Precision Measurement of the Longitudinal Double-Spin Asymmetry for Dijet Production at Intermediate Pseudorapidity in Polarized Proton+Proton Collisions at $\sqrt{s}$ = 200 GeV

TL;DR

This paper presents new measurements of the longitudinal double-spin asymmetry in dijet production at intermediate pseudorapidity in polarized proton-proton collisions at 200 GeV, providing improved constraints on the gluon polarization inside the proton.

Contribution

It reports the preliminary results of $A_{LL}$ for dijet production at intermediate pseudorapidity, with higher precision data from 2015, extending previous measurements and refining the understanding of gluon polarization.

Findings

Results are consistent with previous findings of positive gluon polarization.

New data at intermediate pseudorapidity improve constraints on $ riangle g(x)$ for $x<0.05$.

Comparison with theoretical models suggests better understanding of gluon contribution to proton spin.

Abstract

Measurements of the longitudinal double-spin asymmetry, $A_{LL}$, by the STAR experiment have contributed significantly to our understanding of the gluon helicity distribution, $\Delta g(x)$, inside the proton. Results from the 2009 inclusive jet measurement, when included into global analyses, indicated a substantially positive polarization for gluons with partonic momentum fraction $x$ greater than 0.05. In addition to the inclusive jets, analyses of dijet production extending to higher pseudorapidity (up to $\eta \sim 1.8$) provide better constraints on the $x$ dependent behavior of $\Delta g(x)$. Recently, STAR published several new results at midrapidity (up to $\eta \sim 1.0$) using the $p+p$ data collected in 2012, 2013 and 2015 at both $\sqrt{s}$ = 510 and 200 GeV. These new results confirm the previous findings and provide additional constraints in the largely unexplored region of $x < 0.05$. In this talk, the preliminary results of the $A_{LL}$ for dijet production at intermediate pseudorapidity (up to $\eta \sim 1.8$) based on 2015 data at $\sqrt{s}$ = 200 GeV, with twice the figure-of-merit of the 2009 data, will be presented. The comparison with the theoretical expectations as well as its potential impact on $\Delta g(x)$ will be discussed.