New Analysis of the Single Transverse-Spin Asymmetry for Hadron Production at RHIC

TL;DR

This study investigates the origin of large single transverse-spin asymmetries in inclusive hadron production in proton-proton collisions at RHIC using the collinear twist-3 framework. It incorporates both soft-gluon-pole and soft-fermion-pole contributions from quark-gluon correlators G_F and tilde G_F to describe A_N across pion and kaon channels. The authors perform fits to STAR and BRAHMS data, finding that SFP contributions are necessary for a good description, and that SGP alone cannot capture all features, especially for K channels; the results reveal flavor- and x-dependent structures in the twist-3 correlators and emphasize the need for further data and theoretical constraints on their scale evolution. Overall, the work supports a sizable SFP role in SSA and highlights remaining uncertainties due to neglected terms and limited data.

Abstract

We analyze the single transverse-spin asymmetry (SSAs) for inclusive hadron production in the $pp$ collision at RHIC based on the twist-3 mechanism in QCD. As an origin of SSAs, we take into account of all kinds of pole contributions associated with the twist-3 quark-gluon correlation functions in the polarized proton. By the inclusion of the soft-fermion-pole contribution in addition to the soft-gluon-pole contribution, we find SSAs observed at RHIC are better described for all kinds of mesons. $P_T$-dependence of the asymmetry and the comparison with the FNAL data are also presented.

Figures (6)

• Figure 1: The results of the three fits for $A_N$. The solid lines are for FIT 1, the long-dotted lines are for FIT 2, and the dashed lines are for FIT 3. The result by KQVY06 is also shown by the short-dotted lines.
• Figure 2: The SGP function $G_{F}^{a}(x,x)$ and the SFP function $G_{F}^{a}(0,x)+\widetilde{G}_{F}^{a}(0,x)$ at $Q^2=2.4$ GeV$^2$ for each quark-flavor obtained in FIT 1 in comparison to the unpolarized quark distribution $f_a(x)$ [scaled by factor 1/10].
• Figure 3: SGP functions at $Q^2=2.4$ GeV$^2$ obtained in three fits. The SGP function in KQVY06 is also shown by using the relation $G_{F}^{a}(x,x)=-\frac{\sqrt{4\pi\alpha_s}}{\pi M_N} T_{F,a}(x,x)$ for each quark flavor. For antiquark flavors, the sign of this relation is opposite.
• Figure 4: Separation of $A_N$ in FIT 1 into the SGP (solid lines) and the SFP (dashed lines) contributions.
• Figure 5: Comparison of the $P_{hT}$-dependence of $A_N^{\pi^0}$ with the result of STAR data STAR2.