Abstract
The effect of the chiral symmetry restoration (CSR) on observables from heavy-ion collisions is studied in the energy range within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the non-linear σ − ω model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations. For the pion-nucleon Σ-term we adopt Σπ ≈ 45 MeV which corresponds to some ‘world average’. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at , realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive. Our results provide a microscopic explanation for the “horn” structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to , while the drop at higher energies is associated to the appearance of a deconfined partonic medium.
Highlights
According to Quantum Chromo Dynamics (QCD) [1,2,3,4,5,6], matter changes its phase at high temperature and density and bound hadrons dissolve to interacting quarks and gluons in the Quark-Gluon-Plasma (QGP)
To investigate this proposal we extend the existing Parton-Hadron-String Dynamics (PHSD) transport approach [52] to include essential facets of chiral symmetry restoration in terms of the Schwinger mechanism for string decay
The Parton-Hadron-String Dynamics (PHSD) transport approach [52, 53] is a microscopic covariant dynamical model for strongly interacting systems formulated on the basis of Kadanoff-Baym equations [59,60,61] for Green’s functions in phase-space representation
Summary
According to Quantum Chromo Dynamics (QCD) [1,2,3,4,5,6], matter changes its phase at high temperature and density and bound (colorless) hadrons dissolve to interacting (colored) quarks and gluons in the Quark-Gluon-Plasma (QGP). PHSD incorporates explicit partonic degrees-of-freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD (lQCD) as well as dynamical hadronization and hadronic elastic and inelastic collisions in the final reaction phase This approach has been tested for p + p, p + A and A + A collisions from the SPS to LHC energy regime [52,53,54,55]. The collective flow coefficients vn for the azimuthal angular distributions were found to be well in line with the PHSD calculations as well as the suppression of hard probes such as charm quarks at RHIC energies [56] In all these studies the question of chiral symmetry restoration has been discarded since the observables analyzed were driven by the dominant deconfinement transition and the parton dynamics in the QGP.
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