Quark-antiquark asymmetry of helicity distributions in the nucleon sea

Mengyun Liu,Bo-Qiang Ma

doi:10.1103/physrevd.98.036024

Abstract

We study the helicity distributions of light flavor quark-antiquark ($q \bar{q}$) pairs in the nucleon sea. The valence quarks are handled by adopting the light-cone SU(6) quark-spectator-diquark model and the sea $q \bar{q}$ pairs are treated from statistical consideration by introducing the helicity suppression factors $l_q(x)$ and $\bar l_q(x)$ to parametrize the helicity distributions of q-flavor sea quark and antiquark respectively, while $\Delta l_q(x)=l_q(x)-\bar l_q(x)$ represents a combined effect of helicity contribution due to sea $q \bar{q}$ pairs. From fitting the nucleon polarization asymmetries $A^N_1$ in inclusive deep inelastic scattering processes and the single-spin asymmetries $A^{W^{\pm}}_L$ in Drell-Yan type processes, we find a significant asymmetry between the quark and antiquark helicity distributions of the nucleon sea. Therefore the quark-antiquark asymmetry of helicity distributions of nucleon sea $q \bar{q}$ pairs, i.e., $\Delta q_s(x) \neq \Delta \bar q_s(x)$, plays an important role for a comprehensive understanding of the nucleon spin content.

Highlights

The spin structure of hadrons has received considerable attention since the so-called “proton spin crisis” [1,2], which implies that only a small fraction of the proton spin comes from quark spins
We investigate the quark-antiquark asymmetry of helicity distributions of the nucleon sea with the valence quarks handled by the light-cone SU(6) quarkspectator-diquark model [10,11,51] and the sea quarks treated by introducing new parameters from statistical consideration
The results indicate that the light-cone SU(6) quarkspectator-diquark model, together with the relativistic effect due to quark transversal motions and the quark-antiquark asymmetry of helicity distributions of the nucleon sea with phenomenologically based models and parameters, can provide good descriptions for the physical quantities related to the “proton spin crisis.”

Summary

INTRODUCTION

The spin structure of hadrons has received considerable attention since the so-called “proton spin crisis” [1,2], which implies that only a small fraction (about 30% in recent studies [3,4,5]) of the proton spin comes from quark spins. The sea quark spin contribution to the proton is believed to be nontrivial, as reflected in the recent observations [21,22] of single spin asymmetries of WÆ production in polarized proton-proton collisions [23,24] It is shown [21] that a reasonable description of WÆ single spin asymmetry data [23] can be achieved by adopting sizable helicity distributions of the light-flavor up (u) and down (d) antiquarks. We investigate the quark-antiquark asymmetry of helicity distributions of the nucleon sea with the valence quarks handled by the light-cone SU(6) quarkspectator-diquark model [10,11,51] and the sea quarks treated by introducing new parameters from statistical consideration.

LIGHT-CONE QUARK-SPECTATORDIQUARK MODEL

NUCLEON POLARIZATION ASYMMETRIES AN1 IN POLARIZED DIS PROCESSES

Findings

SUMMARY