Dynamical freeze-out condition in ultrarelativistic heavy ion collisions

TL;DR

This paper introduces a dynamical freeze-out criterion for heavy ion collisions, determining decoupling surfaces by comparing hydrodynamic expansion rates with microscopic scattering rates, and finds consistent decoupling temperatures across RHIC and LHC.

Contribution

It proposes a rate-based dynamical freeze-out condition and demonstrates its effectiveness in describing pion spectra and decoupling temperatures at RHIC and LHC.

Findings

Decoupling occurs when expansion rate matches scattering rate.

Effective decoupling temperature remains stable from RHIC to LHC.

Dynamical freeze-out surfaces improve spectral predictions.

Abstract

We determine the decoupling surfaces for the hydrodynamic description of heavy ion collisions at RHIC and LHC by comparing the local hydrodynamic expansion rate with the microscopic pion-pion scattering rate. The pion $p_T$ spectra for nuclear collisions at RHIC and LHC are computed by applying the Cooper-Frye procedure on the dynamical-decoupling surfaces, and compared with those obtained from the constant-temperature freeze-out surfaces. Comparison with RHIC data shows that the system indeed decouples when the expansion rate becomes comparable with the pion scattering rate. The dynamical decoupling based on the rates comparison also suggests that the effective decoupling temperature in central heavy ion collisions remains practically unchanged from RHIC to LHC.