Local equilibrium in heavy ion collisions. Microscopic model versus statistical model analysis

TL;DR

This study compares microscopic transport model calculations with statistical model predictions in heavy ion collisions, finding near-equilibrium conditions at lower energies but significant deviations at higher energies due to complex decay processes.

Contribution

It provides a detailed analysis of local equilibrium assumptions in heavy ion collisions across a range of energies, highlighting the limitations of statistical models at higher energies.

Findings

Near-equilibrium conditions are approached at AGS energies for t ≥ 10 fm/c.

Deviations from statistical model predictions increase with collision energy.

Violations of local equilibrium are linked to multiparticle decays of strings and resonances.

Abstract

The assumption of local equilibrium in relativistic heavy ion collisions at energies from 10.7 AGeV (AGS) up to 160 AGeV (SPS) is checked in the microscopic transport model. Dynamical calculations performed for a central cell in the reaction are compared to the predictions of the thermal statistical model. We find that kinetic, thermal and chemical equilibration of the expanding hadronic matter are nearly approached late in central collisions at AGS energy for $t \geq 10$ fm/$c$ in a central cell. At these times the equation of state may be approximated by a simple dependence $P \cong (0.12-0.15) \epsilon$. Increasing deviations of the yields and the energy spectra of hadrons from statistical model values are observed for increasing energy, 40 AGeV and 160 AGeV. These violations of local equilibrium indicate that a fully equilibrated state is not reached, not even in the central cell of heavy ion collisions at energies above 10 AGeV. The origin of these findings is traced to the multiparticle decays of strings and many-body decays of resonances.