Jan Steinheimer

Although restoration of chiral symmetry is predicted by quantum chromodynamics to take place at high baryon density, most modeling of neutron star interiors disregards a chiral phase transition. We model neutron star cores with a parity doublet model, which allows for dynamical chiral symmetry restoration and predicts the appearance of the parity partners of nucleons and hyperons at large densities, as well as deconfined quark matter. We study the thermal evolution of neutron stars, focusing for the first time on the impact of Urca processes involving the parity partners in neutron star cooling simulations. We find that Urca processes for the parity partners of the nucleons significantly affect the thermal evolution of massive stars and allow for improved agreement with observed surface temperature and ages.

We present a comparison of the ultrarelativistic quantum molecular dynamics model using a chiral mean field (CMF) equation of state with flow and pion production data in heavy ion collisions at low beam energies E lab = 0.2 – 2.0 A GeV and varying isospin fraction. The CMF model parameters are constrained by known properties of the high density equation of state of quantum chromodynamics at varying isospin fractions. This allows us to calculate the equation of state as well as nuclear interactions for different physical systems like neutron stars and heavy ion collisions in a consistent way. It is found that heavy ion reactions at the upcoming FAIR facility will only have marginal sensitivity on the isospin dependence of the high-density equation of state. At lower beam energies, comparing to FOPI, HADES, and S π RIT data, the sensitivity is higher but the observed deviations and systematic uncertainties of the existing data are larger than the sensitivity to the symmetry energy.

We investigate dimuon production in the context of a first-order phase transition in QCD matter using a chiral fluid dynamics model. This approach incorporates nonequilibrium effects such as entropy production and reheating, which emerge during the dynamical evolution through a first-order phase transition. By comparing equilibrium and nonequilibrium scenarios across a range of beam energies ( s NN = 2.2 – 6.2 GeV), we analyze the resulting invariant mass spectra. Our results reveal a substantial enhancement of dilepton yields in the nonequilibrium scenario, particularly pronounced at lower beam energies, where reheating leads to a prolonged lifetime of the fireball and increased emission. The enhancement persists even after normalizing to pion multiplicities, indicating sensitivity beyond effects of entropy production.

Nuclear experiments near and below the threshold of hyperon production have shown that the production of Kaons is a sensitive probe for the dense QCD equation of state. At beam energies up to 1.5 AGeV, strangeness production can probe the equation of state for densities up to approximately twice nuclear saturation. In this paper, we will discuss the possibilities of extending this range in density by the study of multi-strange baryons as well as charmed hadrons in the SIS100 beam energy range up to 10 AGeV. Here, densities up to five times nuclear saturation can be reached and the production of multi-strange and charmed hadrons shows a strong sensitivity to the equation of state. On the other hand, a precise prediction of the effect of the equation of state will require knowledge of the fundamental production cross-section near the elementary production threshold in p + p collisions which is yet not measured for the hadrons discussed.