Pion emission source shape in UrQMD Au+Au collisions at STAR energies
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arXiv
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| Natura: | Preprint |
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2026
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| _version_ | 1866916015743434752 |
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| author | Molnar, Matyas Kincses, Daniel Csanad, Mate |
| author_facet | Molnar, Matyas Kincses, Daniel Csanad, Mate |
| contents | 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 $\sqrt{s_{NN}}=3$--$27\,\mathrm{GeV}$, taking the RHIC BES-II range of collider and fixed target experiment energies for reference. Using Lévy-type source parameterisations, we extract the pair multiplicity parameter $λ^{*}$ (related to the correlation strength $λ$), Lévy index $α$, 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 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2605_15717 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Pion emission source shape in UrQMD Au+Au collisions at STAR energies Molnar, Matyas Kincses, Daniel Csanad, Mate Nuclear Theory High Energy Physics - Phenomenology Nuclear Experiment 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 $\sqrt{s_{NN}}=3$--$27\,\mathrm{GeV}$, taking the RHIC BES-II range of collider and fixed target experiment energies for reference. Using Lévy-type source parameterisations, we extract the pair multiplicity parameter $λ^{*}$ (related to the correlation strength $λ$), Lévy index $α$, 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 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. |
| title | Pion emission source shape in UrQMD Au+Au collisions at STAR energies |
| topic | Nuclear Theory High Energy Physics - Phenomenology Nuclear Experiment |
| url | https://arxiv.org/abs/2605.15717 |