The high-density equation of state in heavy-ion collisions: Constraints from proton flow

TL;DR

This paper investigates how different equations of state affect flow observables in heavy-ion collisions using transport simulations, providing constraints on phase transitions at high densities based on experimental data.

Contribution

It introduces a method to incorporate various equations of state into transport simulations and identifies observable signatures of phase transitions at high densities.

Findings

Strong phase transitions at densities below 4n_0 are excluded by data.

A phase transition causes a characteristic increase in elliptic flow (v_2).

A minimum in the directed flow slope indicates a low-density phase transition.

Abstract

A set of different equations of state is implemented in the molecular dynamics part of a non-equilibrium transport simulation (UrQMD) of heavy-ion collisions. It is shown how different flow observables are affected by the density dependence of the equation of state. In particular, the effects of a phase transition at high density are explored, including an expected reduction in mean $m_T$. We also show that an increase in $v_2$ is characteristic for a strong softening of the equation of state. The phase transitions with a low coexistence density, $n_{\mathrm{CE}}<4 n_0$, show a distinct minimum in the slope of the directed flow as a function of the beam energy, which would be a clear experimental signal. By comparing our results with experimental data, we can exclude any strong phase transition at densities below $4n_0$.