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

TL;DR

This study investigates how chiral symmetry restoration and deconfinement influence strange hadron production in heavy-ion collisions, revealing the dominance of partonic effects at high energies and explaining the observed maximum in the $K^+/\pi^+$ ratio.

Contribution

It introduces a transport model incorporating chiral symmetry restoration effects alongside deconfinement, providing new insights into the transition dynamics in heavy-ion collisions.

Findings

Partonic degrees-of-freedom dominate above ~80 A GeV.

Supports the existence of 'quarkyonic matter' from 5 to 40 A GeV.

Explains the maximum in the $K^+/\pi^+$ ratio at ~30 A GeV.

Abstract

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 $K^+/\pi^+$ 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 modelling of chiral symmetry restoration is driven by the pion-nucleon $\Sigma$-term in the computation of the quark scalar condensate $<q {\bar q}>$ 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 $<q {\bar q}>$, is evaluated within the nonlinear $\sigma-\omega$ model which is constraint 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 $\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 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.