Abstract
The ASY-EOS experiment at GSI laboratory measured the direct and elliptic flow of neutrons and light charged particles in the reaction 197Au+197 Au at 400 A MeV incident energy. The ratio of elliptic flow of neutrons with respect to that of the light charged particles was used as main experimental observable to probe the density dependence of the symmetry energy term of the nuclear equation of state. Results, obtained by comparison of the experimental data with the UrQMD model predictions, strongly support a moderately soft to linear density dependence of the symmetry energy at suprasaturation densities below 2ρ0.
Highlights
The study of the equation of state EOS(ρ, T, β) in neutron rich matter, where β =/ρ is the isospin asymmetry as a function of neutrons, protons and nuclear matter density respectively, provides fundamental understanding in many aspects of nuclear physics and astrophysics phenomena [1]
Flow parameters were already studied in the past for the 197Au+197Au at 400 A MeV by using the FOPI detector coupled with the LAND neutron array [10]
There are yet different open problems, questions and uncertainties related to the high density behavior of the symmetry energy [25]
Summary
Heavy ion collisions provide unique experimental tools to explore densities larger than the nuclear saturation one in controlled laboratory conditions. The FOPI data on π−/π+ ratio has been used to probe the symmetry term of EOS at suprasaturation density, with contradicting results in the model description of data [8, 9]. A recent re-analysis of these data, despite their low statistical precision, showed that the ratio of the elliptic flow parameters of neutrons with respect to hydrogen isotopes was an observable suitable to determine the strength of the symmetry energy by comparison of the experimental data with the results of the UrQMD transport model [11] or the Tübingen QMD model [12]
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