Examination of directed flow as a signature of the softest point of the equation of state in QCD matter

TL;DR

This paper investigates how the directed flow of protons and pions in high-energy heavy-ion collisions can signal the softest point of the QCD equation of state, showing that models including a softening effect match experimental data.

Contribution

It demonstrates that incorporating a softening of the equation of state in transport models explains the collapse of directed flow observed at certain energies, unlike standard approaches.

Findings

Softening of the equation of state is necessary to reproduce directed flow collapse.

Standard hadronic models fail below 20 GeV incident energy.

The softening occurs at high baryon density stages of the collision.

Abstract

We analyze the directed flow of protons and pions in high-energy heavy-ion collisions in the incident energy range from $\sqrt{s_{{\scriptscriptstyle NN}}}=7.7$ to 27 GeV within a microscopic transport model. Standard hadronic transport approaches do not describe the collapse of directed flow below $\sqrt{s_{{\scriptscriptstyle NN}}}\simeq 20$ GeV. By contrast, a model which simulates effects of a softening of the equation of state, well describes the behavior of directed flow data recently obtained by the STAR Collaboration~\cite{STARv1}. We give a detailed analysis of how directed flow is generated. Particularly, we found that softening of effective equation of state at the overlapping region of two nuclei, i.e. the reaction stages where the system reaches high baryon density state, is needed to explain the observed collapse of proton directed flow within a hadronic transport approach.