Collective Motion and the Asymmetric-Matter Equation-of-State

Abstract

The asymmetric-matter equation-of-state is a topic of highest current interest in nuclear structure and reactions as well as in astrophysics. The importance of the symmetry energy, i.e. of the difference between the energy densities of neutron matter and of symmetric nuclear matter, for many nuclear phenomena implies that a variety of observables can be used for investigating its strength. Most of them, however, probe densities below saturation. Higher densities can be accessed with heavy-ion reactions at sufficiently high energies. The observed collective flow of particles and fragments has appeared as a useful probe of the potential strengths during the high-density phase. Recent measurements of the elliptic-flow ratio of neutrons with respect to light charged particles at the GSI laboratory have led to a new constraint for the symmetry energy at suprasaturation density. It confirms, with higher precision, the moderately soft to linear density dependence of the symmetry energy previously deduced from the FOPI-LAND measurements. New calculations demonstrate the opportunities offered by future experiments at FAIR.