Event-by-event investigation of the two-particle source function in heavy-ion collisions with EPOS

TL;DR

This study uses the EPOS event generator to analyze the two-particle source function in heavy-ion collisions, revealing that individual events exhibit non-Gaussian, Levy-shaped distributions, challenging previous assumptions about event averaging effects.

Contribution

It demonstrates that non-Gaussian features of the source function are intrinsic to individual events, not artifacts of averaging, and provides a detailed event-by-event Levy parameter analysis.

Findings

Individual events show Levy-shaped source distributions.

Non-Gaussian features are inherent, not due to event averaging.

Levy parameters vary with event centrality and particle momentum.

Abstract

Exploring the shape of the pair-source function for particles such as pions or kaons has been an important goal of heavy-ion physics, and substantial effort has been made in order to understand the underlying physics behind the experimental observations of non-Gaussian behavior. In experiments, since no direct measurement of the source function is possible, quantum-statistical momentum correlations are utilized to gain information about the space-time geometry of the particle emitting source. Event generators, such as EPOS, however, provide direct access to the freeze-out coordinates of final state particles, and thus the source function can be constructed and investigated. The EPOS model is a sophisticated hybrid model where the initial stage evolution of the system is governed by Parton-Based Gribov-Regge theory, and subsequently a hydrodynamic evolution is utilized, followed by hadronization and hadron dynamics. EPOS has already proven to be successful in describing several different experimental observations for systems characterized by baryon chemical potential close to zero, but so far the source shape has not been explored in detail. In this paper we discuss an event-by-event analysis of the two-particle source function in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions generated by the EPOS model. We find that when utilizing all stages of the model, L\'evy-shaped distributions (unlike Gaussian distributions) provide a good description of the source shape in the individual events. Hence it is clear that it is not the event averaging that creates the non-Gaussian features in the pair distributions. Based on this observation, we determine L\'evy-parameters of the source as a function of event centrality and particle momentum.