Transport-based initial conditions for heavy-ion collisions at finite densities

TL;DR

This paper develops a transport-based approach to generate event-by-event initial conditions for (3+1)D relativistic hydrodynamics at finite net densities by coupling the SMASH hadronic transport model with MUSIC hydrodynamics and a 4D lattice-QCD-based EOS within the X-SCAPE framework. It extends particlization to finite densities by implementing Grad's moment and Chapman-Enskog corrections for multiple conserved charges ($B$, $Q$, $S$), ensuring conservation laws across the fluid-to-particle transition. The study demonstrates distinctive features of charge fluctuations, particularly large $Q$ and $S$ densities, and shows how covariant smearing and out-of-equilibrium corrections influence final-state hadron spectra and flow across beam energies relevant to BES. These results establish a self-consistent, event-by-event framework for exploring QCD matter at high baryon density and provide a platform for future investigations into diffusion, core-corona dynamics, and BES phenomenology.

Abstract

We employ the SMASH transport model to provide event-by-event initial conditions for the energy-momentum tensor and conserved charge currents in hydrodynamic simulations of relativistic heavy-ion collisions. We study the fluctuations and dynamical evolution of three conserved charge currents (net baryon, net electric charges, and net strangeness) with a 4D lattice-QCD-based equation of state, NEOS-4D, in the hydrodynamic phase. Out-of-equilibrium corrections at the particlization are generalized to finite densities to ensure the conservation of energy, momentum, and the three types of charges. These theoretical developments are integrated within X-SCAPE as a unified framework for studying the nuclear matter properties in the Beam Energy Scan program.

Figures (12)

• Figure 1: Probability distributions of the Lorentz factor $\gamma$ in the covariant smearing kernel Eq. \ref{['eq:K_covariant']} for a single Au+Au collision event at $b=0$, shown for three different center-of-mass energies $\sqrt{s_{\rm NN}}$.
• Figure 2: Schematic diagram of the simulation workflow for SMASH+MUSIC+iSS+SMASH model in the X-SCAPE framework and orchestrated by the BDM module. The simulation begins with SMASH pre-equilibrium dynamics in Minkowski coordinates, proceeds through hydrodynamic evolution and particlization in Milne coordinates, and concludes with hadronic rescattering in the SMASH afterburner.
• Figure 3: Evolution snapshots for scaled entropy density $\tau s$, scaled conserved charge densities ($n_B/s$, $n_Q/s$, $n_S/s$) in the $x-\eta_s$ plane at $y=0$ (left panels) and in the transverse $x-y$ plane at mid-rapidity $\eta_s = 0$ (right panels) for SMASH initial conditions at $\sqrt{s_{\rm NN}}=200\;\mathrm{GeV}$ with the covariant smearing kernel at $\tau = 0.54, 1, 2, 5$ fm/$c$ at impact parameter $b=0$.
• Figure 4: Evolution snapshots for scaled entropy density $\tau s$, scaled conserved charge densities ($n_B/s$, $n_Q/s$, $n_S/s$) in the $x-\eta_s$ plane at $y=0$ (left panels) and in the transverse $x-y$ plane at mid-rapidity $\eta_s = 0$ (right panels) for SMASH initial conditions at $\sqrt{s_{\rm NN}}=19.6\;\mathrm{GeV}$ with the covariant smearing kernel at $\tau = 1.27, 2, 3, 5$ fm/$c$ at a random impact parameter in the range from 6 to 8 fm.
• Figure 5: Evolution snapshots for scaled entropy density $\tau s$, scaled conserved charge densities ($n_B/s$, $n_Q/s$, $n_S/s$) in the $x-\eta_s$ plane at $y=0$ (left panels) and in the transverse $x-y$ plane at mid-rapidity $\eta_s = 0$ (right panels) for SMASH initial conditions at $\sqrt{s_{\rm NN}}=7.7\;\mathrm{GeV}$ with the covariant smearing kernel at $\tau = 3.24, 4, 5, 7$ fm/$c$ at a random impact parameter in the range from 6 to 8 fm.