Two Particle Correlations at Forward Rapidity in STAR

TL;DR

This paper reports on measurements of two-particle azimuthal correlations at forward rapidity in STAR, revealing features consistent with gluon saturation phenomena and comparing results to the Color Glass Condensate model.

Contribution

First measurement of forward rapidity two-particle correlations in STAR with enhanced calorimeter acceptance, probing low-x gluon saturation effects.

Findings

Data show qualitative features expected from gluon saturation.

Results are consistent with predictions of the Color Glass Condensate model.

Enhanced detector acceptance allows sensitivity to gluon densities down to x≈10^{-3}.

Abstract

During the 2008 run RHIC provided high luminosity in both p+p and d+Au collisions at $\sqrt{s_{NN}}= 200$ GeV. Electromagnetic calorimeter acceptance in STAR was enhanced by the new Forward Meson Spectrometer (FMS), and is now almost contiguous from $-1<\eta<4$ over the full azimuth. This large acceptance provides sensitivity to the gluon density in the nucleus down to $x\approx 10^{-3}$, as expected for $2\rightarrow 2$ parton scattering. Measurements of the azimuthal correlation between a forward $\pi^{0}$ and an associated particle at large rapidity are sensitive to the low-x gluon density. Data exhibit the qualitative features expected from gluon saturation. A comparison to calculations using the Color Glass Condensate (CGC) model is presented.