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M. Colonna

Publications and source records attributed to M. Colonna.

At least 73 records · Page 4Linked to original sources
arXiv2011

Hadron-quark phase transition in asymmetric matter with dynamical quark masses

The two-Equation of State (EoS) model is used to describe the hadron-quark phase transition in asymmetric matter formed at high density in heavy-ion collisions. For the quark phase, the three-flavor Nambu--Jona-Lasinio (NJL) effective theory is used to investigate the influence of dynamical quark mass effects on the phase transition. At variance to the MIT-Bag results, with fixed current quark masses, the main important effect of the chiral dynamics is the appearance of an End-Point for the coexistence zone. We show that a first order hadron-quark phase transition may take place in the region T=(50-80)MeV and ρ_B=(2-4)ρ_0, which is possible to be probed in the new planned facilities, such as FAIR at GSI-Darmstadt and NICA at JINR-Dubna. From isospin properties of the mixed phase somepossible signals are suggested. The importance of chiral symmetry and dynamical quark mass on the hadron-quark phase transition is stressed. The difficulty of an exact location of Critical-End-Point comes from its appearance in a region of competition between chiral symmetry breaking and confinement, where our knowledge of effective QCD theories is still rather uncertain.

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arXiv2011

Symmetry Energy Effects on the Mixed Hadron-Quark Phase at High Baryon Density

The phase transition of hadronic to quark matter at high baryon and isospin density is analyzed. Relativistic mean field models are used to describe hadronic matter, and the MIT bag model is adopted for quark matter. The boundaries of the mixed phase and the related critical points for symmetric and asymmetric matter are obtained. Due to the different symmetry term in the two phases, isospin effects appear to be rather significant. With increasing isospin asymmetry the binodal transition line of the (T,ρ_B) diagram is lowered to a region accessible through heavy ion collisions in the energy range of the new planned facilities, e.g. the FAIR/NICA projects. Some observable effects are suggested, in particular an "Isospin Distillation" mechanism with a more isospin asymmetric quark phase, to be seen in charged meson yield ratios, and an onset of quark number scaling of the meson/baryon elliptic flows. The presented isospin effects on the mixed phase appear to be robust with respect to even large variations of the poorly known symmetry term at high baryon density in the hadron phase. The dependence of the results on a suitable treatment of isospin contributions in effective QCD Lagrangian approaches, at the level of explicit isovector parts and/or quark condensates, is finally discussed.

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arXiv2010

Fragmentation paths in dynamical models

We undertake a quantitative comparison of multi-fragmentation reactions, as modeled by two different approaches: the Antisymmetrized Molecular Dynamics (AMD) and the momentum-dependent stochastic mean-field (SMF) model. Fragment observables and pre-equilibrium (nucleon and light cluster) emission are analyzed, in connection to the underlying compression-expansion dynamics in each model. Considering reactions between neutron-rich systems, observables related to the isotopic properties of emitted particles and fragments are also discussed, as a function of the parametrization employed for the isovector part of the nuclear interaction. We find that the reaction path, particularly the mechanism of fragmentation, is different in the two models and reflects on some properties of the reaction products, including their isospin content. This should be taken into account in the study of the density dependence of the symmetry energy from such collisions.

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arXiv2010

Symmetry Energy Effects on Fusion Cross Sections

We investigate the reaction path followed by Heavy Ion Collisions with exotic nuclear beams at low energies. We will focus on the interplay between reaction mechanisms, fusion vs. break-up (fast-fission, deep-inelastic), that in exotic systems is expected to be influenced by the symmetry energy term at densities around the normal value. The evolution of the system is described by a Stochastic Mean Field transport equation (SMF), where two parametrizations for the density dependence of symmetry energy (Asysoft and Asystiff) are implemented, allowing one to explore the sensitivity of the results to this ingredient of the nuclear interaction. The method described here, based on the event by event evolution of phase space quadrupole collective modes will nicely allow to extract the fusion probability at relatively early times, when the transport results are reliable. Fusion probabilities for reactions induced by 132Sn on 64,58Ni targets at 10 AMeV are evaluated. We obtain larger fusion cross sections for the more n-rich composite system, and, for a given reaction, in the Asysoft choice. Finally a collective charge equilibration mechanism (the Dynamical Dipole) is revealed in both fusion and break-up events, depending on the stiffness of the symmetry term just below saturation.

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arXiv2010

Isospin emission and flows at high baryon density: a test of the symmetry potential

High energy Heavy Ion Collisions (HIC) are studied in order to access nuclear matter properties at high density. Particular attention is paid to the selection of observables sensitive to the poorly known symmetry energy at high baryon density, of large fundamental interest, even for the astrophysics implications. Using fully consistent transport simulations built on effective theories we test isospin observables ranging from nucleon/cluster emissions to collective flows (in particular the elliptic, squeeze out, part). The effects of the competition between stiffness and momentum dependence of the Symmetry Potential on the reaction dynamics are thoroughly analyzed. In this way we try to shed light on the controversial neutron/proton effective mass splitting at high baryon and isospin densities. New, more exclusive, experiments are suggested.

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arXiv2010

Probing the Nuclear Symmetry Energy with Heavy Ion Collisions

Heavy Ion Collisions (HIC) represent a unique tool to probe the in-medium nuclear interaction in regions away from saturation. In this report we present a selection of new reaction observables in dissipative collisions particularly sensitive to the symmetry term of the nuclear Equation of State ($Iso-EoS$). We will first discuss the Isospin Equilibration Dynamics. At low energies this manifests via the recently observed Dynamical Dipole Radiation, due to a collective neutron-proton oscillation with the symmetry term acting as a restoring force. At higher beam energies Iso-EoS effects will be seen in an Isospin Diffusion mechanism, via Imbalance Ratio Measurements, in particular from correlations to the total kinetic energy loss. For fragmentation reactions in central events we suggest to look at the coupling between isospin distillation and radial flow. In Neck Fragmentation reactions important Iso-EoS information can be obtained from fragment isospin content, velocity and alignement correlations. The high density symmetry term can be probed from isospin effects on heavy ion reactions at relativistic energies (few AGeV range), in particular for high transverse momentum selections of the reaction products. Rather isospin sensitive observables are proposed from nucleon/cluster emissions, collective flows and meson production. The possibility to shed light on the controversial neutron/proton effective mass splitting in asymmetric matter is also suggested. A large symmetry repulsion at high baryon density will also lead to an "earlier" hadron-deconfinement transition in n-rich matter. The binodal transition line of the (T,ρ_B) diagram is lowered to a region accessible through heavy ion collisions in the energy range of the new planned facilities, e.g. the FAIR/NICA projects. Some observable effects of the formation of a Mixed Phase are suggested, in particular a Neutron Trapping mechanism. The dependence of the results on a suitable treatment of the isovector part of the interaction in effective QCD Lagrangian approaches is critically discussed. We stress the interest of this study in nuclear astrophysics, in particular for supernovae explosions and neutron star structure, where the knowledge of the Iso-EoS is important at low as well as at high baryon density.

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arXiv2010

Break-up mechanisms in heavy ion collisions at low energies

We investigate reaction mechanisms occurring in heavy ion collisions at low energy (around 20 MeV/u). In particular, we focus on the competition between fusion and break-up processes (Deep-Inelastic and fragmentation) in semi-peripheral collisions, where the formation of excited systems in various conditions of shape and angular momentum is observed. Adopting a Langevin treatment for the dynamical evolution of the system configuration, described in terms of shape observables such as quadrupole and octupole moments, we derive fusion/fission probabilities, from which one can finally evaluate the corresponding fusion and break-up cross sections. The dependence of the results on shape, angular momentum and excitation energy is discussed.

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arXiv2010

Isospin Diffusion in $^{58}$Ni-Induced Reactions at Intermediate Energies

Isospin diffusion is probed as a function of the dissipated energy by studying two systems $^{58}$Ni+$^{58}$Ni and $^{58}$Ni+$^{197}$Au, over the incident energy range 52-74\AM. Experimental data are compared with the results of a microscopic transport model with two different parameterizations of the symmetry energy term. A better overall agreement between data and simulations is obtained when using a symmetry term with a potential part linearly increasing with nuclear density. The isospin equilibration time at 52 \AM{} is estimated to 130$\pm$10 fm/$c$.

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arXiv2009

Probing the symmetry energy at high baryon density with heavy ion collisions

The nuclear symmetry energy at densities above saturation density ($ρ_0\sim 0.16 fm^{-3}$) is poorly constrained theoretically and very few relevant experimental data exist. Its study is possible through Heavy Ion Collisions (HIC) at energies $E/A> 200$ MeV, particularly with beams of neutron-rich radioactive nuclei. The energy range implies that the momentum dependence of the isospin fields, i.e. the difference of the effective masses on protons and neutrons, also has to be investigated before a safe constraint on $\esy(ρ)$ is possible. We discuss the several observables which have been suggested, like $n/p$ emission and their collective flows and the ratio of meson yields with different isospin projection, $π^-/π^+$ and $K^0/K^+$. We point out several physical mechanisms that should be included in the theoretical models to allow a direct comparison to the more precise experiments which will be able to distinguish the isospin projection of the detected particles: CSR/Lanzhou, FAIR/GSI, RIBF/RIKEN, FRIB/MSU.

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arXiv2009

Elliptic Flow at Finite Shear Viscosity in a Kinetic Approach at RHIC

Within a covariant parton cascade, we discuss the impact of both finite shear viscosity $η$ and freeze-out dynamics on the elliptic flow generated at RHIC. We find that the enhancement of $η/s$ in the cross-over region of the QGP phase transition cannot be neglected in order to extract the information from the QGP phase. We also point out that the elliptic flow $v_2(p_T)$ for a fluid at $η/s \sim 0.1-0.2$ is consistent with the one needed by quark number scaling drawing a nice consistency between the nearly perfect fluid property of QGP and the coalescence process.

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arXiv2009

Dynamical Phase Trajectories in Baryon and Isospin Density Spaces

We review recent results obtained for charge asymmetric systems at Fermi and intermediate energies, ranging from 30 MeV/u to 1 GeV/u. Observables sensitive to the isospin dependent part of nuclear interaction are discussed, providing information on the symmetry energy behavior from sub- to supra-saturation densities.

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arXiv2009

The Dynamical Dipole Mode in Fusion Reactions with Exotic Nuclear Beams

We report the properties of the prompt dipole radiation, produced via a collective bremsstrahlung mechanism, in fusion reactions with exotic beams. We show that the gamma yield is sensitive to the density dependence of the symmetry energy below/around saturation. Moreover we find that the angular distribution of the emitted photons from such fast collective mode can represent a sensitive probe of its excitation mechanism and of fusion dynamics in the entrance channel.

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arXiv2008

Isospin diffusion in semi-peripheral $^{58}Ni$ + $^{197}Au$ collisions at intermediate energies (I): Experimental results

Isospin diffusion in semi-peripheral collisions is probed as a function of the dissipated energy by studying two systems $^{58}Ni$ + $^{58}Ni$ and $^{58}Ni$ + $^{197}Au$, over the incident energy range 52-74\AM. A close examination of the multiplicities of light products in the forward part of phase space clearly shows an influence of the isospin of the target on the neutron richness of these products. A progressive isospin diffusion is observed when collisions become more central, in connection with the interaction time.

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arXiv2008

Isospin diffusion in semi-peripheral $^{58}Ni$ + $^{197}Au$ collisions at intermediate energies (II): Dynamical simulations

We study isospin effects in semi-peripheral collisions above the Fermi energy by considering the symmetric $^{58}Ni$ + $^{58}Ni$ and the asymmetric reactions $^{58}Ni$ + $^{197}Au$ over the incident energy range 52-74 A MeV. A microscopic transport model with two different parameterizations of the symmetry energy term is used to investigate the isotopic content of pre-equilibrium emission and the N/Z diffusion process. Simulations are also compared to experimental data obtained with the INDRA array and bring information on the degree of isospin equilibration observed in Ni + Au collisions. A better overall agreement between data and simulations is obtained when using a symmetry term which linearly increases with nuclear density.

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arXiv2008

Competition of breakup and dissipative processes in peripheral collisions at Fermi energies

Heavy ion collisions in the Fermi energy regime may simultaneously show features of direct and dissipative processes. To investigate this behavior in detail, we study isotope and velocity distributions of projectile-like fragments in the reactions $^{18}$O (35 $A\cdot$MeV) + $^9$Be($^{181}$Ta) at forward angles. We decompose the experimental velocity distributions empirically into two contributions: a direct, `breakup' component centered at beam velocity and a dissipative component at lower velocities leading to a tail of the velocity distributions. The direct component is interpreted in the Goldhaber model, and the widths of the velocity distributions are extracted. The dissipative component is then successfully described by transport calculations. The ratio of the yields of the direct and the dissipative contributions can be understood from the behavior of the deflection functions. The isotope distributions of the dissipative component agree qualitatively with the data, but the modification due to secondary de-excitation needs to be considered. We conclude, that such reactions are of interest to study the equilibration mechanism in heavy ion collisions.

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arXiv2008

Anisotropies in momentum space at finite Shear Viscosity in ultrarelativistic heavy-ion collisions

Within a parton cascade we investigate the dependence of anisotropies in momentum space, namely the elliptic flow $v_2= $ and the $v_4= $, on both the finite shear viscosity $η$ and the freeze-out (f.o.) dynamics at the RHIC energy of 200 AGeV. In particular it is discussed the impact of the f.o. dynamics looking at two different procedures: switching-off the collisions when the energy density goes below a fixed value or reducing the cross section according to the increase in $η/s$ from a QGP phase to a hadronic one. We address the relation between the scaling of $v_2(p_T)$ with the eccentricity $ε_x$ and with the integrated elliptic flow. We show that the breaking of the $v_2(p_T)/ε_x$ scaling is not coming mainly from the finite $η/s$ but from the f.o. dynamics and that the $v_2(p_T)$ is weakly dependent on the f.o. scheme. On the other hand the $v_4(p_T)$ is found to be much more dependent on both the $η/s$ and the f.o. dynamics and hence is indicated to put better constraints on the properties of the QGP. A first semi-quantitative analysis show that both $v_2$ and $v_4$ (with the smooth f.o.) consistently indicate a plasma with $4πη/s \sim 1-2$.

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