Marcus Bleicher

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.