Circumstantial Evidence for a Soft Nuclear Symmetry Energy at Suprasaturation Densities

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 , Nucl. Phys. A 781, 459 (2007)] provide circumstantial evidence suggesting a rather soft nuclear symmetry energy E-sym(rho) at rho >= 2 rho(0) compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.

National Natural Science Foundation of China 10675148 10575071 10675082 MOE of China NCET-05-0392 Shanghai Rising-Star Program 06QA14024 SRF for ROCS SEM of China National Basic Research Program of China (973 Program) 2007CB815004 US National Science Foundation PHY-0652548 PHY-0757839 Research Corporation 7123 Texas Coordinating Board of Higher Education 003565-0004-2007