Free-streaming approximation in early dynamics of relativistic heavy-ion collisions

TL;DR

This paper proposes a free-streaming approximation for the early stage of relativistic heavy-ion collisions, showing it effectively models the transition to hydrodynamics and reproduces key experimental observables.

Contribution

It introduces a free-streaming approach that smoothly matches to hydrodynamics, allowing delayed hydrodynamic start times and accurate description of collision observables.

Findings

Initial azimuthal flow develops due to sudden equilibration.

Smooth matching of energy-momentum tensor from free streaming to transverse hydrodynamics.

Reproduces transverse-momentum spectra, elliptic flow, and HBT radii accurately.

Abstract

We investigate an approximation to early dynamics in relativistic heavy-ion collisions, where after formation the partons are free streaming and around the proper time of 1 fm/c undergo a sudden equilibration described in terms of the Landau matching condition. We discuss physical and formal aspects of this approach. In particular, we show that initial azimuthally asymmetric transverse flow develops for non-central collisions as a consequence of the sudden equilibration. Moreover, the energy-momentum tensor from the free-streaming stage matches very smoothly to the form used in the transverse hydrodynamics, whereas matching to isotropic hydrodynamics requires a more pronounced change in the energy-momentum tensor. After the hydrodynamic phase statistical hadronization is carried out with the help of THERMINATOR. The physical results for 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 within our approach. The agreement is equally good for a purely hydrodynamic evolution started at an early proper time of 0.25 fm/c, or for the free streaming started at that time, followed by the sudden equilibration at tau ~1 fm/c and then by perfect hydrodynamics. Thus, the inclusion of free streaming allows us to delay the start of hydrodynamics to more realistic times of the order of 1 fm/c.