Measuring the thermalization time

TL;DR

This paper proposes using directed flow as a novel measure to determine the early pressure anisotropy in heavy-ion collisions, revealing rapid equilibration within 0.25 fm/c.

Contribution

It introduces a new method for measuring early pressure anisotropy using directed flow, which is more sensitive than traditional observables.

Findings

Transverse momentum spectra and elliptic flow are insensitive to early anisotropy.

Directed flow effectively measures early pressure differences.

Pressures equilibrate in less than 0.25 fm/c.

Abstract

A new method of measuring the pressure anisotropy in the early stage of heavy-ion collisions is discussed. It is shown that the transverse momentum spectra, elliptic flow and interferometry radii are not sensitive to the early anisotropy. We propose the directed flow as a measure of the early transverse and longitudinal pressures. Calculations indicate that the transverse and longitudinal pressures equilibrate in less than 0.25 fm/c.