Rapidity correlations in the RHIC Beam Energy Scan Data

TL;DR

This paper studies rapidity correlations in heavy-ion collisions at RHIC, decomposing them into Legendre polynomials to explore collision dynamics and compare experimental data with models across various energies.

Contribution

It provides the first detailed measurement of rapidity correlations in Au+Au collisions over a range of energies and compares these with UrQMD model predictions.

Findings

Rapidity correlations vary with beam energy.

Decomposition into Legendre polynomials reveals collision dynamics.

Comparison with UrQMD shows qualitative agreement.

Abstract

A pair-normalized two-particle covariance versus the rapidity of the two particles, called R$_2$, was originally studied in ISR and FNAL data in the 1970's. This variable has recently seen renewed interest for the study of the dynamics of heavy-ion collisions in the longitudinal direction. These rapidity correlations can be decomposed into a basis set of Legendre polynomials with prefactors $\langle a_{mn}\rangle$, which can be considered the rapidity analog of the decomposition of azimuthal anisotropies into a set of cosine functions with prefactors v$_{\rm n}$. The $\langle a_{mn}\rangle$ values have been suggested to be sensitive to the number of particle emitting sources, baryon stopping, viscosities, and critical behavior. The rapidity correlations have been measured by the STAR collaboration as a function of the beam energy for 0-5% central Au$+$Au collisions with beam energies ranging from 7.7 to 200 $\mathrm{GeV}$. The experimental results and comparisons to the UrQMD model are presented.