arXiv subjects

M. Baldo

Publications and source records attributed to M. Baldo.

At least 91 records · Page 5Linked to original sources

arXiv2001

The Local Potential Approximation for the Brueckner G-matrix

The Brueckner G-matrix for a slab of nuclear matter is analyzed in the singlet $^1S$ and triplet $^3S+^3D$ channels. The complete Hilbert space is split into two domains, the model subspace $S_0$, in which the two-particle propagator is calculated explicitly, and the complementary one, $S'$, in which the local potential approximation is used. This kind of local approximation was previously found to be quite accurate for the $^1S$ pairing problem. A set of model spaces $S_0(E_0)$ with different values of the cut-off energy $E_0$ is considered, $E_0$ being the upper limit for the single-particle energies of the states belonging to $S_0$. The independence of the G-matrix of $E_0$ is assumed as a criterion of validity of the local potential approximation. Such independence is obtained within few percent for $E_0=10 ÷20$ MeV for both the channels under consideration.

nucl-th↗

arXiv2000

Strange hadronic stellar matter within the Brueckner-Bethe-Goldstone theory

In the framework of the non-relativistic Brueckner-Bethe-Goldstone theory, we derive a microscopic equation of state for asymmetric and $β$-stable matter containing $Σ^-$ and $Λ$ hyperons. We mainly study the effects of three-body forces (TBFs) among nucleons on the hyperon formation and the equation of state (EoS). We find that, when TBFs are included, the stellar core is almost equally populated by nucleons and hyperons. The resulting EoS, which turns out to be extremely soft, has been used in order to calculate the static structure of neutron stars. We obtain a value of the maximum mass of 1.26 solar masses (1 solar mass $M_o \simeq 1.99 \cdot 10^{33} g$). Stellar rotations increase this value by about 12%.

nucl-th↗

arXiv2000

Single particle spectrum and binding energy of nuclear matter

In non-relativistic Brueckner calculations of nuclear matter, the self-consistent single particle potential is strongly momentum dependent. To simplify the calculations, a parabolic approximation is often used in the literature. The variation in the binding energy value introduced by the parabolic approximation is quantitatively analyzed in detail. It is found that the approximation can introduce an uncertainty of 1-2 MeV near the saturation density.

nucl-th↗

arXiv2000

Dispersive effects in neutron matter superfluidity

The explicit energy dependence of the single particle self-energy (dispersive effects), due to short range correlations, is included in the treatment of neutron matter superfluidity. The method can be applied in general to strong interacting fermion systems, and it is expected to be valid whenever the pairing gap is substantially smaller than the Fermi kinetic energy. The results for neutron matter show that dispersive effects are strong in the density region near the gap closure.

nucl-th↗

arXiv1999

Hyperon stars in the Brueckner-Bethe-Goldstone theory

In the framework of the Brueckner-Bethe-Goldstone theory, we determine a fully microscopic equation of state for asymmetric and $β$-stable nuclear matter containing $\sim$ and $\la$ hyperons. We use the Paris and the new Argonne $Av_{18}$ two-body nucleon interaction, whereas the nucleon-hyperon interaction is described by the Njimegen soft-core model. We stress the role played by the three-body nucleon interaction, which produces a strong repulsion at high densities. This enhances enormously the hyperon population, and produces a strong softening of the equation of state, which turns out almost independent on the nucleon-nucleon interaction. We use the new equation of state in order to calculate the structure of static neutron stars. We obtain a maximum mass configuration with $M_{\rm max}$ = 1.26 (1.22) when the Paris ($Av_{18}$) nucleon potential is adopted. Central densities are about 10 times normal nuclear matter density. Stellar rotations, treated within a perturbative approach, increase the value of the limiting mass by about 12%.

nucl-th↗

arXiv1999

A simple model for the microscopic effective pairing interaction

The microscopic effective pairing interaction in the $^1S_0$-channel is investigated for two different values of the chemical potential $μ$ starting from the separable form of the Paris NN-potential. It is shown that, within a high accuracy, this effective interaction can be approximated by the off-shell free T-matrix taken at the negative energy $E=2μ$.

nucl-th↗

arXiv1998

Surface behaviour of the pairing gap in semi-infinite nuclear matter

The $^1S_0$-pairing gap in semi-infinite nuclear matter is evaluated microscopically using the effective pairing interaction recently found explicitly in the coordinate representation starting from the separable form of the Paris NN-potential. Instead of direct iterative solution of the gap equation, a new method proposed by V.A.Khodel, V.V.Khodel and J.W.Clark was used which simplifies the procedure significantly. The gap $Δ$ obtained in our calculations exibits a strong variation in the surface region with a pronounced maximum near the surface.

nucl-th↗

arXiv1998

One-body dissipation and chaotic dynamics in a classical simulation of a nuclear gas

In order to understand the origin of one-body dissipation in nuclei, we analyze the behavior of a gas of classical particles moving in a two-dimensional cavity with nuclear dimensions. This "nuclear" billiard has multipole-deformed walls which undergo periodic shape oscillations. We demonstrate that a single particle Hamiltonian containing coupling terms between the particles' motion and the collective coordinate induces a chaotic dynamics for any multipolarity, independently on the geometry of the billiard. If the coupling terms are switched off the "wall formula" predictions are recovered. We discuss the dissipative behavior of the wall motion and its relation with the order-to-chaos transition in the dynamics of the microscopic degrees of freedom.

nucl-th↗

arXiv1998

Multi-connected Momentum Distribution and Fermion Condensation

The structure of the ground state beyond the instability point of the quasiparticle system with Fermi-step momentum distribution is studied within the model of a Fermi liquid with a strong repulsive interaction. A ground state rearrangement occurs as the interaction strength is increased beyond a definite critical value. Numerical investigation of the initial stage of this structural transition shows that there are two temperature regions, corresponding to different scenarios of the rearrangement. While for temperature T larger than some characteristic temperature T_0 the behaviour of the system is the same as that in the case of the fermion condensation, for T<T_0 the intermediate structure with multi-connected quasiparticle momentum distribution arises. The transition of this structure to the fermion condensate at increasing interaction strength is discussed.

cond-mat↗

arXiv1998

Generalized entropy and temperature in nuclear multifragmentation

In the framework of a 2D Vlasov model, we study the time evolution of the "coarse-grained" Generalized Entropy (GE) in a nuclear system which undergoes a multifragmentation (MF) phase transition. We investigate the GE both for the gas and the fragments (surface and bulk part respectively). We find that the formation of the surface causes the growth of the GE during the process of fragmentation. This quantity then characterizes the MF and confirms the crucial role of deterministic chaos in filling the new available phase-space: at variance with the exact time evolution, no entropy change is found when the linear response is applied. Numerical simulations were used also to extract information about final temperatures of the fragments. From a fitting of the momentum distribution with a Fermi-Dirac function we extract the temperature of the fragments at the end of the process. We calculate also the gas temperature by averaging over the available phase space. The latter is a few times larger than the former, indicating a gas not in equilibrium. Though the model is very schematic, this fact seems to be very general and could explain the discrepancy found in experimental data when using the slope of light particles spectra instead of the double ratio of isotope yields method in order to extract the nuclear caloric curve.

nucl-th↗

arXiv1998

3P_2-3F_2 pairing in neutron matter with modern nucleon-nucleon potentials

We present results for the $^3P_2 - ^3F_2$ pairing gap in neutron matter with several realistic nucleon-nucleon potentials, in particular with recent, phase-shift equivalent potentials. We find that their predictions for the gap cannot be trusted at densities above $ρ\approx 1.7ρ_0$, where $ρ_0$ is the saturation density for symmetric nuclear matter. In order to make predictions above that density, potential models which fit the nucleon-nucleon phase shifts up to about 1 GeV are required.

nucl-th↗

arXiv1998

Neutron star properties with nuclear BHF equations of state

We study the properties of static and rotating neutron stars adopting non-relativistic equations of state (EOS) for asymmetric nuclear matter based on the Brueckner-Hartree-Fock (BHF) scheme. The BHF calculation, with the continuous choice for the single particle potential, appears to be very close to the full EOS, which includes the three-hole line contribution calculated by solving the Bethe-Fadeev equations within the gap choice for the single particle potential. Three-body forces are included in order to reproduce the correct saturation point for nuclear matter. A comparison with fully relativistic many-body calculations of nuclear matter EOS is made.

nucl-th↗

arXiv1997

Microscopic nuclear equation of state with three-body forces and neutron star structure

We calculate static properties of non-rotating neutron stars (NS's) using a microscopic equation of state (EOS) for asymmetric nuclear matter, derived from the Brueckner-Bethe-Goldstone many-body theory with explicit three-body forces. We use the Argonne AV14 and the Paris two-body nuclear force, implemented by the Urbana model for the three-body force. We obtain a maximum mass configuration with $ M_{max} = 1.8 M_{\sun}$ ($M_{max} = 1.94 M_{\sun}$) when the AV14 (Paris) interaction is used. They are both consistent with the observed range of NS masses. The onset of direct Urca processes occurs at densities $n \geq 0.65~fm^{-3}$ for the AV14 potential and $n \geq 0.54~fm^{-3}$ for the Paris potential. Therefore, NS's with masses above $M^{Urca} = 1.4 M_{\sun}$ for the AV14 and $M^{Urca} = 1.24 M_{\sun}$ for the Paris potential can undergo very rapid cooling, depending on the strength of superfluidity in the interior of the NS. The comparison with other microscopic models for the EOS shows noticeable differences.

astro-ph↗

arXiv1996

Microscopic Nuclear Equation of State with Three-Body Forces and Neutron Star Structure

We calculate static properties of non-rotating neutron stars (NS's) using a microscopic equation of state (EOS) for asymmetric nuclear matter. The EOS is computed in the framework of the Brueckner--Bethe--Goldstone many--body theory. We introduce three-body forces in order to reproduce the correct saturation point of nuclear matter. A microscopic well behaved EOS is derived. We obtain a maximum mass configuration with $M_{max} = 1.8 M_\odot$, a radius $R = 9.7$ km and a central density $n_c = 1.34~fm^{-3}$. We find the proton fraction exceeds the critical value $x^{Urca}$, for the onset of direct Urca processes, at densities $n \geq 0.45~fm^{-3}$. Therefore, in our model, NS's with masses above $M^{Urca} = 0.96 M_\odot$ can undergo very rapid cooling depending on whether or not nucleon superfluidity in the interior of the NS takes place. A comparison with other microscopic models for the EOS is done, and neutron star structure is calculated for these models too.

nucl-th↗

arXiv1996

Dynamical chaos and critical behavior in Vlasov simulations of nuclear multifragmentation

We discuss the presence of both dynamical chaos and signals of a second--order phase transition in numerical Vlasov simulations of nuclear multifragmentation. We find that chaoticity and criticality are strongly related and play a crucial role in the process of fragments formation. This connection is not limited to our model and seems a rather general feature.

nucl-th↗

arXiv1996

Chaoticity and Dissipation of Nuclear Collective Motion in a Classical Model

We analyze the behavior of a gas of classical particles moving in a two-dimensional "nuclear" billiard whose multipole-deformed walls undergo periodic shape oscillations. We demonstrate that a single particle Hamiltonian containing coupling terms between the particles' motion and the collective coordinate induces a chaotic dynamics for any multipolarity, independently on the geometry of the billiard. The absence of coupling terms allows us to recover qualitatively the "wall formula" predictions. We also discuss the dissipative behavior of the wall motion and its relation with the order-to-chaos transition in the dynamics of the microscopic degrees of freedom.

nucl-th↗

arXiv1995

Chaos vs. Linear Instability in the Vlasov Equation: A Fractal Analysis Characterization

In this work we discuss the most recent results concerning the Vlasov dynamics inside the spinodal region. The chaotic behaviour which follows an initial regular evolution is characterized through the calculation of the fractal dimension of the distribution of the final modes excited. The ambiguous role of the largest Lyapunov exponent for unstable systems is also critically reviewed.

nucl-th↗

arXiv1995

Building scars for integrable systems

It is shown, by means of a simple specific example, that for integrable systems it is possible to build up approximate eigenfunctions, called {\it asymptotic eigenfunctions}, which are concentrated as much as one wants to a classical trajectory and have a lifetime as long as one wants. These states are directly related to the presence of shell structures in the quantal spectrum of the system. It is argued that the result can be extended to classically chaotic system, at least in the asymptotic regime.

quant-ph↗