Centrality dependence of Lévy-stable two-pion Bose-Einstein correlations in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions

The PHENIX collaboration
Abdulameer, N.J. , Acharya, U. , Adare, A. , Aidala, C. , Ajitanand, N.N. , Akiba, Y. , Akimoto, R. , Al-Ta'ani, H. , Alexander, J. , Angerami, A.
Phys.Rev.C 110 (2024) 064909, 2024.

Abstract (data abstract)
The PHENIX experiment measured the centrality dependence of two-pion Bose-Einstein correlation functions in $\sqrt{s_{NN}}$=200~GeV Au+Au collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The data are well represented by Lévy-stable source distributions. The extracted source parameters are the correlation-strength parameter $\lambda$, the Lévy index of stability $\alpha$, and the Lévy-scale parameter $R$ as a function of transverse mass $m_T$​ and centrality. The $\lambda(m_T)$ parameter is constant at larger values of $m_T$​, but decreases as $m_T$​ decreases. The Lévy scale parameter $R(m_T​)$ decreases with $m_T$​ and exhibits proportionality to the length scale of the nuclear overlap region. The Lévy exponent $\alpha(m_T)$ is independent of $m_T$ within uncertainties in each investigated centrality bin, but shows a clear centrality dependence. At all centralities, the Lévy exponent $\alpha$ is significantly different from that of Gaussian ($\alpha=2$) or Cauchy ($\alpha=1$) source distributions. Comparisons to the predictions of Monte-Carlo simulations of resonance-decay chains show that in all but the most peripheral centrality class (50\%-60\%), the obtained results are inconsistent with the measurements, unless a significant reduction of the in-medium mass of the $\eta'$ meson is included. In each centrality class, the best value of the in-medium $\eta'$ mass is compared to the mass of the $\eta$ meson, as well as to several theoretical predictions that consider restoration of $U_A​(1)$ symmetry in hot hadronic matter.

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