Interplay of longitudinal and transverse expansion in the kinetic dynamics of heavy-ion collisions

TL;DR

This paper models the early non-equilibrium stage of heavy-ion collisions using kinetic equations, revealing how longitudinal and transverse expansion interplay affects flow asymmetries and can indicate non-equilibrium effects.

Contribution

It introduces a kinetic model for non-boost invariant systems to study the interplay of longitudinal and transverse expansion in heavy-ion collisions.

Findings

Flow asymmetry is highly sensitive to non-equilibrium conditions.

Rapidity odd directed flow can serve as a probe for non-equilibrium effects.

The effect on final transverse momentum observables is minimal.

Abstract

The early dynamics in heavy-ion collisions involves a rapid, far from equilibrium evolution. This early pre-equilibrium stage of the dynamics can be modeled using kinetic equations. The effect of this pre-equilibrium stage on final observables derived from transverse momenta of emitted particles is small. The kinetic equations in the relaxation time approximation for a non-boost invariant system are solved. The asymmetry of the flow with respect to the reaction plane at different rapidities is found to be very sensitive to the degree of non-equilibrium in the evolution. This suggests that the rapidity odd directed flow could be studied to identify the occurrence of non-equilibrium effects and to estimate the asymmetry of the pressure between the longitudinal and transverse directions in the collision.