No abstract

We propose a method to introduce Fock term contributions in relativistic models of fermions coupled to mesons, including self-interactions for the mesonic fields. We show that effects on equilibrium properties and on the dynamical response of the fermionic system can be consistently accounted for. Some implications on equilibrium properties of asymmetric nuclear matter are discussed. In particular an indication is emerging for a reduced contribution of charged mesons to the symmetry term of the nuclear equation of state around normal density.

We perform a study of the fragmentation path of excited nuclear sources, within the framework of a stochastic mean-field approach. We consider the reaction 129 Xe + 119 Sn at two beam energies: 32 and 50 MeV/A, for central collisions. It is observed that, after the compression phase the system expands towards a dilute configuration from which it may recontract or evolve into a bubble-like structure. Then fragments are formed through the development of volume and/or surface instabilities. The two possibilities co-exist at 32 MeV/A, leading to quite different fragment partitions, while at 50 MeV/A the hollow configuration is observed in all events. Large variances are recovered in a way fully consistent with the presence of spinodal decomposition remnants. Kinematical properties of fragments are discussed and suggested as observables very sensitive to the dominant fragment production mechanism. A larger radial collective flow is observed at 50 MeV/A, in agreement with experiments.

The Equation of State (EOS) for asymmetric nuclear matter is discussed starting from a phenomenological hadronic field theory of Serot-Walecka type including exchange terms. In a model with self interactions of the scalar sigma-meson we show that the Fock terms naturally lead to isospin effects in the nuclear EOS. These effects are quite large and dominate over the contribution due to isovector mesons. We obtain a potential symmetry term of "stiff" type, i.e. increasing with baryon density and an interesting behaviour of neutron/proton effective masses of relevance for transport properties of asymmetric dense matter.Phenomenological hadronic field theories (Quantum Hadrodynamics, QHD) are widely used in dense nuclear matter studies since relativistic effects are expected to increase with baryon density [1]. In most of the previous works on the subject, the Relativistic Mean Field (RMF ) approximation of QHD has been followed. In the RMF the meson fields are treated as classical fields and consequently a Hartree reduction of one body density matrices is used.Although the model has driven a large amount of results on relativistic effects in nuclear structure and dynamics [2-6], the lack of exchange terms has implied some non satisfying features of the theory and some efforts have been done to try to cure this problem [7][8][9][10]. In the RMF theory each meson field is introduced, with appropriated readjusted couplings, just to describe the dynamics of a corresponding degree of freedom, without mixing due to many-body effects. Neutral σ and ω mesons are in charge of saturation properties, isospin effects are carried by isovector δ [a 0 (980)] and ρ mesons and finally spin terms are coming from pseudoscalar π and η fields. In a sense the model represents a straightforward extension of the One-Boson-Exchange (OBE) description of nucleon-nucleon scattering.The aim of this letter is to introduce explicit many-body effects just evaluating exchange term contributions. We will get qualitative new features of equilibrium (EOS) and dynamical properties of asymmetric nuclear matter. In particular a new density dependence of the symmetry term is expected, at variance with the simple linear increase predicted by the ρ−exchange mechanism in the Hartree scheme.