Circumstantial evidence for a soft nuclear symmetry energy at supra-saturation 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 {\it et al.}, NPA {\bf 781}, 459 (2007)) provide circumstantial evidence suggesting a rather soft nuclear symmetry energy \esym 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.

Comments: Version to appear in Phys. Rev. Lett