Symmetry energy from elliptic flow in Au + Au

The elliptic-flow ratio of neutrons with respect to protons or light complex particles in reactions of neutron-rich systems at relativistic energies is proposed as an observable sensitive to the strength of the symmetry term in the equation of state at supra-normal densities. The results obtained from the existing FOPI/LAND data for ¹⁹⁷Au + ¹⁹⁷Au collisions at 400 MeV/nucleon in comparison with the UrQMD model favor a moderately soft symmetry term with a density dependence of the potential term proportional to γ(ρ/ρ₀)${(\rho /{\rho }_0)}^{\gamma }$ with γ=0.9±0.4$\gamma =0.9\pm 0.4$.