Transverse Single Spin Asymmetries of Forward π0 and Jet-like Events in s = 500 GeV Polarized Proton Collisions at STAR

Yuxi Pan

doi:10.1142/s2010194516600375

Abstract

The large transverse single spin asymmetries (SSA) of high [Formula: see text] inclusive hadrons produced in polarized proton collisions are usually explained by means of collinear twist-3 multi-parton correlations. In this picture these asymmetries can originate from initial-state twist-3 parton distributions in the polarized proton and/or through the coupling between proton transversity and twist-3 fragmentation functions. The measurement of SSA for forward inclusive hadrons produced in [Formula: see text] collisions out to high transverse momentum helps to examine the validity and interplay of these initial- and final-state models. These models can be further explored by investigating the dependence of the SSA on event topologies. We present our latest status on the measurement of SSA for forward inclusive [Formula: see text] detected within [Formula: see text] in [Formula: see text] = 500 GeV [Formula: see text] collisions as well as its dependence on event topologies. We will also present our analysis of Sivers and Collins asymmetries for forward jet-like events consisting of multi-photon final states. The measurements are based on the data taken in 2011 with integrated luminosity [Formula: see text] 22 [Formula: see text].

Highlights

The measurement of transverse single spin asymmetries (TSSA) in polarized pp collisions provides critical information on the higher-twist effects and transverse momentum dependent phenomena in parton distributions[1] and fragmentation processes.[2]
It has been shown in theory that this type of asymmetry is generated by the coupling of transversity distribution of the proton and Collins fragmentation function.[8]
Summary and Outlook STAR has measured the transverse single spin asymmetries of forward inclusive π0 which can be readily compared with theory predictions

Summary

Introduction

The measurement of transverse single spin asymmetries (TSSA) in polarized pp collisions provides critical information on the higher-twist effects and transverse momentum dependent phenomena in parton distributions[1] and fragmentation processes.[2] A global analysis of TSSA in hadronic and semi-inclusive DIS processes makes it possible to examine the universality of polarized distribution[3] and fragmentation functions as well as their evolutions. This is an Open Access article published by World Scientific Publishing Company. To that end STAR has measured the forward inclusive π0 production in terms transverse single spin of its dependences on asymmetrie√s of xF (=2pz/ s), transverse momentum pT and event topologies. Running with η < 4.0 are reconstructed through the π0 → γγ channel, where the photons are recorded by STAR’s Forward Meson Spectrometer (FMS).[4]

Analysis

Inclusive π0 single spin asymmetries