Chemical Freezeout in Heavy Ion Collisions

TL;DR

This paper investigates how chemical freezeout affects the hydrodynamic modeling of heavy ion collisions, showing it alters temperature relations but not flow velocities, and analyzing its impact on particle spectra and freezeout conditions.

Contribution

It introduces a hadronic equation of state consistent with chemical freezeout and examines its effects on flow and particle spectra in heavy ion collision models.

Findings

Chemical freezeout does not change pressure-energy density relation.

Final spectra are insensitive to the switching temperature from hydrodynamics to cascade.

Pion spectra indicate a freezeout temperature around 100 MeV.

Abstract

We construct a hadronic equation of state consistent with chemical freezeout and discuss how such an equation of state modifies the radial and elliptic flow in a hydrodynamic + hadronic cascade model of relativistic heavy ion collisions at the SPS. Incorporating chemical freezeout does not change the relation between pressure and energy density. However, it does change the relation between temperature and energy density. Consequently, when the hydrodynamic solution and freezeout are expressed in terms of energy density, chemical freezeout does not modify the hydrodynamic radial and elliptic flow velocities studied previously. Finally, we examine chemical freezeout within the hadronic cascade (RQMD). Once chemical freezeout is incorporated into the hydrodynamics, the final spectra and fireball lifetimes are insensitive to the temperature at which the switch from hydrodynamics to cascade is made. Closer inspection indicates that the pion spectrum in chemically frozen hydrodynamics is significantly cooler than in the hydro+cascade model. This difference is reflected in $v_{2}(p_{T})$. We extract the freezeout hadron density in RQMD and interpret it in thermal terms; the freezeout hadron density corresponds to a freezeout temperature of $T_{f}\approx100 $ MeV and $\mu_{\pi} \approx 80 $ MeV.