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U. Lombardo

Publications and source records attributed to U. Lombardo.

At least 19 recordsLinked to original sources
arXiv2026

Microscopic Optical Potential from Brueckner-Hartree-Fock Theory

Modern Brueckner-Hartree-Fock (BHF) calculations are very successful in describing various properties of symmetric and asymmetric nuclear matter. Within BHF theory a microscopic optical potential (MOP) for nucleon-nucleus scattering is developed. First, we parametrize the energy and density dependence of complex optical potentials in nuclear matter based on BHF calculations and then we construct the MOP for finite nuclei with the local density approximation extended to include the finite-range effects. The density distribution and the spin-orbit contribution are calculated from the Hartree-Fock (HF) approximation with LNS5 Skyrme interaction, the latter being constrained by the BHF results. The central real and imaginary potentials turn out to be quantitatively consistent with the phenomenological global Koning-Delaroche (KD) potentials. The performance of MOP is evaluated by considering neutron/proton scattering on $^{40,48}$Ca. The elastic scattering differential cross sections, analyzing powers and total/reaction cross sections are analyzed in the energy below 200 MeV. A good agreement between the theoretical results and the measurements is achieved. Since our results are presented in the analytic forms, they can thus be used easily in the analysis of the experimental data of the nucleon scattering on exotic nuclei.

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arXiv2018

Proton-proton 1S0 pairing in neutron star

The onset of 1S0 proton spin-singlet pairing in neutron-star matter is studied in the framework of the BCS theory including medium polarization effects. The strong three-body coupling of the diproton pairs with the dense neutron environment and the self-energy effects severely reduce the gap magnitude, so to reshape the scenario of the proton superfluid phase inside the star. The vertex corrections due to the medium polarization are attractive in all isospin-asymmetry range at low density and tend to favor the pairing in that channel. However quantitative estimates of their effect on the energy gap do not give significant changes. Implications of the new scenario on the role of pairing in neutron-star cooling is briefly discussed.

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arXiv2018

On the spin-isospin decomposition of nuclear symmetry energy

The decomposition of nuclear symmetry energy into spin and isospin components is discussed to elucidate the underlying properties of the NN bare interaction. This investigation was carried out in the framework of the Brueckner-Hartree-Fock theory of asymmetric nuclear matter with consistent two and three body forces. It is shown the interplay among the various two body channels in terms of isospin singlet and triplet components as well as spin singlet and triplet ones. The broad range of baryon densities enables to study the effects of three body force moving from low to high densities.

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arXiv2016

Medium polarization in asymmetric nuclear matter

The influence of the core polarization on the effective nuclear interaction of asymmetric nuclear matter is calculated in the framework of the induced interaction theory. The strong isospin dependence of the density and spin density fluctuations is studied along with the interplay between the neutron and proton core polarizations. Moving from symmetric nuclear matter to pure neutron matter the crossover of the induced interaction from attractive to repulsive in the spin singlet state is determined as a function of the isospin imbalance.The density range in which it occurs is also determined. For the spin triplet state the induced interaction turns out to be always repulsive. The implications of the results for the neutron star superfluid phases are shortly discussed.

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arXiv2015

Role of nucleonic Fermi surface depletion in neutron star cooling

The Fermi surface depletion of beta-stable nuclear matter is calculated to study its effects on several physical properties which determine the neutron star thermal evolution. The neutron and proton Z factors measuring the corresponding Fermi surface depletions, are calculated within the Brueckner-Hartree-Fock approach employing the AV18 two-body force supplemented by a microscopic three body force. Neutrino emissivity, heat capacity and, in particular, neutron 3PF2 superfluidity turn out to be reduced, especially at high baryonic density, to such an extent that the cooling rates of young neutron stars are significantly slowed

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arXiv2013

3PF2 pairing in high-density neutron matter

The onset of the 3PF2 superfluid phase in high-density neutron matter is studied within the BCS framework with two and three body forces. Owing to the strong correlations the energy gap is so sizeably quenched as to demand to reconsider the role of superfluidity in neutron-star phenomena.

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arXiv2012

An ab initio theory of double odd-even mass differences in nuclei

Two aspects of the problem of evaluating double odd-even mass differences D_2 in semi-magic nuclei are studied related to existence of two components with different properties, a superfluid nuclear subsystem and a non-superfluid one. For the superfluid subsystem, the difference D_2 is approximately equal to 2Δ, the gap Δbeing the solution of the gap equation. For the non-superfluid subsystem, D_2 is found by solving the equation for two-particle Green function for normal systems. Both equations under consideration contain the same effective pairing interaction. For the latter, the semi-microscopic model is used in which the main term calculated from the first principles is supplemented with a small phenomenological addendum containing one phenomenological parameter supposed to be universal for all medium and heavy atomic nuclei.

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arXiv2011

Semi-microscopic model of pairing in nuclei

A semi-microscopic model for nucleon pairing in nuclei is presented starting from the ab intio BCS gap equation with Argonne v18 force and the self-consistent Energy Density Functional Method basis characterized with the bare nucleon mass. The BCS theory is formulated in terms of the model space S0 with the effective pairing interaction calculated from the first principles in the subsidiary space S0. This effective interaction is supplemented with a small phenomenological addendum containing one phenomenological parameter universal for all medium and heavy atomic nuclei. We consider the latter as a phenomenological way to take into account both the many-body corrections to the BCS theory and the effective mass effects. For protons, the Coulomb interaction is introduced directly. Calculations made for several isotopic and isotonic chains of semi-magic nuclei confirm the validity of the model. The role of the self-consistent basis is stressed.

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arXiv2010

On limits of ab initio calculations of pairing gap in nuclei

A brief review of recent microscopic calculations of nuclear pairing gap is given. A semi-microscopic model is suggested in which the ab-initio effective pairing interaction is supplemented with a small phenomenological addendum. It involves a parameter which is universal for all medium and heavy nuclei. Calculations for several isotopic and isotonic chains of semi-magic nuclei confirm the relevance of the model.

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arXiv2010

Transport parameters in neutron stars from in-medium NN cross sections

We present a numerical study of shear viscosity and thermal conductivity of symmetric nuclear matter, pure neutron matter and $β$-stable nuclear matter, in the framework of the Brueckner theory. The calculation of in-medium cross sections and nucleon effective masses is performed with a consistent two and three body interaction. The investigation covers a wide baryon density range as requested in the applications to neutron stars. The results for the transport coefficients in $β$-stable nuclear matter are used to make preliminary predictions on the damping time scales of non radial modes in neutron stars.

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arXiv2010

Microscopic evaluation of the pairing gap

We discuss the relevant progress that has been made in the last few years on the microscopic theory of the pairing correlation in nuclei and the open problems that still must be solved in order to reach a satisfactory description and understanding of the nuclear pairing. The similarities and differences with the nuclear matter case are emphasized and described by few illustrative examples. The comparison of calculations of different groups on the same set of nuclei show, besides agreements, also discrepancies that remain to be clarified. The role of the many-body correlations, like screening, that go beyond the BCS scheme, is still uncertain and requires further investigation.

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arXiv2009

Neutron star properties in density-dependent relativistic Hartree-Fock theory

With the equations of state provided by the newly developed density dependent relativistic Hartree-Fock (DDRHF) theory for hadronic matter, the properties of the static and $β$-equilibrium neutron stars without hyperons are studied for the first time, and compared to the predictions of the relativistic mean field (RMF) models and recent observational data. The influences of Fock terms on properties of asymmetric nuclear matter at high densities are discussed in details. Because of the significant contributions from the $σ$- and $ω$-exchange terms to the symmetry energy, large proton fractions in neutron stars are predicted by the DDRHF calculations, which strongly affect the cooling process of the star. The critical mass about 1.45 $M_\odot$, close to the limit 1.5 $M_\odot$ determined by the modern soft X-ray data analysis, is obtained by DDRHF with the effective interactions PKO2 and PKO3 for the occurrence of direct Urca process in neutron stars. The maximum masses of neutron stars given by the DDRHF calculations lie between 2.45 M$_\odot$ and 2.49 M$_\odot$, which are in reasonable agreement with high pulsar mass $2.08 \pm 0.19 M_\odot$ from PSR B1516+02B. It is also found that the mass-radius relations of neutron stars determined by DDRHF are consistent with the observational data from thermal radiation measurement in the isolated neutron star RX J1856, QPOs frequency limits in LMXBs 4U 0614+09 and 4U 1636-536, and redshift determined in LMXBs EXO 0748-676.

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arXiv2009

Neutron Stars in the Relativistic Hartree-Fock Theory and Hadron-Quark Phase Transition

Based on the density-dependent relativistic Hartree-Fock theory (DDRHF) for hadronic matter, the properties of neutron stars have been studied and compared with the results from the density-dependent relativistic mean field theory (DDRMF). Though similar equations of state are obtained, DDRHF calculations give larger fractions of proton, electron and muon at high baryon density for neutron star matter than the ones from DDRMF. The maximum masses of neutron stars lie between 2.3 M$_\odot$ and 2.5 M$_\odot$, and the corresponding radii between 11.7 km and 12.5 km. In addition, the phase transition from hadronic matter to quark matter in neutron stars is studied by using the MIT bag model for the quark phase. The transition is studied in both Gibbs and Maxwell constructions.

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arXiv2009

Spatial correlation properties of the anomalous density matrix in a slab of nuclear matter with realistic NN-forces

Spatial correlation characteristics of the anomalous density matrix $κ$ in a slab of nuclear matter with the Paris and Argonne v18 forces are calculated. A detailed comparison with predictions of the effective Gogny force is made. It is found that the two realistic forces lead to very close results which are qualitatively similar to those for the Gogny force. At the same time, the magnitude of $κ$ for realistic forces is essentially smaller than the one for the Gogny force. The correlation characteristics are practically independent of the magnitude of $κ$ and turn out to be quite close for the three kinds of the force. In particular, all of them predict a small value of the local correlation length at the surface of the slab and a big one, inside. These results are in agreement with those obtained recently by Pillet at al. for finite nuclei with the Gogny force.

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arXiv2008

Temperature dependence of single-particle properties in nuclear matter

The single-nucleon potential in hot nuclear matter is investigated in the framework of the Brueckner theory by adopting the realistic Argonne V18 or Nijmegen 93 two-body nucleon-nucleon interaction supplemented by a microscopic three-body force. The rearrangement contribution to the single-particle potential induced by the ground state correlations is calculated in terms of the hole-line expansion of the mass operator and provides a significant repulsive contribution in the low-momentum region around and below the Fermi surface. Increasing temperature leads to a reduction of the effect, while increasing density makes it become stronger. The three-body force suppresses somewhat the ground state correlations due to its strong short-range repulsion, increasing with density. Inclusion of the three-body force contribution results in a quite different temperature dependence of the single-particle potential at high enough densities as compared to that adopting the pure two-body force. The effects of three-body force and ground state correlations on the nucleon effective mass are also discussed.

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arXiv2008

Deconfinement phase transition in hybrid neutron stars from the Brueckner theory with three-body forces and a quark model with chiral mass scaling

We study the properties of strange quark matter in equilibrium with normal nuclear matter. Instead of using the conventional bag model in quark sector, we achieve the confinement by a density-dependent quark mass derived from in-medium chiral condensates. In nuclear matter, we adopt the equation of state from the Brueckner-Bethe-Goldstone approach with three-body forces. It is found that the mixed phase can occur, for a reasonable confinement parameter, near the normal nuclear saturation density, and goes over into pure quark matter at about 5 times the saturation. The onset of mixed and quark phases is compatible with the observed class of low-mass neutron stars, but it hinders the occurrence of kaon condensation.

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arXiv2006

Microscopic three-body forces and kaon condensation in cold neutrino-trapped matter

We investigate the composition and the equation of state of the kaon condensed phase in neutrino-free and neutrino-trapped star matter within the framework of the Brueckner-Hartree-Fock approach with three-body forces. We find that neutrino trapping shifts the onset density of kaon condensation to a larger baryon density, and reduces considerably the kaon abundance. As a consequence, when kaons are allowed, the equation of state of neutrino-trapped star matter becomes stiffer than the one of neutrino free matter. The effects of different three-body forces are compared and discussed. Neutrino trapping turns out to weaken the role played by the symmetry energy in determining the composition of stellar matter, and thus reduces the difference between the results obtained by using different three-body forces.

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