Gluon polarization measurements from longitudinally polarized proton-proton collisions at STAR

Zilong Chang

doi:10.1088/1742-6596/1643/1/012184

Abstract

The gluon polarization contribution to the proton spin is an integral part to solve the longstanding proton spin puzzle. At the Relativistic Heavy Ion Collider (RHIC), the STAR experiment has measured jets produced in mid-pseudo-rapidity, |η| < 1.0, and full azimuth, ϕ, from longitudinally polarized pp collisions to study the gluon polarization in the proton. At center of mass energies = 200 and 510 GeV, jet production is dominated by hard QCD scattering processes such as gluon-gluon (gg) and quark-gluon (qg), thus making the longitudinal double-spin asymmetry (ALL ) sensitive to the gluon polarization. Early STAR inclusive jet ALL results at = 200 GeV provided the first evidence of the non-zero gluon polarization at momentum fraction x > 0.05. The higher center of mass energy = 510 GeV allows to explore the gluon polarization as low as x ∼ 0.015. In this talk we will present the recent STAR inclusive jet and dijet ALL results at = 510 GeV, and discuss the relevant new analysis techniques for the estimation of trigger bias and reconstruction uncertainty, the underlying event correction on the jet energy and its effect on jet ALL . Dijet results are shown for different topologies in regions of pseudo-rapidity, effectively scanning the x-dependence of the gluon polarization.

Highlights

Deep inelastic scattering (DIS) experiments in the 1980s showed that quarks inside the proton make only a small contribution to its total spin [1]
The next-to-leading order (NLO) perturbative quantum chromodynamics calculations show that the qg and gg
In summary, the inclusive jet measurements at Solenoidal√Tracker at RHIC (STAR) probe the magnitude of the ∆g(x) over a wide range of x

Summary

Introduction

Deep inelastic scattering (DIS) experiments in the 1980s showed that quarks inside the proton make only a small contribution to its total spin [1]. The kinematics space in x − Q2 covered by the polarized fixed target experiments through DIS processes only provide limited constraints on the gluon polarization inside the proton [3]. At RHIC, either transversely or longitudinally polarized proton beams collide at both s = 200 and 510 GeV. To explore the gluon polarizations, we measure the longitudinal double-spin asymmetry, ALL, for jets, defined as the fractional difference of the jet cross sections when beams have the same and the opposite helicities. The ALL can be expressed as the sum of convolutions of the polarized PDFs and partonic longitudinal double-spin asymmetry aLL over all possible partonic processes. √ processes dominate jet production at both s = 200 and 510 GeV [7] Both qg and gg processes have sizable aLL [8], jet ALL are sensitive to gluon polarizations.

Inclusive jet and dijet ALL measurements at STAR

Other measurements and STAR forward upgrade

Conclusion