Two Particle Correlations at Forward Rapidity in STAR

TL;DR

This study uses STAR's expanded forward acceptance via the Forward Meson Spectrometer to measure di-hadron azimuthal correlations across a broad rapidity range in p+p and d+Au collisions at 200 GeV. The analysis focuses on forward π0 triggers and associated particles to probe low-x gluons in the nucleus, comparing results to CGC-based expectations. Central d+Au collisions exhibit a strong suppression and broadening of the away-side peak relative to p+p, consistent with saturation effects and CGC predictions, while near-side correlations remain largely intact. Comprehensive systematic checks indicate the observed suppression is not due to multiplicity or combinatorial background, underscoring forward-rapidity di-hadron correlations as a robust signature of gluon saturation at RHIC.

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 $-1<η<4$ 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 $π^{0}$ 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.

Figures (5)

• Figure 1: On the left: schematic view of the STAR experimental hall. Gold beam is coming from the West (right in figure) side. On the right: PYTHIA Sjostrand:2001yu simulation of di-pion production at large $\eta$ p+p collisions at $\sqrt{s}=200$ GeV. The $\eta$ of the associated particle is strongly correlated to the $x$ value of the soft parton probed in the partonic scattering. Figure from Bland:2005uu.
• Figure 2: Uncorrected coincidence probability versus azimuthal angle difference between two forward neutral pions in p+p collisions (left) compared to peripheral (center) and central d+Au collisions (right). Data are shown with statistical errors and fit with a constant plus two Gaussian functions (in red). CGC expectations Marquet200741 have been superimposed (in blue) to data for central d+Au collisions.
• Figure 3: Comparison between parton density functions and their effect on azimuthal correlations. Left: comparison of density distributions of gluons using Duke-Owen (blue), CTEQ3L (red) and CTEQ5L (black) PDF sets. Right: uncorrected forward di-pion correlations in p+p as simulated in PYTHIA 6.222, using the three different PDF sets.
• Figure 4: Uncorrected azimuthal correlations between two forward neutral pions as simulated in p+p by PYTHIA 6.222 (left). On the central and rightmost panel: PYTHIA events have been embedded into minimum bias d+Au data to emulate the effect of the larger multiplicity in d+Au. Each probability is scaled by a common arbitrary factor.
• Figure 5: Off-peak analysis. On the left panel invariant mass spectra for leading and sub-leading pion candidates are shown. The off-mass selection for sub-leading pions is indicated in blue. Following, uncorrected azimuthal correlations between two forward pion candidates (center) and between a forward pion candidate and a pair of clusters with invariant mass in the range $0.25<M_{\gamma\gamma}<0.45$ GeV/c$^{2}$ (right).