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Sivers vs. Collins effect in azimuthal single spin asymmetries in pion production in SIDIS

A. V. Efremov, K. Goeke, P. Schweitzer

TL;DR

This paper analyzes whether the Sivers effect can compete with the Collins effect in azimuthal single-spin asymmetries (A_UL) for pion production in SIDIS. Using phenomenological Sivers functions and favoured fragmentation, the authors find the Sivers contribution to A_UL with a longitudinally polarized target is negligible in HERMES kinematics and argue the same holds for CLAS and COMPASS, supporting Collins-only interpretations. They emphasize that transverse-target measurements can cleanly separate Sivers and Collins contributions, with estimates suggesting a modest Sivers role (~5%) in such contexts. The appendix discusses weighting schemes for asymmetries and the implications of different modeling approaches on the extracted signals.

Abstract

Recently it has been argued that the transverse momentum dependent twist-2 Sivers distribution function does not vanish in QCD. Therefore both, the Collins and Sivers effects, should be considered in order to explain the azimuthal single spin asymmetries A(UL) in pion production in semi-inclusive deeply inelastic lepton scattering of a longitudinally polarized target. On the basis of presently available phenomenological information on the Sivers function we estimate that for those asymmetries A(UL) in the kinematic region of the HERMES experiments the Sivers effect can be neglected with respect to the Collins effect. It is argued that the same feature holds also for the COMPASS and CLAS experiments. This justifies theoretical approaches to understand the HERMES data on the basis of the Collins effect only.

Sivers vs. Collins effect in azimuthal single spin asymmetries in pion production in SIDIS

TL;DR

This paper analyzes whether the Sivers effect can compete with the Collins effect in azimuthal single-spin asymmetries (A_UL) for pion production in SIDIS. Using phenomenological Sivers functions and favoured fragmentation, the authors find the Sivers contribution to A_UL with a longitudinally polarized target is negligible in HERMES kinematics and argue the same holds for CLAS and COMPASS, supporting Collins-only interpretations. They emphasize that transverse-target measurements can cleanly separate Sivers and Collins contributions, with estimates suggesting a modest Sivers role (~5%) in such contexts. The appendix discusses weighting schemes for asymmetries and the implications of different modeling approaches on the extracted signals.

Abstract

Recently it has been argued that the transverse momentum dependent twist-2 Sivers distribution function does not vanish in QCD. Therefore both, the Collins and Sivers effects, should be considered in order to explain the azimuthal single spin asymmetries A(UL) in pion production in semi-inclusive deeply inelastic lepton scattering of a longitudinally polarized target. On the basis of presently available phenomenological information on the Sivers function we estimate that for those asymmetries A(UL) in the kinematic region of the HERMES experiments the Sivers effect can be neglected with respect to the Collins effect. It is argued that the same feature holds also for the COMPASS and CLAS experiments. This justifies theoretical approaches to understand the HERMES data on the basis of the Collins effect only.

Paper Structure

This paper contains 8 sections, 16 equations, 3 figures.

Table of Contents

  1. Introduction.
  2. Sivers effect in the HERMES longitudinal target polarization experiment.
  3. CLAS and COMPASS experiments.
  4. Sivers effect in SIDIS with transversely polarized target.
  5. Comment on the calculations of $A_{UL}$ by the present authors.
  6. Conclusions.
  7. Acknowledgement.
  8. Appendix.

Figures (3)

  • Figure 1: Kinematics of the process $lp\rightarrow l'h X$ in the lab frame.
  • Figure 2: The "ratio of Sivers to Collins effect" as defined in Eq. (\ref{['Ratio']}) vs. $x$ for the kinematics of the HERMES experiment
  • Figure 3: (a) The single spin azimuthal asymmetry $A_{UT}^{\sin(\phi-\phi_s)k_\perp/M_{\hbox{\tiny N}}}(x)$ for $\pi^\pm$ and $\pi^0$ for the HERMES kinematics as a function of $x$. Note that the weight $k_\perp/M_{\hbox{\tiny N}}$ provides an artificial suppression, see text. (b) The single spin azimuthal asymmetry $A_{UT}^{\sin(\phi-\phi_s)}(x)$ (i.e. weighted without the factor $k_\perp/M_{\hbox{\tiny N}}$) for the HERMES kinematics as a function of $x$.