Results of the ASY-EOS experiment at GSI: The symmetry energy at suprasaturation density

TL;DR

This paper reports experimental measurements of neutron and light charged particle flows in gold-gold collisions at high energy, providing new constraints on the density dependence of nuclear symmetry energy at supra-saturation densities.

Contribution

It presents the first detailed experimental constraint on the symmetry energy at densities beyond twice saturation, using flow measurements and comparison with transport model predictions.

Findings

The symmetry energy density dependence parameter bb = 0.72 b1 0.19.

Densities reached exceed twice the nuclear saturation density.

Results confirm a moderately soft to linear symmetry energy at high densities.

Abstract

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation.