We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range √ sNN =3-20 GeV 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 √ sNN =3-20 GeV, 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 √ sNN ≈ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the 'horn'-structure is investigated as a function of the system size and collision centrality. We close this work by an analysis of strangeness production in the (T, µB)-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.PACS numbers: 25.75. Nq, 25.75.Ld, 24.85.+p, 12.38.Mh † In this work we adopt natural units, henceh = c = 1.

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 + /π + and the (Λ + Σ 0 )/π − 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 Σ-term in the computation of the quark scalar condensate < qq > that serves as an order parameter for CSR and also scales approximately with the effective quark masses ms and mq. Furthermore, the nucleon scalar density ρs, which also enters the computation of < qq >, is evaluated within the nonlinear σ − ω 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 ∼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 + /π + 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.

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We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range √ s NN = 3-20 GeV 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 particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at √ s NN = 3-20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. 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 √ s NN ≈ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the horn structure is investigated as a function of the system size. We additionally present an analysis of strangeness production in the (T, µ B )-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.