Sivers function in light-cone quark model and azimuthal spin asymmetries in pion electroproduction

TL;DR

This work develops a relativistic, gauge-invariant treatment of the Sivers effect in SIDIS by constructing a light-cone proton wave function including both scalar and vector diquark spectators and applying Melosh-Wigner rotation. Final-state interactions modeled via one-gluon exchange generate nonzero Sivers functions for up and down quarks, with a positive u-quark and negative d-quark Sivers function, and these are used to predict weighted single-spin asymmetries for π^+, π^−, and π^0 in SIDIS under HERMES-like kinematics. The analysis highlights the role of vector diquarks in producing a nonzero d-quark Sivers function and compares with existing models, noting quantitative differences due to coupling choices and diquark structure. The results offer testable predictions for azimuthal spin asymmetries in pion electroproduction and illustrate how relativistic effects and final-state interactions drive single-spin phenomena in the nucleon.

Abstract

We perform a calculation of Sivers function in a light-cone SU(6) quark-diquark model with both scalar diquark and vector diquark spectators. We derive the transverse momentum dependent light-cone wave function of the proton by taking into account the Melosh-Wigner rotation. By adopting one-gluon exchange, we obtain a non-vanishing Sivers function of $down$ quark from interference of proton spin amplitudes. We analyze the $\frac{|P_{h\perp}|}{M}$ weighted Sivers asymmetries in $π^+$, $π^-$ and $π^0$ electroproduction off transverse polarized proton target, averaged and not averaged by the kinematics of HERMES experiment.

Figures (2)

• Figure 1: The $\mathbf{r}_\perp^2$-moments of Sivers functions of $u$ and $d$ quarks in SU(6) light-cone quark-diquark model.
• Figure 2: Single-spin Sivers asymmetries measured in SIDIS off proton target. The asymmetries have been weighted by $\frac{|P_{h\perp}|}{M}$. The first, second and third rows correspond to the asymmetries of $\pi^+$, $\pi^-$ and $\pi^0$ production respectively. The left column corresponds to the $x$-dependent asymmetries, the solid and dotted curves correspond to the asymmetries averaged and not averaged by the HERMES kinematics $0.2<z<0.7$. The right column corresponds to the $z$-dependent asymmetries, the solid and dotted curves correspond to the asymmetries averaged and not averaged by the HERMES kinematics $0.023<x<0.4$.