Initial condition for hydrodynamics, partonic free streaming, and the uniform description of soft observables at RHIC

TL;DR

This paper demonstrates that using a simple Gaussian initial condition and including partonic free-streaming in hydrodynamic models enables a consistent description of soft observables at RHIC, such as spectra and flow, with flexible starting times.

Contribution

It introduces a uniform initial condition based on a Gaussian profile and incorporates partonic free-streaming to improve hydrodynamic modeling of RHIC data.

Findings

Gaussian initial profile effectively describes soft observables.

Partonic free-streaming delays hydrodynamics start time without degrading fit quality.

Both early hydrodynamics and delayed start with free-streaming fit RHIC data well.

Abstract

We investigate the role of the initial condition used for the hydrodynamic evolution of the system formed in ultra-relativistic heavy-ion collisions and find that an appropriate choice motivated by the models of early-stage dynamics, specifically a simple two-dimensional Gaussian profile, leads to a uniform description of soft observables measured in the Relativistic Heavy-Ion Collider (RHIC). In particular, the transverse-momentum spectra, the elliptic-flow, and the Hanbury-Brown--Twiss correlation radii, including the ratio R_out/R_side as well as the dependence of the radii on the azimuthal angle (azHBT), are properly described. We use the perfect-fluid hydrodynamics with a realistic equation of state based on lattice calculations and the hadronic gas at high and low temperatures, respectively. We also show that the inclusion of the partonic free-streaming in the early stage allows to delay the start of the hydrodynamical description to comfortable times of the order of 1 fm/c. Free streaming broadens the initial energy-density profile, but generates the initial transverse and elliptic flow. The data may be described equally well when the hydrodynamics is started early, or with a delay due to partonic free-streaming.