Chiral symmetry restoration versus deconfinement in heavy-ion collisions at high baryon density

摘要

We study the production of strange hadrons in nucleus-nucleus collisions from 4 to 160 A GeV within the parton-hadron-string dynamics (PHSD) transport approach that is extended to incorporate essentials aspects of chiral symmetry restoration (CSR) in the hadronic sector (via the Schwinger mechanism) on top of the deconfinement phase transition as implemented in PHSD. Especially the K+/ pi p(+) and the (Lambda + Sigma 0)/ pi(-) ratios in central Au+Au collisions are found to provide information on the relative importance of both transitions. The modeling of chiral symmetry restoration is driven by the pion-nucleon Sigma term in the computation of the quark scalar condensate (q) over barq > that serves as an order parameter for CSR and also scales approximately with the effective quark masses m(s) and m(q). Furthermore, the nucleon scalar density rho(s), which also enters the computation of (q) over barq >, is evaluated within the nonlinear sigma -omega model which is constrained by Dirac-Brueckner calculations and low-energy heavy-ion reactions. The Schwinger mechanism (for string decay) fixes the ratio of strange to light quark production in the hadronic medium. We find that above similar to 80 A GeV the reaction dynamics of heavy nuclei is dominantly driven by partonic degrees of freedom such that traces of the chiral symmetry restoration are hard to identify. Our studies support the conjecture of "quarkyonic matter" in heavy-ion collisions from about 5 to 40 A GeV and provide a microscopic explanation for the maximum in the K+/ pi(+) ratio at about 30 A GeV, which only shows up if a transition to partonic degrees of freedom is incorporated in the reaction dynamics and is discarded in the traditional hadron-string models.