Bjorken Initial Energy Density and Viscous Longitudinal Hydrodynamic Evolution in Xe-Xe Collisions

TL;DR

This study estimates the initial energy density in Xe-Xe collisions at 5.44 TeV, explores viscous hydrodynamic evolution of the quark-gluon plasma, and compares it with Pb-Pb collisions to understand system-size effects.

Contribution

It introduces a generalized method for estimating initial energy density beyond central collisions and analyzes viscous effects on QGP evolution in small systems.

Findings

Viscous effects slow longitudinal expansion and increase QGP lifetime.

Initial energy density primarily controls the evolution pattern.

Xe-Xe and Pb-Pb systems show similar evolution when initial energy densities are comparable.

Abstract

We present a systematic study of the Bjorken initial energy density in Xe-Xe collisions at $\sqrt{s_{NN}} = 5.44$ TeV, estimated using charged-particle multiplicity data and a generalized transverse overlap geometry applicable beyond the most central collisions. The dependence of the extracted energy density is examined by adopting both a constant formation time and a centrality-dependent formation time derived from Pb-Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV. Corresponding Bjorken energy density estimates for Pb-Pb collisions are also presented for comparison. Taking the Bjorken energy density and formation time as initial conditions, the subsequent longitudinal evolution of the quark-gluon plasma (QGP) formed in these collisions is studied. Both ideal and first-order viscous boost-invariant hydrodynamics are employed to assess the influence of dissipation. We observe that viscous effects slow the longitudinal expansion and lead to entropy production dominated by early-time dynamics. The lifetime of the QGP is observed to increase with centrality and is substantially enhanced by viscous effects. These effects are highly sensitive to the choice of formation time, particularly in peripheral collisions. A comparative analysis of Xe-Xe and Pb-Pb collisions demonstrates that the longitudinal evolution is primarily controlled by the initial energy density scale set by the Bjorken prescription. Consequently, when this scale is comparable, both systems exhibit nearly identical evolution patterns, while appreciable distinctions emerge in peripheral collisions due to system-size and geometric effects.