the Spin Muon Collaboration.

Sea quark content of nucleons. Flavour distribution asymmetry or isospin symmetry breaking

It is shown that the violation of the Gottfried sum rule could be due to the flavour distribution asymmetry in the sea of nucleons while preserving the isospin symmetry between the proton and the neutron, or alternatively, due to the isospin symmetry breaking between the proton and the neutron while preserving the flavour distribution symmetry in the sea of nucleons. A new sum is suggested to distinguish between the two possible explanations.

Flavor distributions in the nucleons: SU(2) sea asymmetry or isospin symmetry breaking

The Gottfried sum-rule violation reported by the New Muon Collaboration was interpreted as an indication for a flavor asymmetry of the sea quark in the nucleon. We investigate the alternative possibility that isospin symmetry between the proton and the neutron is breaking for small [ital x]. We examine systematically the consequences of this possibility for several processes, namely, neutrino deep inelastic scattering, the charged pion Drell-Yan process, the proton Drell-Yan process, and semi-inclusive deep inelastic scattering, and conclude that a decision between the two alternative explanations is possible.

Asymmetric quark / anti-quark hadronization in e+ e- annihilation

Abstract We point out that the fragmentation of a strange quark into nucleons versus anti-nucleons is not necessarily identical D p s (z, Q 2 ) ≠ D p s (z, Q 2 ) , even though the perturbative contributions from gluon splitting and evolution are p ↔ p symmetric. The observation of such asymmetries in the hadronization of strange and other heavy quarks can provide insight into the non-perturbative mechanisms underlying jet fragmentation in QCD.

The strange quark spin of the proton in semi-inclusive Λ leptoproduction

Abstract Considering the possible interpretation of the Ellis-Jaffe sum rule violation that the strange quark in the polarized proton is remarkably polarized in the opposite direction of the proton spin, we investigate its implication in the semi-inclusive Λ production in deep inelastic process with the electron beam unpolarized and proton target polarized longitudinally. As a result, we find that the measurement of the Λ polarization in the process considered can provide clean information about the strange quark spin distribution in the proton.

Strange magnetic moment and isospin symmetry breaking

Abstract The small mass difference m n − m p = 1.3 MeV between the proton and neutron leads to an excess of n = π − p over p = π + n fluctuations which can be calculated by using a light-cone meson-baryon fluctuation model of intrinsic quark-antiquark pairs of the nucleon sea. The Gottfried sum rule violation may partially be explained by isospin symmetry breaking between the proton and neutron and the same effect introduces correction terms to the anomalous magnetic moments and the anomalous weak magnetic moments of the proton and neutron. We also evaluated the strange magnetic moment of the nucleon from the lowest strangeness KΛ fluctuation and found a non-trivial influence due to isospin symmetry breaking in the experimental measurements of the strange magnetic moment of the nucleon.

Parton sum rules and improved scaling variable

Abstract The effect from quark masses and transversal motion on the Gottfried, Bjorken, and Ellis-Jaffe sum rules is examined by using a quark-parton model of nucleon structure functions based on an improved scaling variable. Its use results in corrections to the Gottfried, Bjorken, and Ellis-Jaffe sum rules. We use the Brodsky-Huang-Lepage prescription of light-cone wavefunctions to estimate the size of the corrections. We constrain our choice of parameters by the roughly known higher twist corrections to the Bjorken sum rule and find that the resulting corrections to the Gottfried and Ellis-Jaffe sum rules are relevant, though not large enough to explain the observed sum rule violations.

The x-dependent helicity distributions for valence quarks in nucleons [Phys. Lett. B 375 (1996) 320]

We derive simple relations between the polarized and unpolarized valence quark distributions in a light-cone SU(6) quark-spectator model for nucleons. The explicit x-dependent Wigner rotation effect for the lightflavor quarks is calculated. It is shown that the mass difference between the scalar and vector spectators could reproduce the up and down valence quark asymmetry that accounts for the observed ratio F 2 /F p 2 . The proton, neutron, and deuteron polarization asymmetries, Ap1, A n 1 , and A d 1, are in agreement with the available data by taking into account the Wigner rotation effect. The calculated x-dependent helicity distributions of the up and down valence quarks are compared with the recent results from semiinclusive hadron asymmetries in polarized deep inelastic scattering by the Mailing addressWe derive simple relations between the polarized and unpolarized valence quark distributions in a light-cone SU(6) quark-spectator model for nucleons. The explicit x-dependent Wigner rotation effect for the light-flavor quarks is calculated. It is shown that the mass difference between the scalar and vector spectators could reproduce the up and down valence quark asymmetry that accounts for the observed ratio

The non-perturbative effect on R=σL/σT from QCD vacuum1

F_2^{n}/F_2^{p}