Jet measurements in polarized p+p collisions at STAR at RHIC

TL;DR

This paper reports on jet measurements in polarized proton-proton collisions at RHIC, providing insights into gluon contributions to proton spin through new asymmetry measurements and cross section analyses.

Contribution

It presents new jet asymmetry and cross section measurements at 200 and 510 GeV, improving constraints on gluon polarization and unpolarized gluon distributions.

Findings

Evidence of non-zero gluon contribution to proton spin

Consistent x_T scaling of asymmetry measurements

Enhanced constraints on gluon polarization in the 0.05<x<0.2 range

Abstract

Jet production in polarized $p+p$ collisions at $\sqrt{s} = 200$ GeV and $\sqrt{s} = 500$ GeV provides a powerful probe to study gluons inside the proton. The Solenoidal Tracker at RHIC (STAR) has the capability, with nearly full azimuthal ($2\pi$) coverage, to reconstruct jets at mid-rapidity ($|\eta| < 1$). The latest STAR inclusive jet longitudinal double-spin asymmetry $A_{LL}$ measured in 200 GeV $p+p$ collisions provides better constraints on the polarized gluon distribution $\Delta g(x)$ for $0.05<x<0.2$ than previous measurements. A recent global QCD fit (DSSV 2014) which includes the 2009 RHIC results provides the first evidence of non-zero gluon contribution to the proton spin. A new inclusive jet cross section using the anti-$k_{T}$ algorithm provides potential insights into the unpolarized gluon distribution function, and the new inclusive jet $A_{LL}$ measurement in 510 GeV $p+p$ collisions shows consistent $x_{T}$ scaling with the 200 GeV result. Future measurements with continuing high energy polarized proton-proton running at $\sqrt{s} = 500$ GeV at RHIC and detector upgrades in the forward direction will explore the gluonic contribution to the proton spin at low $x$ region.