Beam energy dependence of the expansion dynamics in relativistic heavy ion collisions: Indications for the critical end point?

TL;DR

This study investigates how the expansion dynamics in relativistic heavy ion collisions vary with beam energy, revealing non-monotonic patterns that suggest proximity to the critical end point in the nuclear matter phase diagram.

Contribution

It provides the first comprehensive analysis of flow harmonic and emission source radii across a wide energy range, indicating potential signals of the critical end point in nuclear matter.

Findings

Viscosity estimates are higher at LHC energies compared to RHIC.

Non-monotonic energy dependence observed in expansion parameters.

Possible indication of the critical end point in the phase diagram.

Abstract

The flow harmonic $v_{n}$ and the emission source radii $R_{\text{out}}$, $R_{\text{side}}$ and $R_{\text{long}}$ are studied for a broad range of centrality selections and beam collision energies in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions at RHIC and the LHC respectively. They validate the acoustic scaling patterns expected for hydrodynamic-like expansion over the entire range of beam energies studied. The combined data sets allow estimates for the \sqsn\ dependence of the mean expansion speed $\left<c_s\right>$, emission duration $\left<\Delta\tau\right>$ and the viscous coefficients $\left<\beta''\right>$ that encode the magnitude of the specific shear viscosity $\left<\eta/s\right>$. The estimates indicate initial-state model independent values of $\left<\eta/s\right>$ which are larger for the plasma produced at 2.76 TeV (LHC) compared to that produced at 200 GeV (RHIC) ($\left<4\pi\eta/s\right>_{\text{LHC}}=2.2\pm 0.2$ and $\left<4\pi\eta/s\right>_{\text{RHIC}}=1.3\pm 0.2$). They also show a non-monotonic \sqsn\ dependence for $\left<\beta''\right>$, $\left<c_s\right>$ and $\left<\Delta\tau\right>$, with minima for $\left<\beta''\right>$ and $\left<c_s\right>$, and a complimentary maximum for $\left<\Delta\tau\right>$. These dependencies signal a significant change in reaction dynamics in a narrow span of $\sqrt{s_{NN}}$, which may be linked to reaction trajectories close to the critical end point (CEP) in the phase diagram for nuclear matter.