Pion emission source shape in UrQMD Au+Au collisions at STAR energies

TL;DR

This study uses UrQMD simulations to analyze pion emission source shapes in Au+Au collisions across a range of energies, revealing collective expansion and energy-dependent source characteristics.

Contribution

It provides a systematic femtoscopic analysis of pion source parameters in UrQMD simulations over a broad energy range, incorporating Lévý source parameterizations.

Findings

Source radii decrease with increasing transverse mass.

Source radii increase with collision energy, indicating collective expansion.

Lévý index decreases with collision energy, showing more Lévy-like sources at higher energies.

Abstract

Femtoscopic measurements of two-pion Bose--Einstein correlations have established that particle-emitting sources in heavy-ion collisions are well described by L\'evy $\alpha$-stable distributions, motivating systematic studies across a wide range of collision energies. In this work, we present a three-dimensional femtoscopic analysis of pion pairs in Au+Au collisions simulated with the UrQMD model for collision energies $\sqrt{s_{NN}}=3$--$27\,\mathrm{GeV}$, taking the RHIC BES-II range of collider and fixed target experiment energies for reference. Using L\'evy-type source parameterisations, we extract the pair multiplicity parameter $\lambda^{*}$ (related to the correlation strength $\lambda$), L\'evy index $\alpha$, and three-dimensional radii $R_\mathrm{out}$, $R_\mathrm{side}$, and $R_\mathrm{long}$. We investigate their dependence on the transverse mass ($m_T$) and collision energy, along with derived quantities such as the radius difference $R_{\mathrm{diff}}^2=R_{\mathrm{out}}^2-R_\mathrm{side}^2$ and the ratio $R_\mathrm{out}/R_\mathrm{side}$. We find that $R_\mathrm{out,side,long}$ all decrease with increasing $m_T$ and increase with collision energy, consistent with collective expansion, $R_\mathrm{long}$ showing the strongest and $R_\mathrm{side}$ the weakest energy dependence. The L\'evy index $\alpha$ decreases with collision energy, with a larger $m_T$-dependence towards higher energies. The $\lambda^{*}$ parameter is consistent with a constant close to unity in the absence of pions from long-lived resonances. These results provide a baseline for future comparisons with experimental measurements from the STAR Collaboration, contributing to constraints on the QCD phase diagram.