Theoretical Nuclear Physics in Italy

PREFACE

CONTENTS

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We present an overview of the contributions that Adelchi Fabrocini gave to the field of many-body physics during the last thirty years. He has left us while he was still in full activity, and his work, which is certainly a reference for all of us, will motivate and guide the work of future research in many-body physics for a long time.

Highlights on the recent research activity, carried out by the Italian Community involved in the Few-Nucleon Systems field, will be presented.

The Hyperspherical Adiabatic Method are used to describe the scattering states of a three-body system. We exploit the well know fact that the Adiabatic Functions describe asymptotically the scattering states of the system, both below and above the breakup threshold. In order to obtain an accurate description of the Adiabatic basis at large ρ we follow closely the procedure indicated by Nielsen and co-workers. The scattering wave function is expanded in terms of the Adiabatic basis and the S-matrix is obtained variationally by means of the Khon variational principle. We show two simple numerical applications of the method for the study of a three-nucleon system with a pure central interaction and a realistic potential.

We present an exploratory study consisting in the formulation of a relativistic quantum mechanics to describe the few-nucleon system at low energy, starting from the quantum field theoretical chiral Lagrangian involving pions and nucleons. To this aim we construct a Bakamjian-Thomas mass operator and perform a truncation of the Fock space which respects at each stage the relativistic covariance. Such truncation is justified, at sufficiently low energy, in the framework of a systematic chiral expansion. As an illustration we discuss the low-energy phaseshifts of the nucleon-nucleon and pion-nucleon scattering.

An overview over the field of few-nucleon systems is presented. Special emphasis is laid on the construction of a unified approach to hadronic and electromagnetic reactions on few-nucleon systems, necessary for studying the borderline between quark-gluon and effective descriptions. We briefly outline the Lorentz integral transform (LIT) method which offers a unique possibility to tackle this ambitious task.

The contribution of isospin admixtures in the ground state of the ⁴He nucleus is studied using wave functions derived from the most modern nuclear interactions, including isospin symmetry breaking terms. The present calculations show that this contribution is larger than previous estimates had indicated. Its effect on parity violating elastic scattering of polarized electrons from ⁴He is investigated. In particular, a simple analysis of the recently measured left-right asymmetry at low Q² shows that the contribution of these isospin admixtures is of comparable magnitude to that associated with strangeness components in the nucleon electric form factor.

No abstract received.

We present an accurate numerical study of the equation of state of nuclear matter based on realistic nucleon–nucleon interactions by means of Auxiliary Field Diffusion Monte Carlo (AFDMC) calculations. The AFDMC method samples the spin and isospin degrees of freedom allowing for quantum simulations of large nucleonic systems and represents an important step forward towards a quantitative understanding of problems in nuclear structure and astrophysics.

The research activity performed during the last two years by the Italian physicists in the field of theoretical nuclear structure is reviewed. Though covering the whole range of nuclear structure, the different approaches appear to complement each other forming a coherent research project.

A set of equations of motion are derived within a subspace spanned by states which are tensor products of n Tamm Dancoff phonons and solved iteratively starting from the particle-hole vacuum to generate a basis of microscopic multiphonon states. Such a basis is then used to diagonalize the full Hamiltonian. An numerical implementation of the method on ¹⁶O is presented for illustrative purposes.

Momentum distributions, spectroscopic factors and quasi-hole wave functions of medium-heavy doubly closed shell nuclei have been calculated in the framework of the Correlated Basis Function theory, by using the Fermi hypernetted chain resummation techniques. The calculations have been done by using microscopic two-body nucleon-nucleon potentials of Argonne type, together with three-body interactions. Operator dependent correlations, up to the tensor channels, have been used.

We call projectile fragmentation of neutron halo nuclei the elastic breakup (diffraction) reaction, when the observable studied is the neutron-core relative energy spectrum. This observable has been measured in relation to the Coulomb breakup on heavy target and recently also on light targets. Such data enlighten the effect of the neutron final state interaction with the core of origin. Projectile fragmentation is studied here by a time dependent model for the excitation of a nucleon from a bound state to a continuum resonant state in a neutron-core complex potential which acts as a final state interaction. The final state is described by an optical model S-matrix so that both resonant and non resonant states of any continuum energy can be studied as well as deeply bound initial states. It turns out that due to the coupling between the initial and final states, the neutron-core free particle phase shifts are modified, in the exit channel, by an additional phase. Some typical numerical calculations for the relevant observables are presented and compared to experimental data. It is suggest that the excitation energy spectra of an unbound nucleus might reflect the structure of the parent nucleus from whose fragmentation they are obtained.

The ground-state energy of the doubly magic nuclei ⁴He and ¹⁶O has been calculated within the framework of the Goldstone expansion starting from modern nucleon-nucleon potentials. A low-momentum potential V_low–k has been derived from the bare potential by integrating out its high-momentum components beyond a cutoff Λ. We have employed a simple criterion to relate this cutoff momentum to a boundary condition for the two-nucleon model space spanned by a harmonic-oscillator basis. Convergence of the results has been obtained with a limited number of oscillator quanta.

The interaction induced by the exchange of low-lying surface vibrations between pairs of orbitals close to the Fermi surface provides an important contribution to pairing correlations in superfluid nuclei. We study the spatial dependence of the pairing field produced by this induced interaction in ¹²⁰Sn.

The activity of the Italian community in the period 2004-2006 in the field of compact stars is reviewed.

We address the problem of vortex nucleus interaction in the inner crust of neutron stars. The relevance of this problem is connected with the observed glitches in pulsars that can be explained with an exchange of angular momentum from the superfluid neutrons in the inner crust to the outer crust. The problem is solved within the Hartree-Fock-Bogoliubov theory. We find that the properties of the vortex are strongly influenced by finite size effects, leading to qualitative differences with respect to results based on semiclassical approximations.

We discuss the temporal structure of the Gamma-Ray-Bursts (GRBs) light curves and we analyse the occurrence of quiescent times which are long periods within the prompt emission in which the inner engine is not active. We show that if a long quiescent time is present, it is possible to divide the total duration of GRBs into three periods: the pre-quiescence emission, the quiescent time and the post-quiescence emission. We then discuss a model of the GRBs inner engine based on the formation of quark phases during the life of an hadronic star. Within this model the pre-quiescence emission is interpreted as due to the deconfinement of quark inside an hadronic star and the formation of 2SC quark matter or unpaired quark matter (UQM). The post-quiescence emission is due to the conversion of 2SC (or UQM) into the Color-Flavor-Locking (CFL) phase. The temporal delay between these two processes is connected with the nucleation time of the CFL phase in the 2SC (UQM) phase and it can be associated with the observed quiescent times in the GRBs light curves.

We study the hadron-quark phase transition in the interior of protoneutron stars. For the hadronic sector, we use a microscopic equation of state involving nucleons and hyperons derived within the finite-temperature Brueckner-Bethe-Goldstone many-body theory, with realistic two-body and three-body forces. For the description of quark matter, we employ the MIT bag model both with a constant and a density-dependent bag parameter. We calculate the structure of protostars within a static approach. In particular we focus on a suitable temperature profile, suggested by dynamical calculations, which plays a fundamental role in determining the value of the minimum gravitational mass. The maximum mass instead depends only upon the equation of state employed.

We study the hydrodynamical transition from an hadronic star into a quark or a hybrid star. We discuss the possible mode of burning, using a fully relativistic formalism and realistic Equations of State. We take into account the possibility that quarks form a diquark condensate. We find that the conversion process always corresponds to a deflagration and never to a detonation. Hydrodynamical instabilities can develop on the front but the increase in the conversion's velocity is not sufficient to transform the deflagration into a detonation. Concerning convection, it does not always develop. Instead the process of conversion from ungapped quark matter to gapped quarks always allows the formation of a convective layer.

The research activitities carried out in Italy during the last two years in the field of theoretical nuclear physics with electroweak probes are reviewed. Different models for electron-nucleus and neutrino-nucleus scattering are compared. The results obtained for electromagnetic reactions on few-nucleon systems and on complex nuclei are discussed. The recent developments in the study of electron-and photon-induced reactions with one and two-nucleon emission are presented.

The photodisintegration cross section of ⁴He is calculated using the realistic nucleon-nucleon potential Argonne V18 (AV18) and the three-body force Urbana IX (UIX). Final State interaction is taken into account via the Lorentz Integral Transform method. A comparison with other potential models and with the available experimental data is discussed.

Superscaling analysis of electroweak nuclear response functions is done for momentum transfer values from 300 to 700 MeV/c. Some effects, absent in the Relativistic Fermi Gas model, where the superscaling holds by construction, are considered. From the responses calculated for the ¹²C, ¹⁶O and ⁴⁰Ca nuclei, we have extracted a theoretical universal superscaling function similar to that obtained from the experimental responses. Theoretical and empirical universal scaling functions have been used to calculate electron and neutrino cross sections. These cross sections have been compared with those obtained with a complete calculation and, for the electron scattering case, with the experimental data.

The role of the sea quarks to ground state nucleon properties with electroweak probes is discussed. A relativistic Green's function approach to parity violating electron scattering and a distorted-wave impulse-approximation applied to charged- and neutral-current neutrino-nucleus quasi-elastic scattering are presented in view of the possible determination of the strangeness content of the nucleon.

Some of the most relevant and recent results of the italian theoretical groups active in hadronic physics are briefly reviewed.

We use th Interacting Instanton Liquid Model to investigate the non-perturbative interaction associated to spontaneous chiral symmetry breaking. In particular, we study how hadrons' structure changes as a function of the quark mass. To support the validity of this model in the chiral regime, the quark mass dependencies of several properties are shown to agree with chiral perturbation theory, including the density of eigenmodes of the Dirac operator and the masses of the pion and nucleon. We identify a natural mass scale m* = 80 MeV which sets the boundary of the mass regime where the non-perturbative QCD dynamics is determined by chiral effects.

An approach for a unified description of the nucleon electromagnetic form factors in spacelike and timelike regions is presented. The main ingredients of our model are: i) a Mandelstam formula for the matrix elements of the nucleon electromagnetic current; ii) a 3-dimensional reduction of the problem on the Light-Front performed within the so-called Propagator Pole Approximation (PPA), which consists in disregarding the analytical structure of the Bethe-Salpeter amplitudes and of the quark-photon vertex function in the integration over the minus components of the quark momenta; iii) a dressed photon vertex in the channel, where the photon is described by its spin-1, hadronic component.

We discuss electromagnetic currents in the point-form formulation of relativistic quantum mechanics. The construction is along a spectator model and implies that only one quark is explicitly coupled to the photon, but nevertheless many-body contributions are present in the current operator. Such effects are unavoidable in relativistic constructions and resulting ambiguities are notably reduced by imposing charge normalization and time-reversal invariance. The residual theoretical indetermination introduces small but sizeable changes in the nucleon form-factors, particularly at higher Q² values, with the data generally centered in the middle of the theoretical band.

Within the framework of light-cone quantization we derive the overlap representation of generalized parton distributions for transversely polarized quarks using the Fock-state decomposition in the transverse-spin basis. We apply this formalism to the case of light-cone wave functions in a constituent quark model giving numerical results for the four chiral-odd generalized parton distributions in a region where they describe the emission and reabsorption of a quark by the nucleon. With the transversity distribution obtained in the forward limit of the generalized distribution, we provide some predictions for the double transverse-spin asymmetry in Drell-Yan dilepton production in the kinematics of the experiment.

We present the formalism for a new generation of unquenched quark models in which quark-antiquark pair effects are taken into account in an explicit form via a microscopic QCD-inspired quark-antiquark creation mechanism. No truncation in the sum over all the big tower of states is necessary since these states are automatically generated by means of powerful group-theoretical techniques. An important check on the formalism and the numerical results is provided by the closure limit. As an application, the effect of quark-antiquark pairs on the strange content of the proton spin, Δs, is discussed. The contributions of the up and down quarks, Δu and Δd, are also calculated. This has become possible after solving the difficult problem of permutational symmetry related to quark rearrangements. Finally, we present some preliminary results in the closure limit as well as an outlook for future applications.

A general classification of tetraquark states in terms of the spin-flavour, colour and spatial degrees of freedom has been constructed. The permutational symmetry properties of both the spin-flavour and orbital parts of the quark-quark and antiquark-antiquark subsystems are discussed in short. This classification is model indipendent and useful both for model-builders and experimentalists. An evaluation of the tetraquark spectrum is obtained from a generalization of a Iachello mass formula, originally developed for the mesons. The ground state tetraquark nonet is identified with f₀(600), κ(800), f₀ (980), a₀(980).

No abstract received.

The thermodynamics of N_f= 2 quark flavors is investigated within an updated version of the PNJL model. Quarks interact through chiral four-point couplings and propagate in a homogeneous Polyakov loop background. The input is fixed exclusively by selected pure-gauge lattice QCD results and by pion properties in vacuum. Successful comparison with lattice QCD thermodynamics is obtained, both at zero and at finite quark chemical potential μ. Convergence properties of the Taylor series in μ/T are discussed within our model.

A status report of the Laboratori Nazionali del Sud is provided. Instead of illustrating the details of the activities, focus is put on the more recent achievements mainly in the perspective of the future developments.

To understand the properties of a nucleus, apart from establishing the interaction between its components, it is necessary to determine the arrangement of the nucleons, i.e. the structure of a nucleus. So far our knowledge about the structure of nuclei is mostly limited to nuclei close to the valley of stability, or nuclei with a deficiency of neutrons, which can be produced in fusion-evaporation reactions with stable beams and stable targets. Future perspectives in nuclear structure rely on radioactive ion beams (RIB) as well as on high intensity beams of stable ions (HISB). A world wide effort is presently going on in order to built the next generation radioactive ion beam facilities like the FAIR and the EURISOL projects. The LNL are contributing to such development through the design study of the EURISOL project as well as through the design and construction of the intermediate facility SPES. Concerning the instrumentation, particularly powerful is the combination of large acceptance spectrometers with highly segmented γ-detector arrays. An example is the CLARA γ-ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable with such device complement studies performed with current radioactive beam (RIB) facilities. With this set-up we have recently investigated the stability of the N=50 shell closure. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=31. Also the study of proton rich nuclei can strongly benefit from the use of high intensity stable beams using fusion evaporation reactions at energies close to the Coulomb barrier. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. Among the new instrumentation, the concept of γ-ray tracking has been recently introduced in nuclear spectroscopy. A new γ-ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. The first sub-cluster of AGATA is foreseen to be installed at the PRISMA spectrometer.

The experimental activities in Nuclear Physics carried out by Italian researchers and financially supported by INFN are briefly reviewed. The experimental program covers the most important fields of the modern nuclear physics, namely the study of hadron dynamics, quark gluon plasma, nuclear structure and reaction dynamics and nuclear astrophysics. The experiments are performed at the four national laboratories, at CERN and several others laboratories abroad. In particular, LNL is mainly dedicated to nuclear structure, LNS is strongly involved on the study of equation of state, LNF has a program on hypernuclei and kaonic atoms and LNGS has a facility for measurements of cross sections of astrophysical interest. The large community working on the problem of quark gluon plasma is taking part in the ALICE experiment using the ultrarelativisc heavy ion collisions of LHC. Interdisciplinary researches are also supported. A brief outline of the research and of their present and future objectives is here given.

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