Nuclear Matter Properties at High Densities: Squeezing Out Nuclear Matter Properties from Experimental Data

Abstract

The nuclear equation of state is a topic of highest current interest in nuclear physics and astrophysics. The nuclear equation of state governs the evolution of heavy-ion reactions as well as the characteristics of compact stellar objects like neutron stars, the explosions of supernovae, and the merging of two neutron stars. The symmetry energy is the part of the equation of state which is connected to the asymmetry in the neutron/proton content. During recent years a multitude of experimental and theoretical efforts on different fields have been undertaken to constraint its density dependence at low densities but also above saturation density (\(\rho _0 = 0.16 \mathrm{fm}^{-3}\)). Conventionally, the symmetry energy is described by its magnitude \(S_v\) and the slope parameter L, both at saturation density. Values of \(L \approx \) 44–66 MeV and \(S_v \approx \) 31–33 MeV have been deduced in recent compilations of nuclear structure, heavy-ion reaction, and astrophysics data. Apart from astrophysical data on mass and radii of neutron stars and the gravitational wave signal of neutron star mergers, heavy-ion reactions above incident energies of several 100 MeV are the only means to access the high-density behavior of the symmetry energy. In particular, meson production and collective flows up to about 1 GeV/nucleon are predicted to be sensitive to the slope of the symmetry energy as a function of density. From the measurement of elliptic flow of neutrons with respect to charged particles at GSI, a stringent constraint for the slope of the symmetry energy at supra-saturation densities has been deduced. Future options to reach even higher densities will be discussed.