Sivers and Collins effects in polarized pp scattering processes

TL;DR

The paper investigates transverse single spin asymmetries in polarized proton-proton collisions through Sivers and Collins mechanisms within a phenomenological generalized parton model. It revisits the Collins contribution by correcting a sign error in the relevant partonic channel and incorporates updated SIDIS data and the DSS fragmentation functions, including sea-quark Sivers and new transversity/Collins fits. After the correction, the Collins mechanism can be sizable and can mix with the Sivers effect, with predictions for RHIC sensitive to large-x behavior and fragmentation choices. These results inform universality tests of TMD functions, reveal substantial uncertainties at large x_B, and guide future global analyses combining SIDIS, e+e-, and hadronic data to explain meson SSA data.

Abstract

We summarize the present phenomenology of Sivers and Collins effects for transverse single spin asymmetries in polarized proton-proton collisions within the framework of the generalized parton model (GPM). We will discuss a reassessment of the Collins effect and some preliminary predictions for SSA's in p(pol) p -> pi,K + X processes at RHIC obtained using updated information from SIDIS data and a new set of meson fragmentation functions.

Figures (5)

• Figure 1: Collins contribution (maximized effect) to pion SSA's for E704 kinematics, $\sqrt{s}=20$ GeV, before (left) and after (right) correction. Thin lines are obtained setting to zero all azimuthal phases. The PDF/FF sets are: LSS-BBS/Kretzer.
• Figure 2: Maximized contributions to $A_N(p^\uparrow p\to \pi^0+X)$ for STAR kinematics, $\sqrt{s}=200$ GeV, before (left) and after (right) correction: quark Sivers cont. (solid line); gluon Sivers cont. (dashed line); transversity$\otimes$Collins cont. (dotted line). The PDF/FF sets are: MRST01/KKP.
• Figure 3: Predictions for charged pion SSA's at RHIC-BRAHMS kinematics, $\sqrt{s}=200$ GeV, before (left) and after (right) correction. The parameterizations for the Sivers, transversity and Collins functions are from Refs. [Anselmino:2005ea,Anselmino:2007fs]. Thin lines: Collins contribution alone; thick lines: Sivers + Collins contributions. The PDF/FF sets are: MRST01/Kretzer.
• Figure 4: Predictions for neutral pion SSA at RHIC-STAR kinematics, $\sqrt{s}=200$ GeV, obtained with the previous [Anselmino:2005ea,Anselmino:2007fs] (left) and new [Anselmino:2008sga,Anselmino:2008sj] (right) parameterizations for the Sivers, transversity and Collins functions. Left: Kretzer FF set, Collins + Sivers contributions; right: DSS FF set, Sivers (thick lines) and Collins (thin lines, first panel only) contributions. PDF set: MRST01.
• Figure 5: Predictions for charged pion SSA's at RHIC-BRAHMS kinematics, $\sqrt{s}=200$ GeV, obtained with the previous [Anselmino:2005ea,Anselmino:2007fs] (left) and new [Anselmino:2008sga,Anselmino:2008sj] (right) parameterizations for the Sivers, transversity and Collins functions. Left: Kretzer FF set, Collins (thin lines) and Collins + Sivers (thick lines) contributions; right: DSS FF set, Collins + Sivers contributions. PDF set: MRST01.