Sivers effect and the single spin asymmetry A_N in p(transv. pol.) p --> h X processes

TL;DR

This work assesses whether the Sivers effect, implemented within the Generalised Parton Model (GPM) and constrained by SIDIS data, can explain the large single-spin asymmetries $A_N$ observed in hadronic collisions $p^\uparrow p \to h X$. By parameterising the Sivers function with Gaussian transverse momentum distributions and performing a scan over large-$x$ behavior, the authors generate scan bands that test the SIDIS constraints against RHIC data for pions and kaons. They find that the Sivers mechanism, with SIDIS-extracted functions, can largely reproduce the magnitude and sign of $A_N$ in several RHIC measurements, though some high-$P_T$ results (notably at 500 GeV) hint at additional contributions or evolution effects. Predictions for $A_N$ in jet and direct-photon production provide additional tests of the approach, while acknowledging that TMD evolution and factorisation in hadronic processes remain open questions requiring further data and theory. Overall, the paper supports the viability of the Sivers effect within the GPM to explain key features of hadronic SSAs, but highlights substantial uncertainties and the need for future explorations in both experiment and theory.

Abstract

The single spin asymmetry A_N, for large P_T single inclusive particle production in p(transv. pol.) p collisions, is considered within a generalised parton model and a transverse momentum dependent factorisation scheme. The focus is on the Sivers effect and the study of its potential contribution to A_N, based on a careful analysis of the Sivers functions extracted from azimuthal asymmetries in semi-inclusive deep inelastic scattering processes. It is found that such Sivers functions could explain most features of the A_N data, including some recent STAR results which show the persistence of a non zero A_N up to surprisingly large P_T values.

Figures (12)

• Figure 1: Scan band ( i.e. the envelope of the 81 curves obtained with the scanning procedure) for the Sivers contribution to the charged pion single spin asymmetries $A_N$, at $\sqrt s =$ 200 GeV, as a function of $x_F$ at two different scattering angles, compared with the corresponding BRAHMS experimental data Lee:2007zzh. The shaded scan band is generated, adopting the GRV98 set of collinear PDFs and the Kretzer FFs, following the procedure explained in the text.
• Figure 2: Scan band ( i.e. the envelope of the 81 curves obtained with the scanning procedure) for the Sivers contribution to the neutral pion single spin asymmetry $A_N$, at $\sqrt s =$ 200 GeV, as a function of $x_F$ at two different pseudo-rapidity values, compared with the corresponding STAR experimental data Abelev:2008af. The shaded scan band is generated, adopting the GRV98 set of collinear PDFs and the Kretzer FFs, following the procedure explained in the text.
• Figure 3: The same as in Fig. \ref{['fig:an-star']}, but with the STAR data plotted vs. the pion transverse momentum, $P_T$, for different bins in $x_F$, $\langle x_F \rangle =$ 0.28, 0.37, 0.43 and 0.50.
• Figure 4: Scan band ( i.e. the envelope of the 81 curves obtained with the scanning procedure) for the Sivers contribution to the neutral pion single spin asymmetry $A_N$, as a function of $P_T$ for different $x_F$ values at $\sqrt s = 500$ GeV, compared with the corresponding STAR preliminary experimental data at $\langle x_F \rangle = 0.20, 0.28$Igo:2012. The shaded scan band is generated, adopting the GRV98 set of collinear PDFs and the Kretzer FF set, following the procedure explained in the text.
• Figure 5: Scan band ( i.e. the envelope of the 81 curves obtained with the scanning procedure) for the Sivers contribution to the kaon single spin asymmetry $A_N$, as a function of $x_F$, at $\sqrt s =$ 200 GeV and a fixed scattering angle, compared with the corresponding BRAHMS experimental data Lee:2007zzh. The shaded scan band is generated, adopting the GRV98 set of collinear PDFs and the Kretzer FF set, following the procedure explained in the text.