Probing the Nuclear Equation of State with Peripheral Heavy Ion Collisions at Fermi Energies

A systematic study of quasi-elastic and deep-inelastic collisions at Fermi energies is presented with the aim of obtaining insight into the underlying dynamics and the nuclear equation of state (EOS). Comparisons of experimental heavy-residue data to detailed calculations using the semiclassical microscopic model CoMD (Constrained Molecular Dynamics) are shown. The CoMD code implements an effective interac- tion with a nuclear-matter compressibility of K=200 (soft EOS) or K=380 (stiff EOS) with several forms of the density dependence of the nucleon-nucleon symmetry potential and imposes a constraint in the phase space occupation for each nucleon, restoring the Pauli principle during the collision. Preliminary results from these comparisons point to a soft equation of state (K=200) with a rather stiff density dependence of the isovector part (symmetry energy).

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 are stressed. Reactions induced by unstable 132 Sn 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.

THE DYNAMICAL DIPOLE RADIATION IN DISSIPATIVE COLLISIONS WITH EXOTIC BEAMS

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. We will review in detail all the main properties, yield, spectrum, damping and angular distributions, revealing important isospin effects. Reactions induced by unstable 132Sn beams appear to be very promising tools to test the sub-saturation Isovector EoS. Predictions are also presented for deep-inelastic and fragmentation collisions induced by neutron rich projectiles. The importance of studying violent collisions with radioactive beams at low and Fermi energies is finally stressed.

SPIN CONSTRAINTS ON REGGE PREDICTIONS AND PERTURBATIVE EVOLUTION IN HIGH ENERGY COLLISIONS

Two key issues in the application of perturbative QCD and Regge predictions to high energy processes are whether the hard and soft pomerons should be considered as two separate distinct exchanges and whether the Regge intercepts are Q2 independent or not. Models involving a distinct hard pomeron exchange predict much larger values for the LHC total cross-section. Here we argue that there is a polarized analogue of this issue in the isovector part of the spin structure function g1 and that the spin data appear to favour a distinct hard exchange.

MOMENTUM AND DENSITY DEPENDENCE OF ISOVECTOR PART OF NUCLEAR MEAN FIELD

Momentum and density dependence of the isovector part of nuclear mean field uτ(k, ρ) is studied by using effective interactions having general form. It is found that only the difference between finite range parts of the exchange interactions between like and unlike nucleons accounts for the momentum dependence of uτ(k, ρ). Depending on the choice of the parameters of these exchange interactions, two conflicting trends of momentum dependence of uτ(k, ρ) are noted which lead to two opposite types of splitting of neutron and proton effective masses. The behavior of uτ(k, ρ) at the Fermi momentum k=k F is related to the density dependence of nuclear symmetry energy Jτ(ρ) and effective nucleon mass M*(k=k F , ρ)/M in symmetric nuclear matter. The need for possible theoretical and experimental efforts to constrain the high density behavior of Jτ(ρ) as well as to resolve the puzzle regarding the two opposite types of splitting of neutron and proton effective masses are also stressed.

Optical model analysis of nucleon scattering from isobars A = 92

An optical model analysis of nucleon scattering from isobars, 92Zr and 92Mo, is presented. Simultaneous analysis of either neutron or proton elastic scattering data on both isobars yields results for the isovector part of the optical potential consistent with previous results.