Dynamical freeze-out in event-by-event hydrodynamics

TL;DR

This paper investigates a dynamical freeze-out criterion in hydrodynamic models of heavy-ion collisions, comparing it to traditional constant temperature freeze-out, and finds minimal differences in hadron spectra and flow.

Contribution

It introduces a dynamical freeze-out approach based on expansion and scattering rates and compares its effects to constant temperature freeze-out in event-by-event hydrodynamics.

Findings

Differences between freeze-out criteria are small in all studied cases.

Dynamical freeze-out slightly alters hadron spectra and elliptic flow.

Event-by-event fluctuations influence the freeze-out process.

Abstract

In hydrodynamical modeling of the ultrarelativistic heavy-ion collisions the freeze-out is typically performed at a constant temperature or density. In this work we apply a dynamical freeze-out criterion, which compares the hydrodynamical expansion rate with the pion scattering rate. Recently many calculations have been done using event-by-event hydrodynamics where the initial density profile fluctuates from event to event. In these event-by-event calculations the expansion rate fluctuates strongly as well, and thus it is interesting to check how the dynamical freeze-out changes hadron distributions with respect to the constant temperature freeze-out. We present hadron spectra and elliptic flow calculated using (2+1)-dimensional ideal hydrodynamics, and show the differences between constant temperature and dynamical freeze-out criteria. We find that the differences caused by different freeze-out criteria are small in all studied cases.