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, Nucl. Phys. A 781, 459 (2007)10.1016/j.nuclphysa.2006.10.085] provide circumstantial evidence suggesting a rather soft nuclear symmetry energy E_{sym}(rho) at rho> or =2rho_{0} compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.

Neutron skin thickness in the droplet model with surface width dependence: Indications of softness of the nuclear symmetry energy

“…Recent studies of pure neutron matter at low densities based on universal properties of dilute Fermi gases lead to a similar conclusion [69,70]. One may mention that there exists recent circumstantial evidence [71], derived from π − /π + ratios in central HIC collisions at Ref. [29] n-p emission ratios [20] isoscaling [17][18][19] isospin diffusion [12][13][14][15][16] Thomas-Fermi model [67,68] GDR [25] PDR [24] SIS/GSI energies, hinting at that the nuclear symmetry energy is soft also in the region of suprasaturation densities (ρ 2ρ 0 ).…”

“…[29] n-p emission ratios [20] isoscaling [17][18][19] isospin diffusion [12][13][14][15][16] Thomas-Fermi model [67,68] GDR [25] PDR [24] SIS/GSI energies, hinting at that the nuclear symmetry energy is soft also in the region of suprasaturation densities (ρ 2ρ 0 ). However, further experimental and theoretical confirmations of this fact need to be done [71].…”

Effects of nuclear symmetry energy on η meson production and its rare decay to the dark U-boson in heavy-ion reactions

“…It is seen that our calculations give a reasonable description of the data with x = 0 and 1 while the stiff nuclear symmetry energy with x= -2 leads to a significantly larger number of η mesons. to 1 GeV/nucleon favor the symmetry energy with x=1 [50]. Because η is sensitive to the number of p-n collisions while the π − /π + ratio is determined by the ratio of n-n and p-p colliding pairs, η and the π − /π + ratio provide complementary information about the symmetry energy.…”

“…In fact, findings about the high-density E sym (ρ) from comparing heavyion reaction data with various transport model calculations are still inconsistent, see, e.g., refs. [50][51][52][53]. This is at least partially because of both the uncertainties in the physics inputs and the different numerical techniques in initializing and modeling the transport process of colliding nuclei, see, e.g., refs.…”

Probing the momentum dependence of the symmetry potential by the freen/pratio of pre-equilibrium emission

“…Nowadays, the density dependence of nuclear symmetry energy/potential is still not well known especially at suprasaturation densities [12] and high momentum [13]. Few progress has been made until recently [14]. Also reducing the uncertainties on the constraints of nuclear symmetry energy is of critical importance and remains a big challenge.…”