Out-of-equilibrium phenomena in high energy nuclear collisions

TL;DR

This paper explores out-of-equilibrium phenomena in high energy nuclear collisions, studying disoriented chiral condensates formation and the role of elastic scatterings in gluon kinetic equilibration, revealing new insights into their probabilities and timescales.

Contribution

It introduces a novel sampling strategy for DCC formation probability and challenges the assumption of rapid kinetic equilibration due to elastic collisions in early collision stages.

Findings

DCC formation probability at CERN SPS energies is about 1/1000.

Distinct pion modes behave as separate DCCs, contradicting previous beliefs.

Elastic collisions are less efficient for rapid kinetic equilibration than commonly assumed.

Abstract

In the first part we study the possibility that a Disoriented Chiral Condensate (DCC) forms when hot hadronic matter is quenched, using the linear sigma model. We formulate an original sampling strategy for the initial field configuration in order to get an estimate of the probability that a potentially observable coherent pion field appears in a heavy-ion collision. We obtain a probability of the order of 1/1000 at CERN SPS energies. Next, we study the correlation between isospin orientations of the distinct modes of the pion field emerging after a quench. We show that this correlation is absent: the distinct modes behave as distinct DCCs. This contradicts the common belief that the state produced in the simplest form of the quench scenario - with a fully thermalized initial state - is identical to the originally proposed DCC. In the second part, we investigate the role of elastic scatterings in the process of kinetic equilibration of gluons produced in the very early times of the collision. We compare the two available scenarios describing the initial conditions: saturation and minijet production. We treat the elastic processes in the small scattering angle limit and work in the relaxation time approximation. By measuring the isotropy of different observables as a function of time, we show that the usual assumption that elastic collisions are efficient enough to rapidly achieve kinetic equilibrium is not reliable. Due to the longitudinal expansion at early times, the actual equilibration time is an order of magnitude bigger than the typical 1 fm estimate usually assumed.