Is early thermalization really needed in A+A collisions?

TL;DR

This paper argues that early thermalization in relativistic A+A collisions is not necessary, as transverse flow and anisotropy can develop efficiently during pre-thermal stages without established pressure.

Contribution

It challenges the necessity of early thermalization by showing flow development during pre-thermal stages, expanding understanding of collision dynamics.

Findings

Transverse flow can develop before thermalization.

Early thermalization is not essential for observed flow.

Flow development is driven by system finiteness.

Abstract

In this note we review our ideas, first published in year 2006, and corresponding results, including the new ones, which show that whereas the assumption of (partial) thermalization in relativistic A+A collisions is really crucial to explain soft physics observables, the hypotheses of {\it early} thermalization at times less than 1 fm/c is not necessary. The reason for the later conclusion is that the initial transverse flow in thermal matter as well as its anisotropy, leading to asymmetry of the transverse momentum spectra, could be developed at pre-thermal, either partonic or classical field - Glasma, stage with even more efficiency than in the case of very early perfect hydrodynamics. Such radial and elliptic flows develop no matter whether a pressure already established. The general reason for them is an essential finiteness of the system in transverse direction.