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.

Triton-He-3 relative and differential flows as probes of the nuclear symmetry energy at supra-saturation densities

Using a transport model coupled with a phase-space coalescence afterburner, we study the triton-He-3 (t-He-3) ratio with both relative and differential transverse flows in semicentral Sn-132 + Sn-124 reactions at a beam energy of 400 MeV/nucleon. The neutron-proton ratios with relative and differential flows are also discussed as a reference. We find that similar to the neutron-proton pairs, the t-He-3 pairs also carry interesting information regarding the density dependence of the nuclear symmetry energy. Moreover, the nuclear symmetry energy affects more strongly the t-He-3 relative and differential flows than the pi(-)/pi(+) ratio in the same reaction. The t-He-3 relative flow can be used as a particularly powerful probe of the high-density behavior of the nuclear symmetry energy.

Nuclear matter at a HIRFL-CSR energy regime

We report some recent progress in constraining the symmetry energy E-sym(rho) at high densities using high-energy heavy-ion collisions. Circumstantial evidence of a soft E-sym(rho) at supra-saturation density is obtained by comparing the pion ratio pi(-)/pi(+) measured recently with FOPI at GSI and the IBUU04 model calculations. Detailed studies indicate that the power of determining the E-sym(rho)from pi(-)/pi(+) is enhanced with decreasing the beam energy to near the pion production threshold, showing a correlation to the increasing nuclear stopping. Among several heavy-ion reaction facilities in the world, the cooling storage ring (HIRFL-CSR), newly commissioned at Lanzhou, delivering heavy-ion beams up to 1 A GeV, to be coupled with advanced detectors will contribute significantly to further studies of the equation of state of asymmetric nuclear matter.

Superfluidity in dense nuclear matter

We discuss the onset of superfluidity in neutron stars, where the model of nuclear matter is realized in a high-density and asymmetry state. In particular, we present the study of the effects of microscopic three-body forces on the proton pairing in the 1S0 channel and neutron pairing in 3PF1 channel for β-stable neutron star matter. It is found that the main effects of three-body forces are to shrink the domain of existence of the 1S0 below the threshold of the direct URCA process and to stretch the density range of the 3PF1 pairing in a broad domain so to cover most part of the neutron-star core.