Event-by-event background in estimates of the chiral magnetic effect

TL;DR

This paper uses the PHSD approach to analyze event-by-event fluctuations in azimuthal correlations of charged hadrons, aiming to understand signals of local parity violation in heavy-ion collisions.

Contribution

It introduces a detailed event-by-event PHSD model including electromagnetic and fluctuation effects to interpret azimuthal correlations in heavy-ion collisions.

Findings

STAR data at 7.7 and 11.5 GeV are well reproduced by the model.

Model overestimates correlations at higher energies, indicating missing physics.

Additional fluctuating transverse anisotropy sources are suggested.

Abstract

In terms of the parton-hadron-string-dynamics (PHSD) approach - including the retarded electromagnetic field - we investigate the role of fluctuations of the correlation function in the azimuthal angle $\psi$ of charged hadrons that is expected to be a sensitive signal of local strong parity violation. For the early time we consider fluctuations in the position of charged spectators resulting in electromagnetic field fluctuations as well as in the position of participant baryons defining the event plane. For partonic and hadronic phases in intermediate stages of the interaction we study the possible formation of excited matter in electric charge dipole and quadrupole form as generated by fluctuations. The role of the transverse momentum and local charge conservation laws in the observed azimuthal asymmetry is investigated, too. All these above-mentioned effects are incorporated in our analysis based on event-by-event PHSD calculations. Furthermore, the azimuthal angular correlations from Au+Au collisions observed in the recent STAR measurements within the RHIC Beam-Energy-Scan (BES) program are studied. It is shown that the STAR correlation data at the collision energies of $\sqrt{s_{NN}}$ = 7.7 and 11.5 GeV can be reasonably reproduced within the PHSD. At higher energies the model fails to describe the $\psi$ correlation data resulting in an overestimation of the partonic scalar field involved. We conclude that an additional transverse anisotropy fluctuating source is needed which with a comparable strength acts on both in- and out-of-plane components.