Two-particle azimuthal correlations at forward rapidity in STAR

TL;DR

This work investigates gluon saturation in relativistic heavy-ion collisions by measuring two-particle azimuthal correlations with a forward pion detected by the Forward Meson Spectrometer in STAR. Using p+p and d+Au data at 200 GeV, the analysis probes low-x gluons in the gold nucleus through forward-forward and forward-mid rapidity correlations, comparing to CGC-based predictions (BK/JIMWLK, kT-factorization) and perturbative QCD expectations. The results show no significant away-side broadening in forward-mid correlations, while forward-forward correlations in central d+Au exhibit away-side broadening, consistent with saturation effects at low x and low Q2; centrality enhances the effect, supporting a transition from dilute to saturated regimes. Overall, the study provides qualitative evidence of gluon saturation at RHIC and outlines pathways to extend the exploration at LHC energies where saturation is expected to be more pronounced.

Abstract

During the 2008 run the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven Nation Laboratiory (BNL), NY, provided high luminosity in both p+p and d+Au collisions at $\sqrt{s_{NN}}=200\mathrm{\,GeV}$. 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 allows measurements of the azimuthal correlation between a forward neutral pion and a second particle in a very large rapidity range. Di-hadron azimuthal correlations provide sensitivity to the low longitudinal momentum fraction ($x$) component of the gluon density in the nuclear target. Initial state nonlinear effects are in fact expected to appear in d+Au collisions when the dense part of the nucleus is probed. The analysis in this thesis shows that such correlations and their dependence on rapidity, transverse momentum and collision centrality are qualitative consistent with the existence of gluon saturation effects in the relativistic nucleus. Theoretical expectations for azimuthal correlations between a dilute system (deuteron) and a saturated target (Gold nucleus) have been explored.

Figures (48)

• Figure 2. 1: Left: lowest order description of Deep Inelastic Scattering. Right: pictorial representation of parton density function evolution in $Q^2$ (DGLAP) and $x$ (BFKL), taken from 2010arXiv10020333G. Circles represent the resolved partons whose transverse size $1/Q^2$ decreases with $Q^2$ while density increases with $1/x$.
• Figure 2. 2: Left: geometric scaling at HERA. Figure from Stasto:2000er. Right: limiting fragmentation in Au-Au collisions at RHIC. Figure from PhysRevLett.91.052303.
• Figure 2. 3: Nuclear modification factor for different pseudo-rapidities, from PhysRevLett.93.242303.
• Figure 2. 4: Two-particle azimuthal correlations in p+p and d+Au interactions, as predicted in Kharzeev2005627.
• Figure 3. 5: Layout of the RHIC complex (left, figure from Hahn2003245) and schematics of the STAR detector as used during the 2008 run (right).