Abstract
The PHENIX experiment measured the centrality dependence of two-pion Bose-Einstein correlation functions in √sNN = 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 λ, the Lévy index of stability α, and the Lévy-scale parameter R as a function of transverse mass mT and centrality. The λ(mT ) parameter is constant at larger values of mT , but decreases as mT decreases. The Lévy-scale parameter R(mT ) decreases with mT and exhibits proportionality to the length scale of the nuclear overlap region. The Lévy exponent α(mT ) is independent of mT within uncertainties in each investigated centrality bin, but shows a clear centrality dependence. At all centralities, the Lévy exponent α is significantly different from that of Gaussian (α = 2) or Cauchy (α = 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 η meson is included. In each centrality class, the best value of the in-medium η mass is compared to the mass of the η meson, as well as to several theoretical predictions that consider restoration of UA(1) symmetry in hot hadronic matter.
