arXiv subjects

S. Fantoni

Publications and source records attributed to S. Fantoni.

At least 19 recordsLinked to original sources

arXiv2025

Contrast-enhanced X-ray imaging of articular cartilage: reliability of a cationic contrast agent in combination with high-resolution peripheral quantitative computed tomography system

Articular cartilage showcases distinctive mechanical behaviour, attributable to its biphasic composition and hierarchical organization. Proteoglycans, essential constituents of the extracellular matrix, contribute to tissue swelling, stiffness, and viscoelasticity, thanks to the fixed charge density. Degenerative alterations in proteoglycan content and collagen structure - key indicators of diseases such as osteoarthritis - compromise the mechanical integrity of articular cartilage, underscoring the importance of early detection. Although magnetic resonance imaging provides valuable compositional information, its limited spatial resolution restricts its effectiveness in evaluating thin soft tissues such as articular cartilage. As an alternative, contrast-enhanced X-ray imaging circumvents such limitation, resorting to the use of radiopaque contrast agents. In this context, cationic contrast agents, like CA4+, enable the quantitative assessment of proteoglycan content via electrostatic attraction. High-resolution peripheral quantitative computed tomography offers an optimal compromise between spatial resolution and radiation exposure, making it a promising tool for clinical use. This study aimed to explore the relationship between proteoglycan content - quantified through contrast-enhanced high-resolution peripheral quantitative computed tomography combined with the use of CA4+ - and the viscoelastic properties of healthy bovine articular cartilage, assessed via indentation testing. Results highlighted significant correlations between the enhanced radiopacity of articular cartilage, ascribable to the PG content, with parameters representing the mechanical response of the tissue.

physics.med-ph↗

arXiv2010

Microscopic calculation of the equation of state of nuclear matter and neutron star structure

We present results for neutron star models constructed with a new equation of state for nuclear matter at zero temperature. The ground state is computed using the Auxiliary Field Diffusion Monte Carlo (AFDMC) technique, with nucleons interacting via a semi-phenomenological Hamiltonian including a realistic two-body interaction. The effect of many-body forces is included by means of additional density-dependent terms in the Hamiltonian. In this letter we compare the properties of the resulting neutron-star models with those obtained using other nuclear Hamiltonians, focusing on the relations between mass and radius, and between the gravitational mass and the baryon number.

nucl-th↗

arXiv2009

Equation of state of low--density neutron matter and the $^1S_0$ pairing gap

We report results of the equation of state of neutron matter in the low--density regime, where the Fermi wave vector ranges from $0.4 fm^{-1} \leq k_F \leq 1.0 fm^{-1}$. Neutron matter in this regime is superfluid because of the strong and attractive interaction in the $^1S_0$ channel. The properties of this superfluid matter are calculated starting from a realistic Hamiltonian that contains modern two-- and three--body interactions. The ground state energy and the $^1S_0$ superfluid energy gap are calculated using the Auxiliary Field Diffusion Monte Carlo method. We study the structure of the ground state by looking at pair distribution functions as well as the Cooper-pair wave function used in the calculations.

nucl-th↗

arXiv2009

Quantum Monte Carlo calculation of the equation of state of neutron matter

We calculate the equation of state of neutron matter at zero temperature by means of the auxiliary field diffusion Monte Carlo method (AFDMC) combined with a fixed-phase approximation. The calculation of the energy is carried out by simulating up to 114 neutrons in a periodic box. Special attention was made to reduce finite size effects at the energy evaluation by adding to the interaction the effect due to the truncation of the simulation box, and by performing several simulations using different number of neutrons. The finite size effects due to the kinetic energy were also checked by employing the twist--averaged boundary conditions. We considered a realistic nuclear Hamiltonian containing modern two-- and three--body interactions of the Argonne and Urbana family. The equation of state can be used to compare and to calibrate other many-body calculations and to predict properties of neutron stars.

nucl-th↗

arXiv2008

Critical temperature of non-interacting Bose gases on disordered lattices

For a non-interacting Bose gas on a lattice we compute the shift of the critical temperature for condensation when random-bond and onsite disorder are present. We evidence that the shift depends on the space dimensionality D and the filling fraction f. For D -> infinity (infinite-range model), using results from the theory of random matrices, we show that the shift of the critical temperature is negative, depends on f, and vanishes only for large f. The connections with analogous results obtained for the spherical model are discussed. For D=3 we find that, for large f, the critical temperature Tc is enhanced by disorder and that the relative shift does not sensibly depend on f; at variance, for small f, Tc decreases in agreement with the results obtained for a Bose gas in the continuum. We also provide numerical estimates for the shift of the critical temperature due to disorder induced on a non-interacting Bose gas by a bichromatic incommensurate potential.

cond-mat.stat-mech↗

arXiv2008

Equation of state of superfluid neutron matter and the calculation of $^1S_0$ pairing gap

We present a Quantum Monte Carlo study of the zero temperature equation of state of neutron matter and the computation of the $^1S_0$ pairing gap in the low-density regime with $ρ<0.04$ fm$^{-3}$. The system is described by a non-relativistic nuclear Hamiltonian including both two-- and three--nucleon interactions of the Argonne and Urbana type. This model interaction provides very accurate results in the calculation of the binding energy of light nuclei. A suppression of the gap with respect to the pure BCS theory is found, but sensibly weaker than in other works that attempt to include polarization effects in an approximate way.

nucl-th↗

arXiv2007

Auxiliary Field Diffusion Monte Carlo calculation of nuclei with A<40 with tensor interactions

We calculate the ground-state energy of 4He, 8He, 16O, and 40Ca using the auxiliary field diffusion Monte Carlo method in the fixed phase approximation and the Argonne v6' interaction which includes a tensor force. Comparison of our light nuclei results to those of Green's function Monte Carlo calculations shows the accuracy of our method for both open and closed shell nuclei. We also apply it to 16O and 40Ca to show that quantum Monte Carlo methods are now applicable to larger nuclei.

nucl-th↗

arXiv2006

QMC calculations of symmetric nuclear matter

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.

nucl-th↗

arXiv2006

Quantum Monte Carlo Algorithm Based on Two-Body Density Functional Theory for Fermionic Many-Body Systems: Application to 3He

We construct a quantum Monte Carlo algorithm for interacting fermions using the two-body density as the fundamental quantity. The central idea is mapping the interacting fermionic system onto an auxiliary system of interacting bosons. The correction term is approximated using correlated wave functions for the interacting system, resulting in an effective potential that represents the nodal surface. We calculate the properties of 3He and find good agreement with experiment and with other theoretical work. In particular, our results for the total energy agree well with other calculations where the same approximations were implemented but the standard quantum Monte Carlo algorithm was used

cond-mat.stat-mech↗

arXiv2005

Computational spectroscopy of helium-solvated molecules: effective inertia, from small He clusters toward the nano-droplet regime

Accurate computer simulations of the rotational dynamics of linear molecules solvated in He clusters indicate that the large-size (nano-droplet) regime is attained quickly for light rotors (HCN, CO) and slowly for heavy ones (OCS, N$_2$O, CO$_2$), thus challenging previously reported results. Those results spurred the view that the different behavior of light rotors with respect to heavy ones - including a smaller reduction of inertia upon solvation of the former - would result from the lack of adiabatic following of the He density upon molecular rotation. We have performed computer experiments in which the rotational dynamics of OCS and HCN molecules was simulated using a fictitious inertia appropriate to the other molecule. These experiments indicate that the approach to the nano-droplet regime, as well as the reduction of the molecular inertia upon solvation, is determined by the anistropy of the potential, more than by the molecular weight. Our findings are in agreement with recent infrared and/or microwave experimental data which, however, are not yet totally conclusive by themselves.

cond-mat.other↗

arXiv2005

The spectra of mixed $^3$He-$^4$He droplets

The diffusion Monte Carlo technique is used to calculate and analyze the excitation spectrum of $^3$He atoms bound to a cluster of $^4$He atoms, by using a previously determined optimum filling of single-fermion orbits with well defined orbital angular momentum $L$, spin $S$ and parity quantum numbers. The study concentrates on the energies and shapes of the three kinds of states for which the fermionic part of the wave function is a single Slater determinant: maximum $L$ or maximum $S$ states within a given orbit, and fully polarized clusters. The picture that emerges is that of systems with strong shell effects whose binding and excitation energies are essentially determined over configuration at fixed number of particles and spin, i.e., by the monopole properties of an effective Hamiltonian.

physics.atm-clus↗

arXiv2004

Auxiliary Field Diffusion Monte Carlo calculation of ground state properties of neutron drops

The Auxiliary Field Diffusion Monte Carlo method has been applied to simulate droplets of 7 and 8 neutrons. Results for realistic nucleon-nucleon interactions, which include tensor, spin--orbit and three--body forces, plus a standard one--body confining potential, have been compared with analogous calculations obtained with Green's Function Monte Carlo methods. We have studied the dependence of the binding energy, the one--body density and the spin--orbit splittings of $^7n$ on the depth of the confining potential. The results obtained show an overall agreement between the two quantum Monte Carlo methods, although there persist differences in the evaluation of spin--orbit forces, as previously indicated by bulk neutron matter calculations. Energy density functional models, largely used in astrophysical applications, seem to provide results significantly different from those of quantum simulations. Given its scaling behavior in the number of nucleons, the Auxiliary Field Diffusion Monte Carlo method seems to be one of the best candidate to perform {\sl ab initio} calculations on neutron rich nuclei.

nucl-th↗

arXiv2004

Excitation spectra of a 3He impurity on 4He clusters

The diffusion Monte Carlo technique is used to calculate and analyze the excitation spectrum of a single 3He atom bound to a cluster with N 4He atoms, with the aim of establishing the most adequate filling ordering of single-fermion orbits to the mixed clusters with a large number of 3He atoms. The resulting ordering looks like the rotational spectrum of a diatomic molecule, being classified only by the angular momentum of the level, although vibrational-like excitations appear at higher energies for sufficiently large N.

cond-mat.soft↗

arXiv2003

Hadron Multiplicity in Semi-Inclusive Lepton-Nucleon and Lepton-Nucleus Scattering

We discuss multi-hadron production in both inelastic neutrino-nucleon interactions in the current fragmentation region and neutrino-nucleus collisions in the target fragmentation region. Our analysis, carried out within the framework of the quark-gluon string model, is mainly focused on the difference between these two processes. We show that the $Q^2$ dependence of hadron multiplicity in the current and target fragmentation regions is indeed completely different. The study of inelastic $ν-A$ scattering in the target fragmentation region also provides new information on nuclear structure at small $N-N$ distances. The results of the proposed approach are in satisfactory agreement with the data recently obtained at CERN by the NOMAD Collaboration.

nucl-th↗

arXiv2003

Spin-orbit induced backflow in neutron matter with auxiliary field diffusion Monte Carlo

The energy per particle of zero-temperature neutron matter is investigated, with particular emphasis on the role of the $\vec L\cdot\vec S$ interaction. An analysis of the importance of explicit spin--orbit correlations in the description of the system is carried out by the auxiliary field diffusion Monte Carlo method. The improved nodal structure of the guiding function, constructed by explicitly considering these correlations, lowers the energy. The proposed spin--backflow orbitals can conveniently be used also in Green's Function Monte Carlo calculations of light nuclei.

nucl-th↗

arXiv2003

Neutron matter at zero temperature with auxiliary field diffusion Monte Carlo

The recently developed auxiliary field diffusion Monte Carlo method is applied to compute the equation of state and the compressibility of neutron matter. By combining diffusion Monte Carlo for the spatial degrees of freedom and auxiliary field Monte Carlo to separate the spin-isospin operators, quantum Monte Carlo can be used to simulate the ground state of many nucleon systems $(A\alt 100)$. We use a path constraint to control the fermion sign problem. We have made simulations for realistic interactions, which include tensor and spin--orbit two--body potentials as well as three-nucleon forces. The Argonne $v_8'$ and $v_6'$ two nucleon potentials plus the Urbana or Illinois three-nucleon potentials have been used in our calculations. We compare with fermion hypernetted chain results. We report results of a Periodic Box--FHNC calculation, which is also used to estimate the finite size corrections to our quantum Monte Carlo simulations. Our AFDMC results for $v_6$ models of pure neutron matter are in reasonably good agreement with equivalent Correlated Basis Function (CBF) calculations, providing energies per particle which are slightly lower than the CBF ones. However, the inclusion of the spin--orbit force leads to quite different results particularly at relatively high densities. The resulting equation of state from AFDMC calculations is harder than the one from previous Fermi hypernetted chain studies commonly used to determine the neutron star structure.

nucl-th↗

arXiv2003

Deuteron distribution in nuclei and the Levinger's factor

We compute the distribution of quasideuterons in doubly closed shell nuclei. The ground states of $^{16}$O and $^{40}$Ca are described in $ls$ coupling using a realistic hamiltonian including the Argonne $v_{8}^\prime$ and the Urbana IX models of two-- and three--nucleon potentials, respectively. The nuclear wave function contains central and tensor correlations, and correlated basis functions theory is used to evaluate the distribution of neutron-proton pairs, having the deuteron quantum numbers, as a function of their total momentum. By computing the number of deuteron--like pairs we are able to extract the Levinger's factor and compare to both the available experimental data and the predictions of the local density approximation, based on nuclear matter estimates. The agreement with the experiments is excellent, whereas the local density approximation is shown to sizably overestimate the Levinger's factor in the region of the medium nuclei.

nucl-th↗

arXiv2002

Number--conserving model for boson pairing

An independent pair ansatz is developed for the many body wavefunction of dilute Bose systems. The pair correlation is optimized by minimizing the expectation value of the full hamiltonian (rather than the truncated Bogoliubov one) providing a rigorous energy upper bound. In contrast with the Jastrow model, hypernetted chain theory provides closed-form exactly solvable equations for the optimized pair correlation. The model involves both condensate and coherent pairing with number conservation and kinetic energy sum rules satisfied exactly and the compressibility sum rule obeyed at low density. We compute, for bulk boson matter at a given density and zero temperature, (i) the two--body distribution function, (ii) the energy per particle, (iii) the sound velocity, (iv) the chemical potential, (v) the momentum distribution and its condensate fraction and (vi) the pairing function, which quantifies the ODLRO resulting from the structural properties of the two--particle density matrix. The connections with the low--density expansion and Bogoliubov theory are analyzed at different density values, including the density and scattering length regime of interest of trapped-atoms Bose--Einstein condensates. Comparison with the available Diffusion Monte Carlo results is also made.

cond-mat↗