arXiv subjects

Stefano Gandolfi

Publications and source records attributed to Stefano Gandolfi.

At least 55 records · Page 3Linked to original sources

arXiv2015

Strangeness in nuclei and neutron stars: a challenging puzzle

The prediction of neutron stars properties is strictly connected to the employed nuclear interactions. The appearance of hyperons in the inner core of the star is strongly dependent on the details of the underlying hypernuclear force. We summarize our recent quantum Monte Carlo results on the development of realistic two- and three-body hyperon-nucleon interactions based on the available experimental data for light- and medium-heavy hypernuclei.

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arXiv2015

New insights on the hyperon puzzle from quantum Monte Carlo calculations

In the last years auxiliary field diffusion Monte Carlo has been used to assess the properties of hypernuclear systems, from light- to medium-heavy hypernuclei and hyper-neutron matter. One of the main findings is the key role played by the three-body hyperon-nucleon-nucleon interaction in the determination of the hyperon separation energy of hypernuclei and as a possible solution to the hyperon puzzle. However, there are still aspects of the employed hypernuclear potential that remain to be carefully investigated. In this paper we show that the isospin dependence of the Lambda-NN force, which is crucial in determining the NS structure, is poorly constrained by the available experimental data.

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arXiv2015

Hyperon Puzzle: Hints from Quantum Monte Carlo Calculations

The onset of hyperons in the core of neutron stars and the consequent softening of the equation of state have been questioned for a long time. Controversial theoretical predictions and recent astrophysical observations of neutron stars are the grounds for the so-called hyperon puzzle. We calculate the equation of state and the neutron star mass-radius relation of an infinite systems of neutrons and $Λ$ particles by using the auxiliary field diffusion Monte Carlo algorithm. We find that the three-body hyperon-nucleon interaction plays a fundamental role in the softening of the equation of state and for the consequent reduction of the predicted maximum mass. We have considered two different models of three-body force that successfully describe the binding energy of medium mass hypernuclei. Our results indicate that they give dramatically different results on the maximum mass of neutron stars, not necessarily incompatible with the recent observation of very massive neutron stars. We conclude that stronger constraints on the hyperon-neutron force are necessary in order to properly assess the role of hyperons in neutron stars.

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arXiv2015

Using Neutron Star Observations to Determine Crust Thicknesses, Moments of Inertia, and Tidal Deformabilities

We perform a systematic assessment of models for the equation of state (EOS) of dense matter in the context of recent neutron star mass and radius measurements to obtain a broad picture of the structure of neutron stars. We demonstrate that currently available neutron star mass and radius measurements provide strong constraints on moments of inertia, tidal deformabilities, and crust thicknesses. A measurement of the moment of inertia of PSR J0737-3039A with 10% error, without any other information from observations, will constrain the EOS over a range of densities to within 50%$-$60%. We find tidal deformabilities between 0.6 and $6\times 10^{36}$ g cm$^{2}$ s$^{2}$ (to 95% confidence) for $M=1.4~\mathrm{M}_{\odot}$, and any measurement which constrains this range will provide an important constraint on dense matter. The crustal fraction of the moment of inertia can be as large as 10% for $M=1.4~\mathrm{M}_{\odot}$ permitting crusts to have a large enough moment of inertia reservoir to explain glitches in the Vela pulsar even with a large amount of superfluid entrainment. Finally, due to the uncertainty in the equation of state, there is at least a 40% variation in the thickness of the crust for a fixed mass and radius, which implies that future simulations of the cooling of a neutron star crust which has been heated by accretion will need to take this variation into account.

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arXiv2015

Neutron Matter from Low to High Density

Neutron matter is an intriguing nuclear system with multiple connections to other areas of physics. Considerable progress has been made over the last two decades in exploring the properties of pure neutron fluids. Here we begin by reviewing work done to explore the behavior of very low density neutron matter, which forms a strongly paired superfluid and is thus similar to cold Fermi atoms, though at energy scales differing by many orders of magnitude. We then increase the density, discussing work that ties the study of neutron matter with the determination of the properties of neutron-rich nuclei and neutron-star crusts. After this, we review the impact neutron matter at even higher densities has on the mass-radius relation of neutron stars, thereby making contact with astrophysical observations.

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arXiv2014

From hypernuclei to the Inner Core of Neutron Stars: A Quantum Monte Carlo Study

Auxiliary Field Diffusion Monte Carlo (AFDMC) calculations have been employed to revise the interaction between $Λ$-hyperons and nucleons in hypernuclei. The scheme used to describe the interaction, inspired by the phenomenological Argonne-Urbana forces, is the $ΛN+ΛNN$ potential firstly introduced by Bodmer, Usmani et al.. Within this framework, we performed calculations on light and medium mass hypernuclei in order to assess the extent of the repulsive contribution of the three-body part. By tuning this contribution in order to reproduce the $Λ$ separation energy in $^5_Λ$He and $^{17}_{~Λ}$O, experimental findings are reproduced over a wide range of masses. Calculations have then been extended to $Λ$-neutron matter in order to derive an analogous of the symmetry energy to be used in determining the equation of state of matter in the typical conditions found in the inner core of neutron stars.

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arXiv2014

Accurate determination of the interaction between $Λ$ hyperons and nucleons from auxiliary field diffusion Monte Carlo calculations

An accurate assessment of the hyperon-nucleon interaction is of great interest in view of recent observations of very massive neutron stars. The challenge is to build a realistic interaction that can be used over a wide range of masses and in infinite matter starting from the available experimental data on the binding energy of light hypernuclei. To this end, accurate calculations of the hyperon binding energy in a hypernucleus are necessary. We present a quantum Monte Carlo study of $Λ$ and $ΛΛ$ hypernuclei up to $A=91$. We investigate the contribution of two- and three-body $Λ$-nucleon forces to the $Λ$ binding energy. Ground state energies are computed solving the Schrödinger equation for non-relativistic baryons by means of the auxiliary field diffusion Monte Carlo algorithm extended to the hypernuclear sector. We show that a simple adjustment of the parameters of the $ΛNN$ three-body force yields a very good agreement with available experimental data over a wide range of hypernuclear masses. In some cases no experiments have been performed yet, and we give new predictions. The newly fitted $ΛNN$ force properly describes the physics of medium-heavy $Λ$ hypernuclei, correctly reproducing the saturation property of the hyperon separation energy.

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arXiv2013

Quantum Monte Carlo study of strongly interacting Fermi gases

In recent years Quantum Monte Carlo techniques provided to be a valuable tool to study strongly interacting Fermi gases at zero temperature. We have used QMC methods to investigate several properties of the two-components Fermi gas at unitarity and in the BCS-BEC crossover, both with equal and unequal masses corresponding to the $Li-K$ Fermi mixture. In this paper we present several recent QMC results, including the energy at zero and finite effective range, the contact parameter and the static structure factor, which, at low momentum, depends strongly on the phonons in the unitary Fermi gas.

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arXiv2013

Neutral current interactions of low-energy neutrinos in dense neutron matter

We report the results of a calculation of the response of cold neutron matter to neutral-current interactions with low energy neutrinos, carried out using an effective interaction and effective operators consistently derived within the formalism of Correlated Basis Functions. The neutrino mean free path obtained from the calculated responses turns out to be strongly affected by both short and long range correlations, leading to a sizable increase with respect to the prediction of the Fermi gas model. The consistency between the proposed approach and Landau theory of normal Fermi liquids also has been investigated, using a set of Landau parameters obtained from the matrix elements of the effective interaction.

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arXiv2013

From nuclear structure to neutron stars

Recent progress in quantum Monte Carlo with modern nucleon-nucleon interactions have enabled the successful description of properties of light nuclei and neutron-rich matter. As a demonstration, we show that the agreement between theoretical calculations of the charge form factor of 12C and the experimental data is excellent. Applying similar methods to isospin-asymmetric systems allows one to describe neutrons confined in an external potential and homogeneous neutron-rich matter. Of particular interest is the nuclear symmetry energy, the energy cost of creating an isospin asymmetry. Combining these advances with recent observations of neutron star masses and radii gives insight into the equation of state of neutron-rich matter near and above the saturation density. In particular, neutron star radius measurements constrain the derivative of the symmetry energy.

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arXiv2013

Computational Nuclear Quantum Many-Body Problem: The UNEDF Project

The UNEDF project was a large-scale collaborative effort that applied high-performance computing to the nuclear quantum many-body problem. UNEDF demonstrated that close associations among nuclear physicists, mathematicians, and computer scientists can lead to novel physics outcomes built on algorithmic innovations and computational developments. This review showcases a wide range of UNEDF science results to illustrate this interplay.

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arXiv2013

Precise determination of the structure factor and contact in a unitary Fermi gas

We present a high-precision determination of the universal contact parameter in a strongly interacting Fermi gas. In a trapped gas at unitarity we find the contact to be $3.06 \pm 0.08$ at a temperature of 0.08 of the Fermi temperature in a harmonic trap. The contact governs the high-momentum (short-range) properties of these systems and this low temperature measurement provides a new benchmark for the zero temperature homogeneous contact. The experimental measurement utilises Bragg spectroscopy to obtain the dynamic and static structure factors of ultracold Fermi gases at high momentum in the unitarity and molecular Bose-Einstein condensate (BEC) regimes. We have also performed quantum Monte Carlo calculations of the static properties, extending from the weakly coupled Bardeen-Cooper-Schrieffer (BCS) regime to the strongly coupled BEC case, which show agreement with experiment at the level of a few percent.

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arXiv2012

Effective-Range Dependence of Resonantly Interacting Fermions

We extract the leading effective range corrections to the equation of state of the unitary Fermi gas from ab initio fixed-node quantum Monte Carlo (FNQMC) calculations in a periodic box using a density functional theory (DFT), and show them to be universal by considering several two-body interactions. Furthermore, we find that the DFT is consistent with the best available unbiased QMC calculations, analytic results, and experimental measurements of the equation of state. We also discuss the asymptotic effective-range corrections for trapped systems and present the first QMC results with the correct asymptotic scaling.

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arXiv2012

Quantum Monte Carlo approaches to nuclear and atomic physics

Quantum Monte Carlo methods have proven to be valuable in the study of strongly correlated quantum systems, particularly nuclear physics and cold atomic gases. Historically, such ab initio simulations have been used to study properties of light nuclei, including spectra and form factors, low-energy scattering, and high-momentum properties including inclusive scattering and one- and two-body momentum distributions. More recently they have been used to study the properties of homogeneous and inhomogeneous neutron matter and cold atomic gases. There are close analogies between these seemingly diverse systems, including the equation of state, superfluid pairing, and linear response to external probes. In this paper, we compare and contrast results found in nuclear and cold atom physics. We show updated lattice results for the energy of the homogeneous unitary Fermi gas and comparisons with neutron matter, as well as for the dependence of the cold atom energy on the mass ratio between paired particles, which yields insights on the structure of the ground state. We also provide new lattice and continuum results for the harmonically trapped unitary gas, again comparing neutron matter and cold atoms.

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arXiv2012

Quantum Monte Carlo study of inhomogeneous neutron matter

We present an ab-initio study of neutron drops. We use Quantum Monte Carlo techniques to calculate the energy up to 54 neutrons in different external potentials, and we compare the results with Skyrme forces. We also calculate the rms radii and radial densities, and we find that a re-adjustment of the gradient term in Skyrme is needed in order to reproduce the properties of these systems given by the ab-initio calculation. By using the ab-initio results for neutron drops for close- and open-shell configurations, we suggest how to improve Skyrme forces when dealing with systems with large isospin-asymmetries like neutron-rich nuclei.

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arXiv2012

The equation of state of neutron star matter and the symmetry energy

We present an overview of microscopical calculations of the Equation of State (EOS) of neutron matter performed using Quantum Monte Carlo techniques. We focus to the role of the model of the three-neutron force in the high-density part of the EOS up to a few times the saturation density. We also discuss the interplay between the symmetry energy and the neutron star mass-radius relation. The combination of theoretical models of the EOS with recent neutron stars observations permits us to constrain the value of the symmetry energy and its slope. We show that astrophysical observations are starting to provide important insights into the properties of neutron star matter.

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arXiv2011

Recent progress on the accurate determination of the equation of state of neutron and nuclear matter

The problem of accurately determining the equation of state of nuclear and neutron matter at density near and beyond saturation is still an open challenge. In this paper we will review the most recent progress made by means of Quantum Monte Carlo calculations, which are at present the only ab-inito method capable to treat a sufficiently large number of particles to give meaningful estimates depending only on the choice of the nucleon-nucleon interaction. In particular, we will discuss the introduction of density-dependent interactions, the study of the temperature dependence of the equation of state, and the possibility of accurately studying the effect of the onset of hyperons by developing an accurate hyperon-nucleon and hyperon-nucleon-nucleon interaction.

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arXiv2011

Auxiliary Field quantum Monte Carlo for Strongly Paired Fermions

We demonstrate that the inclusion of a BCS importance function dramatically increases the efficiency of the auxiliary field method for strong pairing. We calculate the ground-state energy of an unpolarized fermi gas at unitarity with up to 66 particles and lattices of up to $27^3$ sites. The method has no fermion sign problem, and an accurate result is obtained for the universal parameter $ξ$. Several different forms of the kinetic energy adjusted to the unitary limit but with different effective ranges extrapolate to the same continuum limit within error bars. The finite effective range results for different interactions are consistent with a linear term proportional to the Fermi momentum times the effective range. The new method described herein will have many applications in superfluid cold atom systems and in both electronic and nuclear structures when pairing is important.

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