Sivers function in a spectator model with axial-vector diquarks

TL;DR

This paper computes the Sivers function within a spectator nucleon model that includes both scalar and axial-vector diquarks, attributing the required final-state interaction to gluon rescattering. The analysis shows that axial-vector diquarks generate a nonzero down-quark Sivers function with an opposite sign and substantially smaller magnitude than the up-quark contribution, and they also substantially damp the up-quark Sivers effect. Using these distributions, the authors estimate SIDIS single-spin asymmetries for π^+, π^−, and π^0, finding reduced asymmetries when axial-vector diquarks are included and only modest differences between π^+ and π^− due to u-quark dominance. They acknowledge limitations such as the absence of sea-quark effects and lack of established Q^2 evolution for the Sivers function, but the results offer qualitative insights into the flavor structure of transverse spin phenomena in SIDIS.

Abstract

We perform a calculation of the Sivers function in a spectator model of the nucleon, with scalar and axial-vector diquarks. We make use of gluon rescattering to produce the nontrivial phases necessary to generate the Sivers function. The inclusion of axial-vector diquarks enables us to obtain a nonzero Sivers function for down quarks. Using the results of our model, we discuss the phenomenology of transvere single spin asymmetries in pi+, pi-, and pi0 production, which are currently analysed by the HERMES and COMPASS collaborations. We find that the inclusion of axial-vector diquarks substantially reduces the asymmetries.

Figures (7)

• Figure 1: Tree-level and one-loop diagrams for the specator-model calculation of the Sivers function. The dashed line indicates both the scalar and axial-vector diquarks.
• Figure 2: Model calculation of $x f_1 (x)$: with scalar diquarks only (dashed line), with scalar and axial-vector diquarks (solid line). The $d$ quark distribution is zero when only scalar diquarks are used.
• Figure 3: Model calculation of $x f_1 (x)$ (solid line) compared to the CTEQ5L parametrization Lai:1999wy (dashed line) at 1 GeV$^2$.
• Figure 4: Model calculation of $x f_{1T}^{\perp (1/2)}(x)$: with scalar diquarks only (dashed line), with scalar and axial-vector diquarks (solid line). The $d$ quark distribution is zero when only scalar diquarks are used.
• Figure 5: Description of the vectors and angles involved in the Sivers asymmetry measurement.