Centrality dependence of the parton bubble model for high-energy heavy-ion collisions and fireball surface substructure at energies available at the BNL relativistic heavy ion collider (RHIC)

Abstract

In an earlier paper we developed a QCD-inspired theoretical parton bubble model (PBM) for RHIC/LHC. The motivation for the PBM was to develop a model that would reasonably quantitatively agree with the strong charged particle pair correlations observed by the STAR Collaboration at RHIC in $\mathrm{Au}+\mathrm{Au}$ central collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV in the transverse momentum range 0.8 to 2.0 GeV/$c$. The model was constructed to also agree with the Hanbury Brown and Twiss (HBT) observed small final-state source size $~2$ fm radii in the transverse momentum range above 0.8 GeV/$c$. The model assumed a substructure of a ring of localized adjoining $~2$ fm radius bubbles perpendicular to the collider beam direction, centered on the beam, at midrapidity. The bubble ring was assumed to be located on the expanding fireball surface of the $\mathrm{Au}+\mathrm{Au}$ collision. These bubbles consist almost entirely of gluons and form gluonic hot spots on the fireball surface. We achieved a reasonable quantitative agreement with the results of both the physically significant charge-independent (CI) and charge-dependent (CD) correlations that were observed. In this paper we extend the model to include the changing development of bubbles with centrality from the most central region where bubbles are very important to the most peripheral where the bubbles are gone. Energy density is found to be related to bubble formation and as centrality decreases the maximum energy density and bubbles shift from symmetry around the beam axis to the reaction plane region, causing a strong correlation of bubble formation with elliptic flow. We find reasonably quantitative agreement (within a few percent of the total correlations) with a new precision RHIC experiment that extended the centrality region investigated to the range 0%\char21{}80% (most central to most peripheral). The characteristics and behavior of the bubbles imply they represent a significant substructure formed on the surface of the fireball at kinetic freezeout.