Charge Conservation and Higher Moments of Charge Fluctuations

TL;DR

This paper investigates how local charge conservation, resonance decays, and other background effects influence higher moments of charge fluctuations in heavy-ion collisions, aiding the interpretation of signals for QCD phase transitions.

Contribution

The paper introduces Monte Carlo models and analytic approaches to quantify background effects on charge fluctuation moments, enhancing understanding of experimental data.

Findings

Charge conservation explains much of the non-Poissonian fluctuations in net-proton data.

Models match STAR net-proton fluctuation data but not net charge fluctuations.

Sensitivity of higher moments to charge conservation parameters is demonstrated.

Abstract

Higher moments of distributions of net charge and baryon number in heavy-ion collisions have been proposed as signals of fundamental QCD phase transitions. In order to better understand background processes for these observables, models are presented which enable one to gauge the effects of local charge conservation, decays of resonances and clusters, Bose symmetrization, and volume fluctuations. Monte Carlo methods for generating samplings of particles consistent with local charge conservation are presented, and are followed by a review of simple analytic models involving a single type of charge with a constant experimental efficiency. The main model consists of thermal emission superimposed onto a simple parameterization of collective flow, known as a blast-wave, with emission being consistent with individual canonical ensembles. The spatial extent of local charge conservation is parameterized by the size and extent over which charge is conserved. The sensitivity of third and fourth order moments, skewness and kurtosis, to these parameters, and to beam energy and baryon density is explored. Comparisons with STAR data show that a significant part of the observed non-Poissonian fluctuations in net-proton fluctuations are explained by charge and baryon-number conservation, but that measurements of the STAR collaboration for fluctuations of net electric charge significantly differ from expectations of the models presented here.