Does non-monotonic behavior of directed flow signal the onset of deconfinement?

TL;DR

This study uses a transport model to analyze directed flow in high-energy nucleus collisions, finding that hadronic models alone cannot explain the observed non-monotonic behavior, suggesting the need for additional physics such as deconfinement.

Contribution

The paper demonstrates that the JAM transport model with mean-field effects cannot fully reproduce the experimental directed flow data, indicating the onset of deconfinement.

Findings

Mean-field effects are crucial at lower energies.

The model's slope of directed flow becomes incompatible with data at higher energies.

Hadronic degrees of freedom alone are insufficient to explain the observed behavior.

Abstract

We investigate the effects of nuclear mean-field as well as the formation and decay of nuclear clusters on the directed flow $v_1$ in high energy nucleus-nucleus collisions from $\sqrt{s_{NN}}=7.7$ GeV to 27 GeV incident energies within a transport model. Specifically, we use the JAM transport model in which potentials are implemented based on the framework of the relativistic quantum molecular dynamics. Our approach reproduces the rapidity dependence of directed flow data up to $\sqrt{s_{NN}}\approx 8$ GeV showing the significant importance of mean-field. However, the slopes of $dv_1/dy$ at mid-rapidity are calculated to be positive at $\sqrt{s_{NN}}=11.7$ and 19.6 GeV, and becomes negative above 27 GeV. Thus the result from the JAM hadronic transport model with nuclear mean-field approach is incompatible with the data. Therefore within our approach, we conclude that the excitation function of the directed flow cannot be explained by the hadronic degree of freedom alone.