Isospin Physics in Heavy-Ion Collisions at Intermediate Energies

Abstract

In nuclear collisions induced by stable or radioactive neutron-rich nuclei a transient state of nuclear matter with an appreciable isospin asymmetry as well as thermal and compressional excitation can be created. This offers the possibility to study the properties of nuclear matter in the region between symmetric nuclear matter and pure neutron matter. In this review, we discuss recent theoretical studies of the equation of state of isospin-asymmetric nuclear matter and its relations to the properties of neutron stars and radioactive nuclei. Chemical and mechanical instabilities as well as the liquid-gas phase transition in asymmetric nuclear matter are investigated. The in-medium nucleon-nucleon cross sections at different isospin states are reviewed as they affect significantly the dynamics of heavy ion collisions induced by radioactive beams. We then discuss an isospin-dependent transport model, which includes different mean-field potentials and cross sections for the proton and neutron, and its application to these reactions. Furthermore, we review the comparisons between theoretical predictions and available experimental data. In particular, we discuss the study of nuclear stopping in terms of isospin equilibration, the dependence of nuclear collective flow and balance energy on the isospin-dependent nuclear equation of state and cross sections, the isospin dependence of total nuclear reaction cross sections, and the role of isospin in preequilibrium nucleon emissions and subthreshold pion production.

Double neutron-proton differential transverse flow as a probe for the high density behavior of the nuclear symmetry energy

“…Heavy-ion reactions induced by neutron-rich nuclei, especially radioactive beams, provide a unique opportunity to constrain the equation of state (EOS) of asymmetric nuclear matter [3,4,5]. Though considerable progress has been made recently in determining the density dependence of the nuclear symmetry energy around the normal nuclear matter density from studying the isospin diffusion in heavy-ion reactions at intermediate energies [6,7,8], much more work is still needed to probe the high-density behavior of the nuclear symmetry energy.…”

Hot nuclear matter equation of state with a three-body force

“…Theoretical simulations [1] indicate that the main reaction dynamics and observables including collective flow, balance energy, isospin equilibrium, pre-equilibrium nucleon emission, isotopic scaling and isospin diffusion are quite sensitive to the isospin dependent part of the nuclear EOS, especially to the density dependence of the symmetry energy.…”

Alpha-cluster model of atomic nuclei

“…The first two terms in (6) are just a standard, 6-parameter form of the EoS approximation commonly used in the present-day experimental and theoretical studies, see, e.g., [19]. The first term describes the symmetric matter in a balanced system (zero isospin and spin), with K 0 being the compressibility of the symmetric nuclear matter.…”

“…As later on we will be searching for the ground-state configurations of the even-even nuclei with N = Z, it can be assumed that the average momenta of the wave packets p k are equal to zero and, thus, the first term in the expression (14) can be neglected. Following a standard approach (see, e.g., [19]) it is assumed that the parametrization of the EoS includes implicitly the kinetic contribution coming from the Fermi motion. As has been demonstrated in [18], the adopted cubic approximation of the EoS is flexible enough to reproduce typical predictions of sophisticated models of the nuclear matter which explicitly take into account the kinetic contribution resulting from the Fermi motion.…”