Theoretical Nuclear Physics in Italy

PREFACE

CONTENTS

The wide activity of the few-nucleon system community in Italy is reviewed. The researches of the different groups are directed towards the solution of common scientific problems and are in close connection with the current experimental activity.

An account of recent studies in the field of theoretical nuclear structure is reported. These studies concern essentially research activities performed under the Italian project “Fisica Teorica del Nucleo e dei Sistemi a Molti Corpi”. Special attention is addressed to results obtained during the last two years as regards the development of new many-body techniques as well as the interpretation of new experimental aspects of nuclear structure.

In this report I will try to illustrate some of the main research themes and “hot topics” in nuclear astrophysics. The particular aim of the present report is to briefly illustrate the research activities, in the field of nuclear astrophysics, performed by the Italian nuclear physicist community within the “Programma di Interesse Nazionale su Fisica Teorica del Nucleo e dei Sistemi a Molti Corpi” (National Research Program on Theoretical Physics of Nuclei and Many Body Systems) supported by the “Ministero dell’Istruzione dell’Università e della Ricerca”.

No abstract received.

No abstract received.

A review of the theoretical activity in Italy in the research field of Hadronic Physics is given. Specific focus is put on phenomenological models based on the effective degrees of freedom of constituent quarks, on parton distributions in hard processes in the Bjorken limit and on the possibility of linking the two concepts via evolution equations. A brief introduction is given also about the socalled generalized parton distributions.

Most of the work about nuclear dynamics developed within the program PRIN 2001–2003 is reviewed here. Special attention has been devoted to underline the original contribution given to each argument.

The organizers of this meeting have asked me to present perspectives of nuclear physics. This means to identify the areas where nuclear physics will be expanding in the next future. In six chapters a short overview of these areas will be given, where I expect that nuclear physics will develop quite fast:

(1) Quantum Chromodynamics and effective field theories in the confinement region.

(2) Nuclear structure at the limits.

(3) High energy heavy ion collisions.

(4) Nuclear astrophysics.

(5) Neutrino physics.

(6) Test of physics beyond the standard model by rare processes.

After a survey over these six points I will pick out a few topics where I will go more in details. There is no time to give for all six points detailed examples. I shall discuss the following examples of the six topics mentionned above:

(1) The perturbative chiral quark model and the nucleon Σ-term.

(2) VAMPIR (Variation After Mean field Projection In Realistic model spaces and with realistic forces) as an example of the nuclear structure renaissance.

(3) Measurement of important astrophysical nuclear reactions in the Gamow peak.

(4) The solar neutrino problem.

As examples for testing new physics beyond the standard model by rare processes I had prepared to speak about the measurement of the electric neutron dipole moment and of the neutrinoless double beta decay. But the time is limited and so I have to skip these points, although they are extremely interesting.

The role of intrinsic transverse momentum both in unpolarized and polarized processes is discussed. We consider inclusive cross sections for pion production in hadronic collisions and for Drell-Yan processes; the results are compared with available experimental data in several different kinematical situations. We reanalyze transverse single spin asymmetries (SSA) observed in inclusive pion production, p^↑ p → π X, in terms of Sivers effect and show how it can be disentangled in polarized Drell-Yan processes by suitably integrating over some final configurations. Estimates for RHIC experiments are given.

We present the main features of a dispersion-relation analysis of real Compton scattering (RCS). Various types of dispersion relations are presented as complementary tools for extracting nucleon polarizabilities from RCS data. The information on nucleon polarizabilities gained in this way are reviewed and the nucleon structure information encoded in these quantities are discussed.

We construct a covariant equation for studying the pair effects in quark models. A minimal, conserved, electromagnetic current is also introduced.

Nuclear and lattice physicists share several topics of common interest, like hadronic masses, electroweak form factors and structure functions of hadrons. The main physics issues that a new collaboration between a nuclear physicist and the lattice group of the SPQ_cdR collaboration is going to address, are summarized and preliminary results obtained for meson and baryon masses are presented. The importance of adopting non-perturbative approaches based on the fundamental theory of the strong interaction, like lattice formulations of QCD, is stressed.

We present a novel description of nuclear many-body systems, both for nuclear matter and finite nuclei, emphasizing the connection with the condensate structure of the QCD ground state and spontaneous chiral symmetry breaking. Lorentz scalar and vector mean-fields are introduced in accordance with QCD sum rules. Nuclear binding arises from pionic fluctuations, using in-medium chiral perturbation theory up to three-loop order. Ground state properties of ¹⁶O and ⁴⁰Ca are calculated. The built-in QCD constraints reduce the number of input parameters significantly in comparison with purely phenomenological relativistic mean-field approaches.

A variational wave function constructed with correlated Hyperspherical Harmonic functions is used to describe the Helium trimer. This system is known to have a deep bound state. In addition, different potential models predict the existence of a shallow excited state that has been identified as an Efimov state. Using the Raylegh-Ritz variational principle the energies and wave functions of both bound states have been obtained by solving a generalized eigenvalue problem. The most recent helium-helium interactions have been investigated.

Preliminary results for the electromagnetic responses of a polarized ³He target, obtained by taking into account the final state interaction effects, are presented. The main features of our approach are discussed for illustrating the model dependence that affects the extraction of the neutron magnetic form factor, , in the few-GeV region recently explored at TJLAB.

Electromagnetic effects are studied in p−d as well as in p−³He scattering. Different observables are considered such as differential cross section and the vector and tensor analyzing powers. As an example calculations of the polarization-transfer coefficients and recently measured in p−d and n−d scattering are shown. Moreover the effect of the magnetic moment interaction is studied in the vector analyzing powers A_y and iT₁₁ at different energies. The calculations have been done using the Argonne v₁₈ two-nucleon plus the Urbana three-nucleon interaction. It is shown that Coulomb effects as well as magnetic moment effects are appreciable and cannot be neglected.

We give an outline of a recently developed algorithm for generating a subset of eigenvalues and eigenvectors of large matrices and show how it can be efficiently implemented for nuclear shell model Hamiltonians.

A low-momentum nucleon-nucleon (NN) potential V_low−k is derived from modern realistic NN potentials by integrating out the high-momentum modes. The smooth V_low−k may be used as input for nuclear structure calculations instead of the usual Brueckner G matrix. Such an approach eliminates the energy dependence one finds in the G-matrix approach, allowing this interaction to be used directly in Hartree-Fock calculations. Bulk properties of ¹⁶O have been calculated starting from different NN potentials. Our results, obtained including up to second order contributions in the Goldstone expansion, are in good agreement with experiment.

Recent advances in the study of warm rapidly rotating nuclei are reported, concerning the determination of the rotational damping width and the violation of the K-quantum number.

The coupling of single-particle motion and of vibrations it is found to be an essential mechanism to explain the inversion of s_1/2 and p_1/2 levels observed in ¹¹Be. The interaction of the two valence neutrons in the ground state of mediated by the exchange of phonons provides 2 MeV of extra binding energy, producing a strongly correlated state.

Spectral statistics of the asymmetric rotor model (ARM) follow the anomalous spectral statistics of the one-dimensional harmonic oscillator. The classical ARM Hamiltonian is integrable with two effective degrees of freedom. For a fixed value of the total angular momentum, the semiclassical energy spectrum can be divided into two sets of energy levels, whose classical analog are librational and rotational motions. The energy levels of each set are quasi equally-spaced and, apart near the separatrix, quantum harmonic approximations reproduce quite well the exact energy spectrum.

In this paper two different topics are discussed. The first topic concerns the collective modes of asymmetric nuclear matter. We use a model based on the Quantum–Hadro–Dynamics effective field picture of nuclear matter. We focus our attention on the effects of the interplay between scalar and vector contributions. The response functions in the isoscalar/isovector channel reveal an interesting “mirror” structure, with a close parallelism in the role respectively played by the compressibility and the symmetry energy. Moreover, almost pure neutron waves are predicted at high densities. The second part of the paper concerns the spinodal decomposition of expanding nuclear matter. The distribution of the liquid domains in the spinodal decomposition is derived. It is found that the formation of the domains can be envisaged as a stationary process. Comparison of the related distribution of the fragment size with experimental data on the nuclear multifragmentation is quite satisfactory.

No abstract received.

In this talk I discuss two types of experiments with exotic nuclei which could be performed at the forthcoming Italian INFN facilities with radioactive beams. First I will discuss nuclear and Coulomb breakup experiments which involve heavy exotic beams and intermediate incident energies, thus being suited for the LNL-SPES proposed facility. Then I will discuss transfer to the continuum reactions aiming at performing spectroscopy in the continuum of light unbound nuclei like ¹⁰Li. Such reactions are best matched if the incident beam energy is very low, as it will be at the LNS-EXCYT facility.

Based on a general approach to binary systems we show that in low density region asymmetric nuclear matter (ANM) is unstable only against isoscalarlike fluctuations. The physical meaning of the thermodynamical chemical and mechanical instabilities is related to the inequality relations verified by the strength of interaction among different components. Relevance of these results in bulk and neck fragmentation is discussed.

The possible signature of the presence of giant pairing states at excitation energy of about 12-14 MeV via two-particle transfer reactions induced by neutron-rich weakly-bound projectiles is studied performing particle-particle RPA calculations on ²⁰⁸Pb and BCS+RPA calculations on ¹¹⁶Sn. We obtain the pairing strength distribution for two particles addition and removal modes for this two nuclei. Estimates of two-particle transfer cross sections can be obtained in the framework of the ‘macroscopic model’ for formfactors and using a DWBA code. The weak-binding nature of the projectile kinematically favours transitions to high-lying states. When the case of (⁶He, ⁴He) transfer reaction is compared to a more conventional (¹⁴C, ¹²C), we find a population of the Giant Pairing Vibration with cross sections of the order of a millibarn, dominating over the mismatched transition to the ground state.

We develop a fully relativistic distorted wave impulse approximation model for electron- and photon-induced one proton knockout reactions. The relativistic mean field for the bound state and the Pauli reduction for the scattering state are used, including a fully relativistic electromagnetic current operator. Results for ¹⁶O(e, e′p) cross section and structure functions are shown in various kinematic conditions and compared with nonrelativistic calculations. Nuclear transparency calculations in a Q² range between 0.3 and 1.8 (GeV/c)² are presented. Results for ¹⁶O(γ,p) differential cross sections are displayed in an energy range between 60 and 150 MeV including two-body seagull contribution in the nuclear current.

We present a model to describe the emission of two protons in electron scattering experiments. The process is induced by one-body electromagnetic operators acting together with short-range correlations, and by two-body Δ currents. The model includes all the diagrams containing a single correlation function. The sensitivity of the cross section to the details of the correlation function is studied by using realistic and schematic correlations. Results for the ¹⁶O nucleus are presented.

A comparison is made between the predictions of the MIT bag model and the Colour Dielectric Model concerning strange quark matter stability: the minimum energy per baryon number is studied as a function of the strangeness fraction of the system: these curves are compared to the masses of hyperons having the same strangeness fraction, and coherently calculated in the model, and for the same parameter values. The MIT bag model and the Double Minimum version of the Colour Dielectric model allow the existence of strangelets, while the Single Minimum version excludes this possibility.

We show that a systematic study of the isospin dependence of the Equation of State of matter at densities of the order of 2–3 times nuclear matter saturation density can give important information concerning quark deconfinement. In particular the region Z/A ~ 0.3 − 0.4 is the most interesting, since for these values of the asymmetry the critical density at which quarks start deconfining can drop dramatically.

We review the observational information on the constancy of the fine structure constant α. We find that small improvements on the measurement of ¹⁸⁷Re lifetime can provide significant progress in exploring the range of variability suggested by QSO data. We also discuss the effects of a time varying α on stellar structure and evolution. We find that radioactive dating of ancient stars can offer a new observational window.

We construct equilibrium sequences of general relativistic neutron star (NS) models in the framework of Hartle’s perturbative formalism. We calculate the maximum rotation frequency of uniformly rotating neutron stars, determined by the mass shedding condition. For that, we use recently proposed equations of state (EOS’s) of stellar matter, derived by describing the hadronic phase within the many-body Brueckner–Bethe–Goldstone formalism, and the quark phase with the MIT bag model. We find that only neutron stars with a deconfined quark phase in their core can rotate at submillisecond periods.

Ultra high energy (UHE) photons can initiate electromagnetic showers in magnetic field. We analyze the two processes that determine the development of the shower, e⁺e⁻ pair creation and synchrotron radiation, and derive formulae for the angular distribution of the produced particles. These formulae are necessary to study the three-dimensional development of the shower.

We investigate in the framework of the Brueckner–Hartree–Fock approximation the influence of neutrino trapping on the composition, equation of state, and structure of neutron stars immediately after birth. We explore also the consequences of neutrino trapping on the early evolution of a neutron star and on the nature of the final compact remnant left by the supernova explosion.

The concepts developed to quantitatively describe the variety of phase transitions displayed by atomic nuclei as a function of mass number, isospin, rotational frequency and temperature (spherical–deformed, normal–superfluid, etc.) taken properly into account fluctuations and quantal size effect are shown to provide not only a powerful framework to describe the phase transition displayed by proteins between the denaturated state and the native, biologically active conformation, but also the basis for a bona fide model solution of the protein folding problem and the design of non–conventional drugs which do not suffer from drug resistance generated by mutations. The extension of these results to real proteins depends on the ability to calculate the interaction between few, strongly interacting (as a rule hydrophobic) amino acids from first priciples, an ability we have developed through the years in mapping out the interdisciplinarity field of research dealing with finite quantal many–body systems like atomic nuclei, metal clusters and fullerenes, lying at the borderline between nuclear and solid state physics.

The experimental Nuclear Physics activity of the Italian researchers is briefly reviewed. The experiments, that are financially supported by the INFN, are done in strict collaboration by more than 500 INFN and University researchers. The experiments cover all the most important field of the modern Nuclear Physics with probes extremely different in energy and interactions. Researches are done in all the four National Laboratories of the INFN even if there is a deeper involvement of the two national laboratories expressly dedicated to Nuclear Physics: the LNL (Laboratorio Nazionale di Legnaro) and LNS (Laboratorio Nazionale del Sud) where nuclear spectroscopy and reaction dynamics are investigated. All the activities with electromagnetic probes develops in abroad laboratories as TJNAF, DESY, MAMI, ESFR and are dedicated to the studies of the spin physics and of the nucleon resonance; hypernuclear and kaon physics is investigated at LNF. A strong community of researchers work in the relativistic and ultra-relativistic heavy ions field in particular at CERN with the SPS Pb beam and in the construction of the ALICE detector for heavy-ion physics at the LHC collider. Experiments of astrophysical interest are done with ions of very low energy; in particular the LUNA accelerator facility at LNGS (Laboratorio Nazionale del Gran Sasso) succeeded measuring cross section at solar energies, below or near the solar Gamow peak. Interdisciplinary researches on anti-hydrogen atom spectroscopy and on measurements of neutron cross sections of interest for ADS development are also supported.

Nuclear structure studies of nuclei far from the stability valley has been one of the main goals of experimental and theoretical nuclear physics in the last years. In particular the interest in the study of nuclei with large variation of the proton/neutron ratios stems from the investigation of the density dependence of the effective interaction of nucleons in nuclei with exotic N/Z ratios whose investigation has become one of the main aims of radioactive nuclear beam projects (like SPES at Legnaro National Laboratories (LNL)). Moderately neutron rich nuclei can be produced making use of the high intensity stable ion beams provided by the LNL accelerator complex (PIAVE + ALPI) by deep-inelastic and multi-nucleon transfer reactions. Spectroscopyc studies will be performed using an array of Clover γ-ray detectors coupled to the PRISMA magnetic spectrometer. The physic aims achievable with this setup will complement studies performed with current radioactive beam (RIB) facilities, being competitive at least until the second generation of RIB facilities (like SPES, SPIRAL II, SIRIUS or RIA) will be fully operational.

The nuclear physics community is presently discussing the future experimental program to be carried out at the two National Laboratories LNL and LNS which are both presently heavily involved in the construction of two radioactive beam facilities of ISOL type. While the EXCYTE facility at LNS uses the fusion evaporation reactions and is mainly devoted to medium light nuclei, the SPES facility, at LNL is based on ²³⁸U fission products and it is designed to provide much larger beam intensities. The SPES facility will provide medium heavy neutron rich beams including the double magic ¹³²Sn nucleus. The research goals for the EXCYTE and SPES fecilities are the studies of the many-body aspects of the structure of nuclei far from stability.

A number of selected examples is here presented and discussed. The focus is the understanding of the evolution of the nuclear properties towards the neutron drip line, evolution which has consequences on the single-particle and collective excitations.

At LNL it has been proposed an Advanced Exotic Ion Beam facility, that will allow a frontier program in RIBs Physics. We illustrate here the concept of the SPES facility, and how this proposal is interconnected with the longer term European project EURISOL.

An overall view of the Laboratori Nazionali del Sud, in terms of activities and major perspectives is provided. The very few examples reported are representative of an intense activity in the relative fields and should help to outline a portrait of our laboratory.

AUTHOR INDEX

LIST OF PARTICIPANTS