Theoretical Nuclear Physics in Italy

PREFACE

CONTENTS

The intense activity of the Italian community devoted to nuclear structure, carried out during the last two years within the framework of the research project “Theoretical Physics of the Nucleus and of the Many-Body Systems”, is reviewed. The important role of the many international collaborations, and of the fruitful exchanges with the experimental groups, is emphasized.

A new approach for deriving the shell-model effective interaction from the free nucleon-nucleon (NN) potential is discussed. It consists in renormalizing the strong repulsive core contained in all modern NN potentials by constructing a low-momentum potential, V_low−k, which is confined within a certain cutoff momentum Λ. Results of shell-model calculations performed within the framework of the V_low−k approach are compared with those obtained by using the usual G-matrix formalism and with experimental data. We also present some results of preliminary calculations with different modern NN potentials and discuss the choice of the cutoff momentum.

A formalism based on the Correlated Basis Functions (CBF) theory is developed to describe nuclei with N ≠ Z in a jj coupling scheme. The binding energies and the density distributions of doubly magic nuclei have been calculated by solving the Fermi Hypernetted Chain (FHNC) integral equations in the single operator chain approximation (SOC). The realistic Argonne two-nucleon potential has been used in the application to the ⁴⁰Ca, ⁴⁸Ca and ²⁰⁸Pb nuclei.

The Auxiliary Field Diffusion Monte Carlo method has been applied to simulate oxygen isotopes in the 1D_5/2 shell using a realistic nucleon-nucleon interactions, which include tensor, spin-orbit and three-body forces. The closed shell ¹⁶O has been replaced by the self-consistent potential obtained by solving Hartree-Fock equations with the effective interaction Skyrme I, and AFDMC was used only on the external neutrons. We report results for energies of the ground-state and of the first excited states.

We have developed a relativistic point-coupling model of nuclear many-body dynamics constrained by the low-energy sector of QCD. The effective Lagrangian is characterized by density-dependent coupling strengths determined by chiral one-and two-pion exchange (with single and double delta isobar excitations) and by large isoscalar background fields that arise through changes of the quark condensate and the quark density at finite baryon density. The model has been tested in the analysis of nuclear ground-state properties along different isotope chains of medium and heavy nuclei. The agreement with experimental data is comparable with purely phenomenological predictions. The built-in QCD constraints and the explicit treatment of pion exchange restrict the freedom in adjusting parameters and functional forms of density-dependent couplings. It is shown that chiral pionic fluctuations play an important role for nuclear binding and saturation mechanism, whereas background fields of about equal magnitude and opposite sign generate the effective spin-orbit potential in nuclei.

In this paper I summarize major recent achievements in the search of Quark Gluon Plasma and I review theoretical research activity carried out in Italy. The main focus will be on phenomenology of relativistic heavy ion collisions.

We investigate two-flavour and two-colour QCD at finite temperature and chemical potential in comparison with a corresponding Nambu and Jona-Lasinio model. By minimizing the thermodynamic potential of the system, we confirm that a second order phase transition occurs at a value of the chemical potential equal to half the mass of the chiral Goldstone mode. For chemical potentials beyond this value the scalar diquarks undergo Bose condensation and the diquark condensate is nonzero. We evaluate the behaviour of the chiral condensate, the diquark condensate, the baryon charge density and the masses of scalar diquark, antidiquark and pion, as functions of the chemical potential. Very good agreement is found with lattice QCD (N_c = 2) results. We also compare with a model based on leading-order chiral effective field theory.

The temporal pseudoscalar meson correlation function in a QCD plasma is investigated in a range of temperatures above T_c of experimental interest. Only the flavour-singlet channel is considered and the imaginary time formalism is employed for the finite temperature calculations. The behaviour of the meson spectral function and of the temporal correlator is first studied in the HTL approximation replacing the free thermal quark propagators with the HTL resummed ones. This procedure satisfactory describes the soft fermionic modes, but its application to the propagation of hard quarks is not reliable. An improved version of the so-called NLA scheme, which allows a better treatment of the hard fermionic modes, is then proposed. The impact of the improved NLA on the pseudoscalar temporal correlator is investigated.

The activity of the Italian nuclear physicists community in the field of Nuclear Astrophysics is reported. The researches here described have been performed within the project “Fisica teorica del nucleo e dei sistemi a multi corpi”, supported by the Ministero dell’Istruzione, dell’Università e della Ricerca.

We calculate the Gravitational Wave emission, induced by r-mode instability, from a rotating Hybrid Star. We explore, moreover, a model in which an Hybrid Star can become a Gravitational Waves burster. The GW bursts are produced by sudden phase transitions, around the quark core of the star, induced by r-mode spinning down. The possible connection between the predictions of this model and the bursts signal found by EXPLORER and NAUTILUS detectors during 2001 is also investigated.

Density and temperature conditions in many stellar core (like the solar core) imply the presence of nonideal plasma effects with memory and long-range interactions between particles. This aspect suggests the possibility that the stellar core could not be in a global thermodynamical equilibrium but satisfies the conditions of a metastable state with a stationary (nonextensive) power law distribution function among ions. The order of magnitude of the deviation from the standard Maxwell-Boltzmann distribution can be derived microscopically by considering the presence of random electrical microfields in the stellar plasma. We show that such a nonextensive statistical effect can be very relevant in many nuclear astrophysical problems.

Recent advances in the theoretical description of few-nucleon systems are reported. This research activity has been performed under the Italian project FISICA TEORICA DEL NUCLEO E DEI SISTEMI A MOLTI CORPI. Bound and scattering states as well as specific reactions are analyzed in connection with the current experimental activity.

Recent advances in the study of pd and nd radiative capture reactions in a wide range of center-of-mass energy below and above deuteron breakup threshold are presented and discussed.

We analyze pion production from nucleon-deuteron collisions, with the outgoing three-nucleon system in bound state. Potentially, these reactions could be used to dissection three-nucleon force diagrams, which lead to three-nucleon potential operators. The experimental data, from threshold up to the Δ resonance, are compared with calculations using accurate nuclear wavefunctions. Pion production amplitudes are obtained through matrix elements involving pion-nucleon rescattering mechanisms in S- and P-waves. We assume the hypothesis that S-wave rescattering includes an isoscalar contribution which is generally suppressed for low-energy pion-nucleon scattering, but is enhanced for pion production because the kinematical regime is different, with involvment of high-momenta contributions. P-wave rescattering includes also explicitly the Δ degrees of freedom. Initial-state interactions (ISI) between the proton and the deuteron have sizable effects on the spin-averaged observables. These ISI effects become important for spin observables such as the deuteron tensor analyzing powers T₂₀. For spin observables involving interference terms amongst the various helicity amplitudes, such as for the nucleon vector analyzing power A_y, ISI effects are very important.

The Discrete Variable Representation (DVR) is used in combination with the Hyperspherical Adiabatic Approximation (HAA) to obtain variational estimates for the solution of the Schrödinger equation for a three-body system. This method is particularly appropriate to study weakly bound systems, such as nuclear or van der Waals complexes. It can be applied to bound states as well as to scattering observables. Applications will be shown for a three-nucleon system using a simple scalar interaction, restricted to the lowest adiabatic curve, and to zero partial angular momenta.

We review some features of heavy ion collisions around the Fermi energy with particular emphasis on results obtained in Italy within a broad theoretical and experimental collaboration. Microscopic vs. macroscopic equation of state (EOS) are discussed and compared to data both in low density and finite temperature regions as well as at higher then normal densities. Fragmentation and a possible liquid gas phase transition is discussed. At higher densities the role of three body forces are discussed and contrasted to microscopic calculations of the EOS and of the binding energy of light nuclei.

We present a general classification scheme for pentaquark states in terms of spin-flavour SU(6) representations and their decomposition in terms of SU_f (3) ⊗ SU_s (2). Special attention is paid to the S₄ permutational symmetry of the spin-flavour part of the four-quark wave functions. This classification is general and useful both for experimentalists and model builders. In particular, it will be useful also for the construction of higher three-quark Fock components or to compare with the nonrelativistic limits of other models. We discuss the spectroscopy of pentaquarks using a Gürsey-Radicati type mass formula, whose coefficients have been determined previously in a study of qqq baryons. Its angular momentum depends on the interplay between the spin-flavour and orbital contributions to the mass operator. The magnetic moment of the Θ⁺(1540) is discussed in the constituent quark model for different values of angular momentum and parity. Finally we construct a model for the five-body problem that can be solved exactly: the properties of the pentaquark are discussed in a collective string-like model.

In this contribution, an approach for a unified description of the pion electromagnetic form factor, in the space- and time-like regions, within a constituent quark model on the light front, will be reviewed. Our approach is based on i) the onshell quark-hadron vertex functions in the valence sector, ii) the dressed photon vertex where a photon decays in a quark-antiquark pair, and iii) the emission and absorption amplitudes of a pion by a quark. Results favorably compare with the existing experimental data.

We discuss the recent experimental results on the ratio bertween the electric and magnetic proton form factors and how they can be described by theoretical models. In particular the calculations performed using the hypercentral Constituent Quark Model are illustrated and shown to compare favourably with the data.

We discuss strong decays of baryon resonances within the concept of relativistic constituent quark models. In particular, we follow a Poincaré-invariant approach along the point form of relativistic quantum mechanics. Here, we focus on pionic decay modes of N and Δ resonances. It is found that the covariant quark-model predictions calculated in the point-form spectator model in general underestimate the experimental data considerably. This points to a systematic defect in the used decay operator and/or the baryon wave functions. From a detailed investigation of the point-form decay operator it is seen that the requirement of translational invariance implies effective many-body contributions. Furthermore, one has to employ a normalization factor in the definition of the decay operator in the point-form spectator model. Our analysis suggests that this normalization factor is best chosen consistently with the one used for the electromagnetic and axial current operators for elastic nucleon form factors.

The nucleon is an ideal laboratory to solve QCD in the nonperturbative regime. There are several experimental observations that still lack a rigorous interpretation; they involve the nucleon as a (polarized) target as well as a beam (in collisions and Drell-Yan processes). These data look like big azimuthal and spin asymmetries, related to the transverse polarization and momentum of the nucleon and/or the final detected particles. They suggest internal reaction mechanisms that are suppressed in collinear perturbative QCD but that are “natural” in Nuclear Physics: quark helicity flips, residual final state interactions, etc.. In my talk, I will give a brief survey of the main results and I will flash the most recent developments and measurements.

We present a calculation of single and double spin asymmetries for inclusive hadron production in hadronic collisions. Our approach is based on Leading Order (LO) perturbative QCD and generalized factorization theorems, with full account of intrinsic parton momentum, κ_⊥, effects. This leads to a new class of spin and κ_⊥-dependent distribution and fragmentation functions. Limiting ourselves to consider leading twist functions, we show how they could play a relevant role in producing non-vanishing spin asymmetries.

Hard exclusive processes, such as deep electroproduction of photons and mesons off nuclear targets, could give access, in the coherent channel, to nuclear generalized parton distributions (GPDs). Here, a realistic microscopic calculation of the unpolarized quark GPD of the ³He nucleus is reviewed. In Impulse Approximation, is obtained as a convolution between the GPD of the internal nucleon and the non-diagonal spectral function, describing properly Fermi motion and binding effects. The obtained formula has the correct limits. Nuclear effects, evaluated by a modern realistic potential, are found to be larger than in the forward case. In particular, they increase with increasing the momentum transfer and the asymmetry of the process. Another feature of the obtained results is that the nuclear GPD cannot be factorized into a Δ²-dependent and a Δ²-independent term, as suggested in prescriptions proposed for finite nuclei. The dependence of the obtained GPDs on different realistic potentials used in the calculation shows that these quantities are sensitive to the details of nuclear structure at short distances.

We discuss the role of non-perturbative spin- and flavor- dependent instanton-induced correlations in light hadrons. We show that the Instanton Liquid Model can reproduce the available data on proton and pion form factors at large momentum transfer and explain the delay of the onset of the perturbative regime in several exclusive reactions. The strong attraction generated by instantons in the diquark channel leads to a quantitative description of non-leptonic decays of hyperons and provides a microscopic dynamical explanation of the Δ I = 1/2 rule.

Some aspects of nuclear dynamics relevant for nuclear structure studies, like the excitation of the double giant resonances and the multi-nucleon transfer reactions, are summarized in some details by using a semi-classical approach that allows a clear separation between the relative motion and the intrinsic degrees of freedom.

We study the spin response function of an infinite homogeneous system of neutrons interacting through a simple spin-spin Heisenberg force in a non-relativistic context. For a ferromagnetic interaction the spin response along the direction of the spontaneous magnetization displays, for not too large momentum transfers, two distinct peaks. The response along the direction orthogonal to the spontaneous magnetization displays a collective mode to be identified with a Goldstone boson of type II. It is shown that the Goldstone boson contributes to the saturation of the energy-weighted sum rule for ≈ 25% when the system becomes fully magnetized.

In this work we have studied the neutrons and protons dynamical behavior in two fragmentation reaction: ⁵⁸Fe+⁵⁸Fe (charge asymmetric, N/Z = 1.23) and ⁵⁸Ni+⁵⁸Ni (charge symmetric, N/Z = 1.07). We note that isospin dynamic processes take place also in the symmetric system ⁵⁸Ni+⁵⁸Ni, that produce more asymmetric fragments and residual nuclei. This is a consequence of the pre-equilibrium phase: we observe a competition between pre-equilibrium evaporation and the phenomenon of the isospin-migration, which is a consequence of the EOS (nuclear equation of state) symmetry term. We have simulated the collision with two different EOS: asy-stiff and asy-soft. Some difference has been noticed, especially about the fragment charge asymmetry. A check of isospin effects has also been done trying to correlate fragment asymmetry with dynamical quantities at the freeze-out time.

We investigate the Lorentz structure of the symmetry energy at supra-normal densities in intermediate energy nucleus-nucleus collisions of asymmetric nuclei. We present several possibilities offered by such dynamical processes to set constraints on the still unknown and very controversial high density behavior of the symmetry energy.

No abstract received.

The INFN (Italian National Institute for Nuclear Physics) has approved a national theoretical network on “Structure and Reactions with Exotic Nuclei”, The project involves the INFN branches of Laboratorio Nazionale del Sud, Padova and Pisa. The aim of the project is to coordinate and homogenize the research already performed in Italy in this field and to strengthen and improve the Italian contribution on the international scenario. Furthermore it aims at creating a solid theoretical structure to support future experimental facilities at the INFN national laboratories such as SPES at LNL and EXCYT at LNS. A review of present and future activities is presented

A new “semiclassical” model of the nuclear matter, composed of u, d colored quarks, is proposed. The approach, named Constrained Molecular Dynamics (CoMD) is based on the molecular dynamics simulation of the quarks, which interact through the Richardson’s potential, and on a constraint due to Pauli blocking. With a suitable choice of the quark masses, some possible Equation of State (EOS) of the nuclear matter, at temperature equal to zero and finite baryon density, are obtained. These equations of state, not only present some known properties of the nuclear matter, as the Quark-Gluon Plasma (QGP) phase transition, but also shown the existence of a new state, the Exotic Color Clustering (ECC) state, in which cluster of quarks with the same color are formed. Some new quantities, “indicators” of the phase transition, are introduced: three order parameters, M_c2, M_c3, M_c4 defined trough the Gell-Mann matrices λ^α, and the lifetime of the J/Ψ particle. The behavior of the J/Ψ particle is studied also in the “finite” systems, obtained by expanding the corresponding “infinite” systems. It seems that the dynamics and the finite size effects do not wash completely the phase transition occurred in infinite systems, and the J/Ψ particle is still a good signature.

We perform molecular dynamics simulations of screening by bound target electrons in low energy nuclear reactions. Quantum effects corresponding to the Pauli and Heisenberg principle are enforced by constraints. We show that the enhancement of the average cross section and of its variance is due to the perturbations induced by the electrons. This gives a correlation between the maximum amplitudes of the inter-nuclear oscillational motion and the enhancement factor. It suggests that the chaotic behavior of the electronic motion affects the magnitude of the enhancement factor.

The last few years activity of the Italian community concerning nuclear physics with electroweak probes is reviewed. Inclusive quasi-elastic electron-scattering, photon end electron induced one- and two-nucleon emission are considered. The scattering of neutrinos off nuclei in the quasi-elastic region is also discussed.

A relativistic Green’s function and a distorted-wave impulse-approximation approach to charged- and neutral-current neutrino-nucleus quasielastic scattering are developed. Results for the neutrino (antineutrino) reactions on ¹⁶O and ¹²C target nuclei are presented and discussed.

The Lorentz integral transform method for exclusive processes is applied to the calculation of electromagnetic reactions leading to ⁴He two-body disintegration. This technique allows to take into account in a rigorous way the final state interaction, without requiring the explicit knowledge of the final state wave function. The obtained results for the cross sections of the ⁴He(γ, n)³He processes as well as the longitudinal response function of the ⁴He(e, e′ p)³H exclusive reaction will be presented.

We shortly review the open problems in the physics of the weak decay of Λ-hypernuclei. For many years the main question has been the discrepancy between theory and experiment for the ratio Γ_n/Γ_p between the neutron- (Λn → nn) and proton-induced (Λp → np) decay rates. Recent indications towards a solution of the puzzle are discussed here. The other open question concerns the asymmetric non-mesonic decay of polarized hyperuclei. While theory predicts negative asymmetries, with a moderate dependence on the hypernucleus, the measurements favor negative values for but small, positive values for . Further theoretical and experimental investigations are needed to clarify this issue.

No abstract received.

A major upgrade of the GSI accelerator complex, presently running in Darmstadt, has been recently funded by the German Government. This new facility will also include a machine for hadronic physics studies giving an intense, high momentum resolution, antiproton beam, with momenta between 1.5 and 15 GeV/c: the High Energy Storage Ring (HESR). This will allow to exploit a wide physics program, mainly devoted to hadron spctroscopy, by means of a general purpose detector (PANDA). In this talk the main topics that might be addressed, in the next future, by the PANDA experiment will be illustrated.

EURONS is an approved Integrated Infrastructure Initiative, which will receive funding from the European Commission Services. The development, the goals, the structure, and the organization of EURONS are outlined. All activities, which will be pursued within the four years after the start of the contract, are briefly described.

We discuss a number of important scientific issues in hadron physics which are addressed in the HAPNET collaboration (Hadronic Physics Network in Experiment and Theory), the successor of the successful HAPHEEP (Hadronic Physics with High Energy Electromagnetic Probes) and ESOP (Electron Scattering off Confined Partons) networks.

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