F. Matera

Fragmentation path of excited nuclear systems

Abstract 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 129Xe + 119Sn 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.

Fock exchange terms in nonlinear quantum hadrodynamics

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.

Influence of vector interactions on the hadron-quark/gluon phase transition

The hadron-quark/gluon phase transition is studied in the two-phase model. As a further study of our previous work, both the isoscalar and isovector-vector interactions are included in the Polyakov loop modified Nambu-Jona-Lasinio model for the quark phase. The relevance of the exchange (Fock) terms is stressed and suitably accounted for. The calculation shows that the isovector-vector interaction delays the phase transition to higher densities and the range of the mixed phase correspondingly shrinks. Meanwhile, the asymmetry parameter of quark matter in the mixed phase decreases with the strengthening of this interaction channel. This leads to some possible observation signals being weakened, although still present. We show that these can be rather general effects of a repulsion in the quark phase due to the symmetry energy. This is also confirmed by a simpler calculation with the MIT-bag model. However, the asymmetry parameter of quark matter is slightly enhanced with the inclusion of the isoscalar-vector interaction, but the phase transition will be moved to higher densities. The largest uncertainty on the phase transition lies in the undetermined coupling constants of the vector interactions. In this respect, new data on the mixed phase obtained from heavy-ion collisions at intermediate energies appear very important.

Self-consistency and search for collective effects in semiclassical pairing theory

Abstract A simple model, in which nuclei are represented as homogeneous spheres of symmetric nuclear matter, is used to study the effects of a self-consistent pairing interaction on the isoscalar nuclear response. Effects due to the finite size of nuclei are suitably taken into account. The semiclassical equations of motion derived in a previous paper for the time-dependent Hartree–Fock–Bogoliubov problem are solved in an improved (linear) approximation in which the pairing field is allowed to oscillate and to become complex. The new solutions are in good agreement with the old ones and also with the result of well-known quantum approaches. The role of the Pauli principle in eliminating one possible set of solutions is also discussed. The density response function is explicitly evaluated and it is shown that the energy-weighted sum rule is restored to its correct value by a part of the fluctuations of the imaginary pairing field. The remaining part of these imaginary fluctuations, together with the fluctuations of the real part, could give rise to collective excitations in the density response function. A detailed analysis of the monopole and quadrupole strength functions shows that there are practically no collective effects in these channels at low excitation energy.

Kinetic equation for finite systems of fermions with pairing

Abstract The solutions of the Wigner-transformed time-dependent Hartree–Fock–Bogoliubov equations are studied in the constant-Δ approximation. This approximation is known to violate particle-number conservation. As a consequence, the density fluctuation and the longitudinal response function given by this approximation contain spurious contributions. A simple prescription for restoring both local and global particle-number conservation is proposed. Explicit expressions for the eigenfrequencies of the correlated systems and for the density response function are derived and it is shown that the semiclassical analogous of the quantum single-particle spectrum has an excitation gap of 2Δ, in agreement with the quantum result. The collective response is studied for a simplified form of the residual interaction.

Effects of self-consistency in semiclassical pairing theory

An extended Vlasov equation, derived from the time-dependent Hartree–Fock–Bogoliubov equation of motion, is used to study the effects of pairing on the nuclear density-density linear response function in semiclassical approximation. Within the approximations adopted here, the fluctuations of the pairing field are purely imaginary and play a crucial role in eliminating a spurious mode that gives fluctuations of the number of particle and in reproducing the correct value of the energy-weighted sum rule. The semiclassical approach seems to have some difficulty in describing pairing vibrations. Further work on this problem is needed.

INSTABILITIES IN A MEAN-FIELD DYNAMICS OF ASYMMETRIC NUCLEAR MATTER

We discuss the features of instabilities in asymmetric nuclear matter, in particular the relation between the nature of fluctuations, the types of instabilities and the properties of the interaction. We show a chemical instability appears as an instability against isoscalar-like fluctuations. Then starting from phenomenological hadronic field theory (QHD), including exchange terms, we discuss the symmetry energy and the relation to the dynamical response inside the spinodal region.