Circumstantial Evidence for a Soft Nuclear Symmetry Energy at Suprasaturation Densities

Abstract

Within an isospin- and momentum-dependent hadronic transport model, it is shown that the recent FOPI data on the ${\pi }^{-}/{\pi }^{+}$ ratio in central heavy-ion collisions at SIS/GSI energies [Willy Reisdorf et al., Nucl. Phys. A 781, 459 (2007)] provide circumstantial evidence suggesting a rather soft nuclear symmetry energy $E_{\mathrm{sym}}(\rho )$ at $\rho \ge 2{\rho }_0$ compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.