H.H. Wolter

Relativistic mean field calculations with density-dependent meson-nucleon coupling

Abstract Nuclear matter and ground state properties for (proton and neutron) semi-closed shell nuclei are described in relativistic mean field theory with coupling constants which depend on the vector density. The parametrization of the density dependence for σ-, ω- and ρ-meson coupling is obtained by fitting to properties of nuclear matter and some finite nuclei. The results are compared to density-dependent coupling constants derived from self-energies of Dirac-Brueckner calculations of nuclear matter. The equation of state for symmetric and asymmetric nuclear matter is discussed. Finite nuclei are described in Hartree approximation, including a charge and a centre-of-mass correction. Pairing is considered in the BCS approximation. Special attention is directed to the predictions for properties at the neutron and proton driplines.

Constraints on the high-density nuclear equation of state from the phenomenology of compact stars and heavy-ion collisions

A new scheme for testing nuclear matter equations of state (EoSs) at high densities using constraints from neutron star (NS) phenomenology and a flow data analysis of heavy-ion collisions is suggested. An acceptable EoS shall not allow the direct Urca process to occur in NSs with masses below 1.5M� , and also shall not contradict flow and kaon production data of heavy-ion collisions. Compact star constraints include the mass

The Bare Astrophysical S(E) Factor of the 7Li(p, α)α Reaction

The astrophysically important 7Li(p, α)α reaction has been studied via the Trojan horse method in the energy range E = 10-400 keV. A new theoretical description, based on the distorted-wave Born approximation approach, allows one to extract information on the bare astrophysical S-factor, Sb(E), with Sb(0) = 55 ± 3 keV barns. The results are compared with direct experimental data leading to a model-independent value of the electron screening potential energy, Ue = 330 ± 40 eV, much higher than the adiabatic limit Uad = 175 eV.

On the Lorentz structure of the symmetry energy

Abstract We investigate in detail the density dependence of the symmetry energy in a relativistic description by decomposing the isovector mean field into contributions with different Lorentz covariant properties. We find important effects of the isovector, scalar channel (i.e., δ-meson like) on the high density behavior of the symmetry energy. Applications to static properties of finite nuclei and to dynamic situations of heavy ion collisions are explored and related to each other. The nuclear structure studies show only moderate effects originating from the virtual δ meson. At variance, in heavy ion collisions one finds important contributions on the reaction dynamics arising from the different Lorentz structure of the high density symmetry energy when a scalar isovector δ field is introduced. Particularly interesting is the related neutron/proton effective mass splitting for nucleon transport effects and for resonance and particle production around the threshold. We show that the δ-like channel turns out to be essential for the production of pions, when comparing with experimental data, in particular for high momentum selections.

A way forward in the study of the symmetry energy: experiment, theory, and observation

The symmetry energy describes how the energy of nuclear matter rises as one goes away from equal numbers of neutrons and protons. This is very important to describe neutron rich matter in astrophysics. This article reviews our knowledge of the symmetry energy from theoretical calculations, nuclear structure measurements, heavy-ion collisions, and astronomical observations. We then present a roadmap to make progress in areas of relevance to the symmetry energy that promotes collaboration between the astrophysics and the nuclear physics communities.

Symmetry Energy of Dilute Warm Nuclear Matter

The symmetry energy of nuclear matter is a fundamental ingredient in the investigation of exotic nuclei, heavy-ion collisions, and astrophysical phenomena. New data from heavy-ion collisions can be used to extract the free symmetry energy and the internal symmetry energy at subsaturation densities and temperatures below 10 MeV. Conventional theoretical calculations of the symmetry energy based on mean-field approaches fail to give the correct low-temperature, low-density limit that is governed by correlations, in particular, by the appearance of bound states. A recently developed quantum-statistical approach that takes the formation of clusters into account predicts symmetry energies that are in very good agreement with the experimental data. A consistent description of the symmetry energy is given that joins the correct low-density limit with quasiparticle approaches valid near the saturation density.

Fluctuations and dynamical instabilities in heavy-ion reactions

Abstract We present a new method to implement fluctuations in the mean-field dynamics. Assuming local thermal equilibrium, we calculate the fluctuation amplitude according to the statistical value and we determine the associated density variance. The method can be applied to study the role of fluctuations in both stable and unstable dynamics. While in stable cases fluctuations are only responsible for the width of the distributions around mean values, their role become crucial in unstable situations, where bifurcations occur and different reaction exit channels are possible. We illustrate this point by describing the break-up of di-nuclear systems exhibiting neck instabilities.

Modelization of the EOS

Abstract.This paper summarizes theoretical predictions for the density and isospin dependence of the nuclear mean field and the corresponding nuclear equation of state. We compare predictions from microscopic and phenomenological approaches. An application to heavy-ion reactions requires to incorporate these forces into the framework of dynamical transport models. Constraints on the nuclear equation of state derived from finite nuclei and from heavy-ion reactions are discussed.

Relativistic effects in the search for high density symmetry energy

Abstract Intermediate energy heavy ion collisions open the unique possibility to explore the equation of state (EOS) of nuclear matter far from saturation, in particular the density dependence of the symmetry energy. Within a relativistic transport model it is shown that the isovector–scalar δ meson, which affects the high density behavior of the symmetry energy density, influences the dynamics of heavy ion collisions in terms of isospin collective flows. The effect is largely enhanced by a relativistic mechanism related to the covariant nature of the fields contributing to the isovector channel. Results for reactions induced by 132 Sn radioactive beams are presented. The elliptic flows of nucleons and light isobars appear to be quite sensitive to microscopic structure of the symmetry term, in particular for particles with large transverse momenta, since they represent an earlier emission from a compressed source. Thus future, more exclusive, experiments with relativistic radioactive beams should be able to set stringent constraints on the density dependence of the symmetry energy far from ground state nuclear matter.

HFB calculations with T = 1 and T = 0 pairing correlations

Abstract In HFB calculations the proton-neutron pairing with T = 0 and T = 1 should be included in addition to the normal pairing between like particles. It is analysed, how one has to choose the quasiparticle transformation to achieve this, and calculations are performed for some doubly even nuclei in the 2s-1d and 2p-1f shells using reaction matrix elements based on a realistic nucleon-nucleon interaction. In N = Z nuclei T = 0 pairing is strong in excited HF states and may lead to a reordering or a near degeneracy of states of different deformation. In N ≠ Z nuclei T = 0 pairing is found only for a small neutron excess, when the HF gaps for both protons and neutrons are small. In these cases N = 0 and T = 1 pairing are about equally strong.