Estimates of the baryon densities attainable in heavy-ion collisions from the beam energy scan program

TL;DR

This paper estimates the baryon and energy densities in heavy-ion collisions during the RHIC Beam Energy Scan, revealing high initial baryon densities in the fragmentation regions that depend on collision energy.

Contribution

It provides new estimates of baryon densities in fragmentation regions across a range of collision energies using a three-fluid dynamics model.

Findings

Approximately 70% of baryon charge is stopped at 39 GeV.

Maximum initial baryon densities occur at 20-40 GeV, reaching about 10 times nuclear saturation density.

High initial baryon densities influence observable effects mainly in fragmentation regions.

Abstract

The baryon and energy densities attained in fragmentation regions in central Au+Au collisions in the energy range of the Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC) are estimated within the model of the three-fluid dynamics. It is shown that a considerable part of the baryon charge is stopped in the central fireball. Even at 39 GeV, approximately 70% of the total baryon charge turns out to be stopped. The fraction of this stopped baryon charge decreases with collision energy rise, from 100% at 7.7 GeV to $\sim$40% at 62 GeV. The highest initial baryon densities of the thermalized matter, $n_B/n_0 \approx$ 10, are reached in the central region of colliding nuclei at $\sqrt{s_{NN}}=$ 20--40 GeV. These highest densities develop up to quite moderate freeze-out baryon densities at the midrapidity because the matter of the central fireball is pushed out to fragmentation regions by one-dimensional expansion. Therefore, consequences of these high initial baryon densities can be observed only in the fragmentation regions of colliding nuclei in AFTER@LHC experiments in the fixed-target mode.