Recent results of gluon and sea quark polarization measurements in polarized proton-proton collisions at STAR

Xuan Li,The Star Collaboration

doi:10.1088/1742-6596/535/1/012003

Abstract

The STAR experiment at RHIC is carrying out a comprehensive high-energy spin physics program to understand the internal structure and dynamics of the proton in polarized proton-proton collisions at = 200 GeV and = 500/510 GeV. STAR has the capability, with nearly full azimuthal coverage, to reconstruct leptons, hadrons and jets in the mid-rapidity region (|η| < 1). The results for inclusive jet longitudinal double spin asymmetries taken during the 2009 RHIC run indicate the first non-zero gluon contribution (Δg(x,Q2)/g(x,Q2)) to the proton spin for 0.05 < x < 1 (Bjorken-x: momentum fraction of partons). Recent longitudinal single-spin asymmetry measurements of W+/− bosons at = 500/510 GeV in polarized proton-proton collisions provide a direct probe of the polarized anti-u and anti-d quark distributions (Δū(x, Q2), Δd̄(x, Q2)). These results better constrain the polarized gluon and sea quark distributions of the proton in the RHIC sensitive kinematic region. Future measurements with continuing high energy polarized proton-proton run at RHIC and detector upgrade will explore the gluonic contribution to the proton spin in extended range.

Highlights

The discovery of the so-called “proton spin crisis” [1] has prompted a variety of investigations to determine precisely the various constituent spin-contributions
The limited Bjorken-x region accessed by the fixed target polarized deep inelastic scattering (DIS) experiments provides poor constraints on the polarized gluon distribution ∆g(x, Q2) and flavor separated polarized anti-quark distributions
Summary and Outlook Latest measurements of the Solenoidal Tracker at RHIC (STAR) longitudinal progam improve our understanding of the proton spin structure

Summary

Introduction

The discovery of the so-called “proton spin crisis” [1] has prompted a variety of investigations to determine precisely the various constituent spin-contributions. STAR inclusive jet double spin asymmetry ALL in 200 GeV p+p collisions, extracts a nonzero gluon spin contributions (∆g(x, Q2)/g(x, Q2)) to the proton in 0.05 < x < 1 [4, 5]. The measured detector jet ALL values are corrected to parton jet ALL in each jet pT bin according to calculations in the simulation with polarized/unpolarized PDFs. Figure 1 shows the 2009 STAR mid-rapidity inclusive jet double spin asymmetry ALL versus the parton jet pT in 200 GeV proton-proton collisions [4]. The latest DSSV update which includes the 2009 STAR inclusive jet ALL [14] and 2009 PHENIX inclusive π0 ALL [15] exhibits a postive polarized gluon helicity function in the momentum fraction region 0.05 < x < 1 as shown in the top panel of Figure 2. Cone alg. (R=0.7) / ET3 > 5GeV ET4 > 8GeV CDoelniveearlegd. (LRu=m0i.n7o) s/ iEtyT3=>150G00epVb-E1 T4 > 8GeV

Bernd Surrow!

Findings

ALL ALL