Chiral symmetry restoration in heavy-ion collisions at intermediate energies

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range root 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 the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the nonlinear sigma-omega 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 Sigma term we adopt Sigma(pi) approximate to 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 root s(NN) = 3-20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the nonstrange 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 so-called horn structure in the excitation function of the K+/pi(+) ratio: The CSR in the hadronic phase produces the steep increase of this particle ratio up to root s(NN) approximate to 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance and disappearance of the horn-structure are investigated as functions of the system size and collision centrality. We close this work by an analysis of strangeness production in the (T, mu(B)) plane (as extracted from the PHSD for central Au+Au collisions) and discuss the possibilities to identify a possible critical point in the phase diagram.