Theoretical analysis of a possible observation of the chiral magnetic effect in Au + Au collisions within the RHIC beam energy scan program

TL;DR

This paper uses the hadron-string-dynamics model to analyze azimuthal correlations in Au+Au collisions, assessing the potential observation of the chiral magnetic effect and the role of electromagnetic fields across different collision energies.

Contribution

It provides a theoretical analysis of the chiral magnetic effect signals in heavy-ion collisions using the HSD model, highlighting energy-dependent dynamics and electromagnetic effects.

Findings

HSD model reproduces STAR data at 7.7 GeV

Deviations increase at higher energies, especially at 39 GeV

At energies around 40 GeV and above, partonic degrees of freedom are necessary

Abstract

In terms of the hadron-string-dynamics (HSD) approach we investigate the correlation function in the azimuthal angle $\psi$ of charged hadrons that is expected to be sensitive to a signal of local strong parity violation. Our analysis of Au+Au collisions is based on the recent STAR data within the RHIC Beam-Energy-Scan (BES) program. The HSD model reasonably reproduces STAR data for $\sqrt{s_{NN}}=$7.7 GeV, while there are some deviations from the experiment at the collision energy of 11.5 GeV and an increase of deviations between theory and experiment at $\sqrt{s_{NN}}=$39 GeV. For reference, the results for $\sqrt{s_{NN}}=$ 200 GeV are given as well. The role of the retarded electromagnetic field is discussed and a compensation effect for the action of its electric and magnetic components is pointed out. We conclude that the recent RHIC BES data at $\sqrt{s_{NN}}=$7.7 and 11.5 GeV can be understood on the hadronic level without involving the idea of a strong parity violation; however, at $\sqrt{s_{NN}}\sim$40 GeV and above one needs to take into consideration explicit partonic (quark-qluon) degrees-of-freedom for a proper treatment of the dynamics.