The symmetry energy in nuclear reactions

TL;DR

This paper investigates the symmetry energy in nuclear reactions at different densities, using experimental data from spectator fragmentation and heavy-ion collisions, and compares results with theoretical models to determine the density dependence of the symmetry term.

Contribution

It provides new experimental insights into the density dependence of the symmetry energy by analyzing spectator fragmentation and nucleon flow data, and compares these with model predictions.

Findings

Symmetry-term coefficient decreases with fragment multiplicity.

Moderately soft symmetry term with gamma ≈ 0.9 is favored.

Different density regimes probed by fragmentation and flow studies.

Abstract

New results for the strength of the symmetry energy are presented which illustrate the complementary aspects encountered in reactions probing nuclear densities below and above saturation. A systematic study of isotopic effects in spectator fragmentation was performed at the ALADIN spectrometer with 124Sn primary and 107Sn and 124La secondary beams of 600 MeV/nucleon incident energy. The analysis within the Statistical Fragmentation Model shows that the symmetry-term coefficient entering the liquid-drop description of the emerging fragments decreases significantly as the multiplicity of fragments and light particles from the disintegration of the produced spectator systems increases. Higher densities were probed in the FOPI/LAND study of nucleon and light-particle flows in central and mid-peripheral collisions of 197Au+197Au nuclei at 400 MeV/nucleon incident energy. From the comparison of the measured neutron and hydrogen squeeze-out ratios with predictions of the UrQMD model a moderately soft symmetry term with a density dependence of the potential term proportional to (rho/rho_0)^{gamma} with gamma = 0.9 +- 0.3 is favored.