Beam-Energy Dependence of Charge Balance Functions from Au+Au Collisions at RHIC

TL;DR

This study measures charge balance functions in Au+Au collisions at RHIC across various energies, revealing that the widths decrease with increasing energy and centrality, supporting the formation of a quark-gluon plasma even at lower energies.

Contribution

It provides the first systematic comparison of balance function widths across a wide energy range from RHIC to LHC, indicating QGP formation at lower energies.

Findings

Balance function widths decrease with energy and centrality.

Shuffled events show little dependence on energy or centrality.

UrQMD simulations produce wider widths than observed.

Abstract

Balance functions have been measured in terms of relative pseudorapidity ($\Delta \eta$) for charged particle pairs at the Relativistic Heavy-Ion Collider (RHIC) from Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7 GeV to 200 GeV using the STAR detector. These results are compared with balance functions measured at the Large Hadron Collider (LHC) from Pb+Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV by the ALICE Collaboration. The width of the balance function decreases as the collisions become more central and as the beam energy is increased. In contrast, the widths of the balance functions calculated using shuffled events show little dependence on centrality or beam energy and are larger than the observed widths. Balance function widths calculated using events generated by UrQMD are wider than the measured widths in central collisions and show little centrality dependence. The measured widths of the balance functions in central collisions are consistent with the delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing of the balance function in central collisions at $\sqrt{s_{\rm NN}}$ = 7.7 GeV implies that a QGP is still being created at this relatively low energy.