Extracting the Sivers function from polarized SIDIS data and making predictions

Abstract

The most recent data on the weighted transverse single spin asymmetry $A_{UT}^{\sin(φ_h-φ_S)}$ from HERMES and COMPASS collaborations are analysed within LO parton model with unintegrated parton distribution and fragmentation functions; all transverse motions are taken into account, with exact kinematics, in the elementary interactions. The overall quality of the data is such that, for the first time, a rather well constrained extraction of the Sivers function for $u$ and $d$ quarks is possible and is performed. Comparisons with models are made. Based on the extracted Sivers functions, predictions for $A_{UT}^{\sin(φ_h-φ_S)}$ asymmetries at JLab are given; suggestions for further measurements at COMPASS, with a transversely polarized hydrogen target and selecting favourable kinematical ranges, are discussed. Predictions are also presented for Single Spin Asymmetries (SSA) in Drell-Yan processes at RHIC and GSI.

Figures (7)

• Figure 1: HERMES data on $A_{UT}^{\sin(\phi_\pi-\phi_S)}$hermnew for scattering off a transversely polarized proton target and charged pion production. The curves are the results of our fit. The shaded area spans a region corresponding to one-sigma deviation at 90% CL (see text for further detail). Predictions for $\pi^0$ (upper panel) and kaon (lower panels) asymmetries are also shown.
• Figure 2: COMPASS data compnew on $A_{UT}^{\sin(\phi_h-\phi_S)}$ for scattering off a transversely polarized deuteron target and the production of positively ($h^+$) and negatively ($h^-$) charged hadrons. The curves are the results of our fit. The shaded area spans a region corresponding to one-sigma deviaton at 90% CL (see text for further detail).
• Figure 3: The $x$-dependence of the first $\hbox{\boldmath $k$}_\perp$ moment, according to Eq. (\ref{['mom']}), of our extracted Sivers functions. The solid line is obtained by using the central values of the parameters in Table I and the shaded area corresponds to varying the parameters within the shaded areas in Figs. 1 and 2. The dot-dashed, dashed and dotted lines show the first $\hbox{\boldmath $k$}_\perp$ moments of the Sivers functions obtained respectively in Refs. efremov, uf and yuan.
• Figure 4: Predictions for $A_{UT}^{\sin(\phi_h-\phi_S)}$ at COMPASS for scattering off a transversely polarized proton target and the production of positively ($h^+$) and negatively ($h^-$) charged hadrons. The plots in the upper panel have been obtained by performing the integrations over the unobserved variables according to the standard COMPASS kinematical cuts ourpaper; results with suggested new cuts, Eq. (\ref{['new cuts']}), are presented in the lower panel.
• Figure 5: Predictions for $A_{UT}^{\sin(\phi_{\pi}-\phi_S)}$ at JLab for the production of $\pi^+$ and $\pi^-$ from scattering off a transversely polarized proton target. Integrations over the unobserved variables have been performed according to the kinematical ranges of Eqs. (\ref{['JLab 6 GeV']}) and (\ref{['JLab 12 GeV']}).