Event-by-Event Simulation of the Three-Dimensional Hydrodynamic Evolution from Flux Tube Initial Conditions in Ultrarelativistic Heavy Ion Collisions

TL;DR

The paper develops an event-by-event, 3+1D hydrodynamic framework for ultrarelativistic heavy-ion collisions using flux-tube initial conditions from EPOS and a cross-over lattice-compatible equation of state. After early hadronization near $T_H ≈ 166$ MeV, the system undergoes a hadronic cascade (UrQMD), with a core-corona separation distinguishing thermalized from non-thermal regions. The approach reproduces a wide range of soft observables, including ridge structures, elliptic flow, identified-particle spectra, yields, and femtoscopy radii, illustrating how initial flux-tube irregularities translate into final-state correlations through realistic dynamics. It highlights that an event-by-event treatment, a realistic EoS, and hadronic rescattering are essential to describe the data, offering a unified picture that connects string models, CGC concepts, and lattice inputs with experimental observables.

Abstract

We present a realistic treatment of the hydrodynamic evolution of ultrarelativistic heavy ion collisions, based on the following features: initial conditions obtained from a flux tube approach, compatible with the string model and the color glass condensate picture; event-by-event procedure, taking into the account the highly irregular space structure of single events, being experimentally visible via so-called ridge structures in two-particle correlations; use of an efficient code for solving the hydrodynamic equations in 3+1 dimensions, including the conservation of baryon number, strangeness, and electric charge; employment of a realistic equation-of-state, compatible with lattice gauge results; use of a complete hadron resonance table, making our calculations compatible with the results from statistical models; hadronic cascade procedure after an hadronization from the thermal matter at an early time.

Figures (49)

• Figure 1: Macroscopic flux tubes (three in this example), made out of many individual ones, of variable length.
• Figure 2: The energy density over $T^{4}$ as a function of the temperature $T$. The dotted line indicates the "hadronization temperature", i.e. end of the thermal phase, when "matter" is transformed into hadrons.
• Figure 3: Particle ratios (hadron yields to $\pi^{+}$ yields) from our model calculations (thick horizontal line) as compared to the statistical model gas1(thin horizontal line), and to data brahmsN1brahmsN2starNpt (points).
• Figure 4: Elementary interaction in the EPOS model.
• Figure 5: Elastic "rescattering" of a ladder parton.