G. Ferini

Aspects of particle production in isospin-asymmetric matter

Abstract The production/absorption rate of particles in compressed and heated asymmetric matter is studied using a Relativistic Mean Field (RMF) transport model with an isospin-dependent collision term. Just from energy conservation in the elementary production/absorption processes we expect to see a strong dependence of the yields on the basic Lorentz structure of the isovector effective interaction, due to isospin effects on the scalar and vector self-energies of the hadrons. This will be particularly evident for the ratio of the rates of particles produced with different charges: results are shown for π + / π − , K + / K 0 yields. In order to simplify the analysis we perform RMF cascade simulations in a box with periodic boundary conditions. In this way we can better pin down all such fine relativistic effects in particle production, that could likely show up even in realistic heavy ion collisions. In fact, the box properties are tuned in order to reproduce the heated dense matter formed during a nucleus–nucleus collision in the few A GeV beam energy region. In particular, K + , 0 production is expected to be directly related to the high density behaviour of the symmetry energy, since kaons are produced very early during the high density stage of the collision and their mean free path is rather large. We show that the K + / K 0 ratio reflects important isospin contributions on the production rates just because of the large sensitivity around the threshold. The results are very promising for the possibility of a direct link between particle production data in exotic Heavy Ion Collisions (HIC) and the isospin-dependent part of the Equation of State (EoS) at high baryon densities.

Scalings of Elliptic Flow for a Fluid at Finite Shear Viscosity

Abstract Within a parton cascade approach we investigate the scaling of the differential elliptic flow v 2 ( p T ) with eccentricity ϵ x and system size and its sensitivity to finite shear viscosity. We present calculations for shear viscosity to entropy density ratio η / s in the range from 1 / 4 π up to 1 / π , finding that the v 2 saturation value varies by about a factor 2. Scaling of v 2 ( p T ) / ϵ x is seen also for finite η / s which indicates that it does not prove a perfect hydrodynamical behavior, but is compatible with a plasma at finite η / s . Introducing a suitable freeze-out condition, we see a significant reduction of v 2 ( p T ) especially at intermediate p T and for more peripheral collisions. This causes a breaking of the scaling for both v 2 ( p T ) and the p T -averaged v 2 , while keeping the scaling of v 2 ( p T ) / 〈 v 2 〉 . This is in better agreement with the experimental observations and shows as a first indication that the η / s should be significantly lower than the pQCD estimates. We finally point out the necessity to include the hadronization via coalescence for a definite evaluation of η / s from intermediate p T data.

In-Medium Effects on Particle Production in Heavy Ion Collisions

Abstract The effect of possible in-medium modifications of nucleon–nucleon ( NN ) cross sections on particle production is investigated in heavy ion collisions (HIC) at intermediate energies. In particular, using a fully covariant relativistic transport approach, we see that the density dependence of the inelastic cross sections appreciably affects the pion and kaon yields and their rapidity distributions. However, the ( π − / π + ) - and ( K 0 / K + ) -ratios depend only moderately on the in-medium behavior of the inelastic cross sections. This is particularly true for kaon yield ratios, since kaons are more uniformly produced in high density regions. Kaon potentials are also suitably evaluated in two schemes, a chiral perturbative approach and an effective meson–quark coupling method, with consistent results showing a similar repulsive contribution for K + and K 0 . As a consequence we expect rather reduced effects on the yield ratios. We conclude that particle ratios appear to be robust observables for probing the nuclear equation of state (EoS) at high baryon density and, particularly, its isovector sector.

Anisotropies in momentum space at finite shear viscosity in ultrarelativistic heavy-ion collisions

Abstract Within a parton cascade we investigate the dependence of anisotropies in momentum space, namely the elliptic flow v 2 = 〈 cos ( 2 ϕ ) 〉 and the v 4 = 〈 cos ( 4 ϕ ) 〉 , on both the finite shear viscosity η and the freeze-out (f.o.) dynamics at the RHIC energy of 200 GeV. In particular the impact of the f.o. dynamics is discussed 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 sensitive to 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 shows that both v 2 and v 4 (with the smooth f.o.) consistently indicate a plasma with 4 π η / s ∼ 1 − 2 .

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.

Does the NJL chiral phase transition affect the elliptic flow of a fluid at fixed η/s?

Abstract We have derived and solved numerically the Boltzmann–Vlasov transport equations that includes both two-body collisions and the chiral phase transition by mean of NJL-field dynamics. The scope is to understand if the field dynamics supply new genuine effects on the build-up of the elliptic flow v 2 , a measure of the asymmetry in the momentum space, and in particular if it can affect the relation between v 2 and the shear viscosity to entropy ratio η / s . Solving the transport equation with a constant cross section for the condition of Au + Au collisions at s NN = 200 A GeV it is shown a sizable suppression of v 2 due to the attractive nature of the field dynamics that generates the constituent mass. However the key result is that if η / s of the system is kept fixed by an appropriate local renormalization of the cross section the v 2 does not depend on the details of the collisional and/or field dynamics and in particular it is not affected significantly by the chiral phase transition.

Isospin effects on strangeness in heavy-ion collisions

Abstract Kaon properties are studied within the framework of a fully covariant transport approach. The kaon–nucleon potential is evaluated in two schemes, a chiral perturbative approach and an effective One-Boson-Exchange model. Isospin effects are explicitly accounted for in both models. The transport calculations indicate a significant sensitivity of momentum distributions and total yields of K 0 , + isospin states on the choice of the kaon–nucleon interaction. Furthermore, isospin effects are rather moderate on absolute kaon yields, but appear on strangeness ratios. This is an important issue in determining the high density symmetry energy from studies of strangeness production in heavy-ion collisions.

Heavy Ion Collisions at Relativistic Energies: Testing a Nuclear Matter at High Baryon and Isospin Density

We show that the phenomenology of isospin effects on heavy ion reactions at intermediate energies (few AGeV range) is extremely rich and can allow a “direct” study of the covariant structure of the isovector interaction in the hadron medium. We work within a relativistic transport frame, beyond a cascade picture, consistently derived from effective Lagrangians, where isospin effects are accounted for in the mean field and collision terms. Rather sensitive observables are proposed from collective flows (“differential” flows) and from pion/kaon production ( π − / π + , K 0 / K + yields). For the latter point relevant non-equilibrium effects are stressed. 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.

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

Abstract 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 ∼ 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.

The high-density symmetry energy in heavy ion collisions

Abstract The nuclear symmetry energy as a function of density is rather poorly constrained theoretically and experimentally both below saturation density, but particularly at high density, where very few relevant experimental data exist. We discuss observables which could yield information on this question, in particular, proton–neutron flow differences, and the production of pions and kaons in relativistic heavy ion collisions. For the meson production we investigate particularly ratios of the corresponding isospin partners π − / π + and K 0 / K + , where we find that the kaons are an interesting probe to the symmetry energy. In this case we also discuss the influence of various choices for the kaon potentials or in-medium effective masses.