The symmetry energy in heavy-ion reactions

The symmetry energy appears in different roles in reaction studies presently performed in many laboratories. Its decreasing strength at sub-saturation densities permits the identification of low-density phenomena at late reaction stages. The less well known behavior at higher-than-normal densities, on the other hand, asks for suitable probes to determine the equation of state of asymmetric nuclear matter. These two complementary aspects will be illustrated with data from experiments conducted at the GSI laboratory. A systematic study of isotopic effects in spectator fragmentation was performed at the Aladin spectrometer with secondary beams of 600 A MeV. The analysis within the Statistical Fragmentation Model reveals that the symmetry-term coefficient needed for the liquid-drop description of the emerging fragments decreases as the disintegration of the spectator system into fragments and light particles increases. Higher densities are probed in more central collisions, and isotopic flows have been proposed as observables sensitive to the strength of the symmetry term. The differential elliptic flows, in particular, have appeared as useful probes from a study of 197Au+197Au collisions at 400 A MeV with the UrQMD model. The preliminary results obtained from a comparison with experimental data of the FOPI/LAND collaboration favor a moderately soft symmetry term with a density dependence of the potential term proportional to (ρ/ρ0)γ with γ=0.9±0.3.