Observables and open problems for NICA

Abstract

The restoration of chiral symmetry in hot dense nuclear systems in competition with a transition to deconfined matter in central nucleus-nucleus collisions at NICA energies is a central problem of nuclear physics. To explore these transitions we study the production of hadrons in nucleus-nucleus collisions from 4 to 160A 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. The modeling of chiral-symmetry restoration in PHSD is driven by the pion-nucleon $\Sigma$ -term in the computation of the quark scalar condensate $\langle q \bar{q} \rangle$ that serves as an order parameter for CSR and is assumed to scale with the effective quark masses $ m_{s}$ and $ m_{q}$ . Furthermore, the nucleon scalar density $\rho_{s}$ , which also enters the computation of $\langle q \bar{q} \rangle$ , is evaluated within the nonlinear $\sigma$ - $ \omega$ model which is constrained by Dirac-Brueckner calculations and low-energy heavy-ion reactions. The essential impact of CSR is found in the Schwinger mechanism (for string decay) which fixes the ratio of strange to light quark production in the hadronic medium. We find that above $\sim 80$ 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 40A GeV and suggest a microscopic explanation for the maximum in the $ K^{+}/\pi^{+}$ ratio at about 30A GeV which only shows up if in addition to CSR a deconfinement transition to partonic degrees-of-freedom is incorporated in the reaction dynamics.