Hydrodynamic afterburner for the Color Glass Condensate and the parton energy loss

TL;DR

The paper develops a unified dynamical framework for relativistic heavy-ion collisions by integrating CGC-based initial gluon production, described via $k_T$-factorization with a saturation scale $Q_s$, into 3D hydrodynamics and a GLV-inspired jet energy-loss model. It demonstrates that CGC initial conditions, when evolved hydrodynamically and coupled to parton energy loss, reproduce the centrality, rapidity, and energy dependences of charged hadron production and midrapidity jet suppression, while forward-rapidity data indicate missing small-$x$ evolution in the initial state. The study provides a coherent picture linking initial gluon saturation to bulk evolution and hard probes, and it identifies key directions for improvement, including more realistic unintegrated gluon distributions, a lattice-inspired equation of state, and a parameter-free jet quenching treatment. Together, these elements move toward a consistent, dynamical description of both soft and hard phenomena in RHIC collisions.

Abstract

We take hydrodynamic initial conditions in relativistic heavy ion collisions from the Color Glass Condensate (CGC) picture through the kT factorization formula. Gluon distributions produced from the CGC are found to provide good initial conditions for the hydrodynamic simulations in Au + Au collisions at Relativistic Heavy Ion Collider (RHIC) energies. We reproduce the centrality, rapidity, and energy dependences of multiplicity within this approach. We also investigate the energy loss of high pT partons in the dense thermalized medium created from colliding two CGC's. We find that our results on the centrality dependence of nuclear modification factors for pions and back-to-back correlation for charged hadrons at midrapidity are consistent with the RHIC data up to semicentral events. Whereas, our approach in which jets are calculated from perturbative QCD 2->2 processes predicts less suppression at forward rapidity region compared to the BRAHMS data in Au+Au collisions at RHIC.

Figures (9)

• Figure 1: (a) Rapidity dependence of initial gluon transverse energy (dashed line) and number distribution (solid line) in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV at $b=2.4$ fm. (b) Gluon number and energy densities as functions of a transverse coordinate. Parameters are $\kappa^2=3.6$, $K=0.7$, and $\lambda = 0.2$. As we will discuss in the next subsection, these parameters correspond to an initialization of hydrodynamic simulations which matches the number density of gluons produced by collisions of two CGC's with the parton distribution in hydrodynamic simulations at initial time. The value of $\kappa^2$ is chosen so that we reproduce the multiplicity of final hadrons as discussed in Sec. IV. In this procedure, the transverse energy distributions represented by dashed lines are not used as hydrodynamic initial conditions. In the next subsection, we will also discuss the other matching procedure in which the transverse energy distribution is used instead of the gluon number density. In that case, the gluon distributions are factor 1.6 smaller than the results in these figures.
• Figure 2: (a) Space-time rapidity dependence of the initial conditions $dN/d\eta_s$ and $dE_T/d\eta_s$ in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. We also compare the initial energy density distribution with the one which we employed previously in Ref. HiranoTsuda. (b) Comparison of the transverse profile from IC-$n$ with those in Refs. HiranoTsuda and Morita.
• Figure 3: Rapidity and pseudorapidity distributions of all hadrons in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV at $b=2.4$ fm are compared to the corresponding transverse energy distributions obtained from hydrodynamic simulations with CGC initial condition.
• Figure 4: Pseudorapidity distributions of charged hadrons in Au + Au collisions at $\sqrt{s_{NN}}=130$ and 200 GeV are compared to the PHOBOS data Back:2002wb. For the choice of initial conditions and impact parameters, see text.
• Figure 5: Mean transverse momenta for pions, kaons and protons as a function of $N_{\mathrm{part}}$. Here contribution only from hydrodynamic components is taken into account. $T^{\mathrm{th}}=100$ MeV is used for all centralities. Data are taken from Ref. phenix:pi