Indications of early thermalization in relativistic heavy-ion collisions

TL;DR

This paper demonstrates that directed flow in relativistic heavy-ion collisions is highly sensitive to early pressure equilibration, suggesting rapid thermalization within approximately 0.25 fm/c.

Contribution

It introduces a 3+1D hydrodynamic model with initial pressure anisotropy to study early thermalization effects in heavy-ion collisions.

Findings

Directed flow is strongly affected by early pressure imbalance.

Transverse momentum spectra and elliptic flow are less sensitive to early pressure anisotropy.

Data indicates rapid thermalization within about 0.25 fm/c.

Abstract

The directed flow of particles emitted from the fireball created in a heavy-ion collision is shown to be a very sensitive measure of the pressure equilibration in the first 1 fm/c of the evolution. Performing a 3+1 dimensional relativistic hydrodynamic calculation with nonequilibrated longitudinal and transverse pressures, we show that the directed flow is strongly reduced if the pressure imbalance survives for even a short time. Transverse momentum spectra, elliptic flow and interferometry correlation radii are not very sensitive to this early pressure anisotropy. Comparison with the data points toward a short equilibration time of the order of $0.25$fm/c or less.