Pion emission source shape in UrQMD Au + Au collisions at STAR energies
Abstract
Femtoscopic measurements of two-pion Bose--Einstein correlations have established that particle-emitting sources in heavy-ion collisions are well described by Lévy α-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 √(sNN)=3--27 GeV, taking the RHIC BES-II range of collider and fixed target experiment energies for reference. Using Lévy-type source parameterisations, we extract the Lévy index α, the three-dimensional radii Rout, Rside, Rlong, and the pair multiplicity parameter λ* (related to the correlation strength λ). We investigate their dependence on the transverse mass (mT) and collision energy, along with derived quantities such as the radius difference Rdiff2=Rout2−Rside2 and the ratio Rout/Rside. We find that Rout,side,long all decrease with increasing mT and increase with collision energy, consistent with collective expansion, Rlong showing the strongest and Rside the weakest energy dependence. The Lévy index α decreases with collision energy, with a larger mT-dependence towards higher energies. The λ* 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.
Versions
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BibTeX
@misc{molnar2026pion,
title = {{Pion emission source shape in UrQMD Au + Au collisions at STAR energies}},
author = {Matyas Molnar and Daniel Kincses and Yan Huang and Mate Csanad},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2605.15717},
eprint = {2605.15717},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.15717},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Pion emission source shape in UrQMD Au + Au collisions at STAR energies
AU - Matyas Molnar
AU - Daniel Kincses
AU - Yan Huang
AU - Mate Csanad
PY - 2026
DA - 2026/10/14
PB - arXiv
DO - 10.48550/arxiv.2605.15717
UR - https://arxiv.org/abs/2605.15717
AB - Femtoscopic measurements of two-pion Bose--Einstein correlations have established that particle-emitting sources in heavy-ion collisions are well described by Lévy α-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 √(s_NN)=3--27 GeV, taking the RHIC BES-II range of collider and fixed target experiment energies for reference. Using Lévy-type source parameterisations, we extract the Lévy index α, the three-dimensional radii R_out, R_side, R_long, and the pair multiplicity parameter λ^* (related to the correlation strength λ). We investigate their dependence on the transverse mass (m_T) and collision energy, along with derived quantities such as the radius difference R_diff^2=R_out^2−R_side^2 and the ratio R_out/R_side. We find that R_out,side,long all decrease with increasing m_T and increase with collision energy, consistent with collective expansion, R_long showing the strongest and R_side the weakest energy dependence. The Lévy index α decreases with collision energy, with a larger m_T-dependence towards higher energies. The λ^* 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.
ER -