Fragment Formation in Central Heavy Ion Collisions at Relativistic Energies

We perform a systematic study of the fragmentation path of excited nuclear matter in central heavy ion collisions at the intermediate energy of $0.4 AGeV$. The theoretical calculations are based on a Relativistic Boltzmann-Uehling-Uhlenbeck ($RBUU$) transport equation including stochastic effects. A Relativistic Mean Field ($RMF$) approach is used, based on a non-linear Lagrangian, with coupling constants tuned to reproduce the high density results of calculations with correlations. At variance with the case at Fermi energies, a new fast clusterization mechanism is revealed in the early compression stage of the reaction dynamics. Fragments appear directly produced from phase-space fluctuations due to two-body correlations. In-medium effects of the elastic nucleon-nucleon cross sections on the fragmentation dynamics are particularly discussed. The subsequent evolution of the primordial clusters is treated using a simple phenomenological phase space coalescence algorithm. The reliability of the approach, formation and recognition, is investigated in detail by comparing fragment momentum space distributions {\it and simultaneously} their yields with recent experimental data of the $FOPI$ collaboration by varying the system size of the colliding system, i.e. its compressional energy (pressure, radial flow). We find an excellent agreement between theory and experiment in almost all the cases and, on the other hand, some limitations of the simple coalescence model. Furthermore, the temporal evolution of the fragment structure is explored with a clear evidence of an earlier formation of the heavier clusters, that will appear as interesting $relics$ of the high density phase of the nuclear Equation of State ($EoS$).Comment: 21 pages, 8 figures, Latex Elsart Style, minor corrections in p.7, two refs. added, Nucl.Phys.A, accepte

Isospin effects on sub-threshold kaon production at intermediate energies

We show that in collisions with neutron rich heavy ions at energies around the production threshold K^0 and K^+ yields might probe the isospin dependent part of the nuclearEquation of State (EoS) at high baryon densities. In particular we suggest the K^0/K^+ ratio as a promising observable. Results obtained in a fully covariant relativistic transport approach are presented for central Au+Au collisions in the beam energy range 0.8-1.8~AGeV. The focus is put on the EoS influence which goes beyond the "collision-cascade" picture. The isovector part of the in-medium interaction affects the kaon multiplicities via two mechanisms: i) a "symmetry potential" effect, i.e. a larger neutron repulsion in n-rich systems (isospin fractionation); ii) a "threshold" effect, due to the change in the self-energies of the particles involved in inelastic processes. Genuine relativistic contributions are revealed, that could allow to directly ``measure'' the Lorentz structure of the effective isovector interaction.Comment: 5 pages, 2 figures, revtex

Temperature and thermodynamic instabilities in heavy ion collisions

We investigate thermodynamic properties and instability conditions in intermediate energy heavy ion reactions. We define locally thermodynamic variables, i.e. density, pressure and temperature, directly from the phase space distribution of a relativistic transport calculation. In particular, temperatures are determined by a fit to two covariant hot Fermi distributions thus taking into account possible anisotropic momentum configurations. We define instability independent from the nuclear matter spinodal by the criterion that the effective compressibility becomes negative. The method is applied to a semi-central Au on Au reaction at 600 MeV/nucleon. We investigate in particular the center of the participant and the spectator matter. In the latter we find a clear indication of instability with conditions of density and temperature that are consistent with experimental determinations.Comment: 20 pages latex, 5 PS-figures, revised version (minor changes) accepted for publication in Nucl. Phys.

Dynamics of Phase Transitions in Asymmetric Nuclear Matter

We present several possibilities offered by the reaction dynamics of dissipative heavy ion collisions to study in detail the symmetry term of the nuclear equation of state, $EOS$. In particular we discuss isospin effects on the nuclear liquid-gas phase transition, {\it Isospin Distillation}, and on collective flows. We stress the importance of a microscopic relativistic structure of the effective interaction in the isovector channel. The possibility of an {\it early} transition to deconfined matter in high isospin density regions is also suggested. We finally select {\it Eleven} observables, in different beam energy regions, that appear rather sensitive to the isovector part of the nuclear $EOS$, in particular in more exclusive experiments.Comment: 8 pages, 7 figures, ISPUN02 Conference, Halong-Vietnam, Nov.20-25 2002, to appear in Nucl.Phys.A. Elsevier Proceedings Styl

Studio di potenziali acquiferi mediante indagini geoelettriche nel villaggio di itigi (Singida - Tanzania)

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Constraining the Symmetry Energy: A Journey in the Isospin Physics from Coulomb Barrier to Deconfinement

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this work we present a selection of reaction observables in dissipative collisions particularly sensitive to the isovector part of the interaction, i.e. to the symmetry term of the nuclear Equation of State (EoS). At low energies the behavior of the symmetry energy around saturation influences dissipation and fragment production mechanisms. We will first discuss the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation during the charge equilibration in fusion and deep-inelastic collisions. Important Iso-EOS effects are stressed. Reactions induced by unstable 132Sn beams appear to be very promising tools to test the sub-saturation Isovector EoS. New Isospin sensitive observables are also presented for deep-inelastic, fragmentation collisions and Isospin equilibration measurements (Imbalance Ratios). The high density symmetry term can be derived from isospin effects on heavy ion reactions at relativistic energies (few AGeV range), that can even allow a ``direct'' study of the covariant structure of the isovector interaction in the hadron medium. Rather sensitive observables are proposed from collective flows and from pion/kaon production. The possibility of the transition to a mixed hadron-quark phase, at high baryon and isospin density, is finally suggested. Some signatures could come from an expected ``neutron trapping'' effect. The importance of studying violent collisions with radioactive beams from low to relativistic energies is finally stressed.Comment: 15 pages, 5 figures, Int.Workshop on Nuclear Dynamics in Heavy Ion Reactions and Neutron Stars, Beijing Normal Univ. July 07, to appear in Int.Journ.Modern Physics E (2008

Wave Speed Measurements in Non-Ideal Compressible Flows Using the Flexible Asymmetric Shock Tube (FAST)

Non-ideal compressible fluid dynamics (NICFD) are defined as compressible fluid flows occurring in the dense vapour, dense vapour-liquid equilibrium or supercritical thermodynamic region. This type of flow can occur in expanders of organic Rankine cycle power plants. In order to study NICFD, a Ludwieg tube-type facility has been designed and constructed at Delft University of Technology. A large variety of fluids can be employed in the facility, but for this study D6 siloxane is chosen as working fluid due to its high thermal stability and the possibility of encountering non-classical gasdynamic phenomena. This compound belongs to the siloxane class, which are also used as working fluids in ORC power systems. Gasdynamic experiments within the NICFD region are presented from which the wave speed and speed of sound can be inferred using the time-of-flight technique. These data can be used to improve and validate thermodynamic models

Isospin Dynamics in Heavy Ion Collisions: EoS-sensitive Observables

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation and at high nucleon momenta. In this report we present a selection of reaction observables particularly sensitive to the isovector part of the interaction, i.e. to the symmetry term of the nuclear Equation of State (EoS) At low energies the behavior of the symmetry energy around saturation influences dissipation and fragment production mechanisms. Predictions are shown for deep-inelastic and fragmentation collisions induced by neutron rich projectiles. Differential flow measurements will also shed lights on the controversial neutron/proton effective mass splitting in asymmetric matter. The high density symmetry term can be derived from isospin effects on heavy ion reactions at relativistic energies (few AGeV range), that can even allow a ``direct'' study of the covariant structure of the isovector interaction in the hadron medium. Rather sensitive observables are proposed from collective flows and from pion/kaon production. The possibility of the transition to a mixed hadron-quark phase, at high baryon and isospin density, is finally suggested. Some signatures could come from an expected ``neutron trapping'' effect.Comment: 10 pages, 5 figures; espcrc1 style; IX Int.Conf. on Nucleus-Nucleus Collisions, Rio de Janeiro Aug.2006; to appear in Nucl.Phys.