Single-spin asymmetries in inclusive deep inelastic scattering and multiparton correlations in the nucleon

TL;DR

The paper investigates transverse single-spin asymmetries in inclusive deep-inelastic scattering arising from multiphoton exchange, emphasizing two-photon diagrams where photons couple to different quarks. It develops a collinear twist-3 framework linking the quark-photon-quark correlator F_{FT} to the ETQS quark-gluon-quark correlator T_F and analyzes proton and neutron data to test the Sivers-related mechanisms. Using three distinct T_F inputs, the authors compute A_{UT}^p and A_{UT}^n, finding that Sivers-based inputs can describe proton data while neutron data require specific sign and flavor structure; the neutron results tend to favor Sivers-linked interpretations, while some hadronic SSA analyses imply additional sources beyond the Sivers effect. The work suggests that the observed DIS SSAs are consistent with a Sivers-related mechanism in which final-state interactions are crucial, but it also indicates that a complete explanation of SSAs in hadronic reactions likely involves multiple mechanisms beyond the Sivers effect alone.

Abstract

Transverse single-spin asymmetries in inclusive deep inelastic lepton-nucleon scattering can be generated through multiphoton exchange between the leptonic and the hadronic part of the process. Here we consider the two-photon exchange, and mainly focus on the transverse target spin asymmetry. In particular, we investigate the case where two photons couple to different quarks. Such a contribution involves a quark-photon-quark correlator in the nucleon, which has a (model-dependent) relation to the Efremov-Teryaev-Qiu-Sterman quark-gluon-quark correlator T_F. Using different parametrizations for T_F we compute the transverse target spin asymmetries for both a proton and a neutron target and compare the results to recent experimental data. In addition, potential implications for our general understanding of single-spin asymmetries in hard scattering processes are discussed.

Figures (8)

• Figure 1: Left panel: Two-photon exchange contribution (box graph) to inclusive DIS in the parton model. The Hermitian conjugate diagram, not shown in the figure, has to be considered as well. The so-called crossed box graph does not contribute to $A_{UT}$. Right panel: Sample diagram for two-photon exchange contribution involving a $qgq$ correlator. Such diagrams contribute to the leading power of the SSA for a transversely polarized target.
• Figure 2: Two-photon exchange contributions to inclusive DIS where the photons couple to different quarks. (The Hermitian conjugate diagrams are not shown.) Such contributions can be expressed through a $q \gamma q$ correlator in the nucleon. Particle lines that can go on-shell are indicated by a short dash (see text for more details).
• Figure 3: Feynman diagrams for the $q \gamma q$-correlation function $F_{FT}$ in a diquark model of the nucleon. Diagram (c) has to be considered in the case of the proton. Only diagram (a) gives a nonzero contribution.
• Figure 4: Numerical result for our model for the $q \gamma q$ correlator $F_{FT}$ for the proton at the scale $\mu^2 = 2\,\textrm{GeV}^2$, based on three different inputs for $T_F$. Left panel: Up quarks. Right panel: Down quarks.
• Figure 5: Proton target asymmetry for three inputs for $F_{FT}$. Left panel: Comparison to data from the HERMES Collaboration :2009wj. The asymmetry has been evaluated for the average values $\langle x \rangle$ and $\langle Q^2 \rangle$ of the data bins. Right panel: Results for typical HERMES kinematics at $y = 0.5$, where the lowest $x$ value for the curves corresponds to $Q^2 = 1\,\textrm{GeV}^2$.