Abstract
The effect of the chiral symmetry restoration (CSR) on observables from heavy-ion collisions is studied in the energy range [see formula in PDF]=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 ∑-term we adopt ∑π ≈ 45 MeV which corresponds to a ’world average’. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at [see formula in PDF]=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 ones. 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 [see formula in PDF] ≈ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium.
Highlights
The strange particle production has always been suggested as one of the most sensitive observables in heavy-ion collisions (HIC) that could indicate the creation of a Quark-Gluon-Plasma (QGP) during the early stages of the reaction
We point out that our Parton-Hadron-String Dynamics (PHSD) calculations provide a microscopic interpretation of the "horn" structure in the excitation function of the K+/π+ ratio in central Au+Au collisions
The steep rise of this ratio at Alternating-Gradient Synchrotron (AGS) energies is associated to chiral symmetry restoration (CSR) in the hadronic phase, while the drop at higher SPS energies is due to the appearance of the QGP phase in an increasing volume of the interaction region
Summary
The strange particle production has always been suggested as one of the most sensitive observables in heavy-ion collisions (HIC) that could indicate the creation of a Quark-Gluon-Plasma (QGP) during the early stages of the reaction. Lattice QCD (lQCD) calculations show that at vanishing baryon chemical potential (μB=0) the CSR takes place at roughly the same critical temperature and energy density as the deconfinement phase transition which is a crossover. At finite baryon chemical potential lQCD calculations cannot be performed due to the sign problem and one must rely on effective models (or extrapolations) in order to study the QCD phase transitions [12,13,14]. The strangeness production at Alternating-Gradient Synchrotron (AGS) and lower SPS energies was suggested to be a signature of CSR [9]. In this contribution we will report on the main results from Refs. In this contribution we will report on the main results from Refs. [9,10,11] where a systematic study has been performed on effects of the CSR on final particle distributions within the PHSD approach
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