arXiv subjects

Panagiota Papakonstantinou

Publications and source records attributed to Panagiota Papakonstantinou.

29 records · Page 2Linked to original sources

arXiv2021

Density dependence of the nuclear symmetry energy and neutron skin thickness in the KIDS framework

The KIDS framework for the nuclear equation of state (EoS) and energy density functional (EDF) offers the possibility to explore systematically the effect of EoS parameters on predictions for a variety of observables. The EoS parameters can be varied independently of each other and independently of assumptions regarding the in-medium nucleon effective mass. Here I present a pilot study of the neutron skin thickness (NST) in nuclei of current interest. The results indicate that variations of the symmetry energy slope parameter L by roughly 10 MeV and variations of the droplet-model counterpart of the curvature parameter $K_τ$ by roughly 20 MeV affect predictions by comparable amounts. However, structural details may also have sizable effects on predictions, notably in the cases of $^{68}$Ni and $^{208}$Pb. This work is part of a systematic investigation of the NST within the KIDS framework and of a broader effort to constrain the density dependence of the nuclear symmetry energy.

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arXiv2021

Constraints on the curvature of nuclear symmetry energy from recent astronomical data within the KIDS framework

We investigate the density dependence of the nuclear symmetry energy $S(ρ) $ in the KIDS (Korea-IBS-Daegu-SKKU) framework for the nuclear equation of state (EoS) and energy-density functional (EDF). The aim is to constrain the value of the curvature parameter ($K_{\rm sym}$) based on recent astronomical data. First, assuming a standard saturation point, we calculate bulk nuclear properties within KIDS-EDF for different values of the compression modulus of symmetric nuclear matter ($K_0$) and of the leading-order symmetry energy parameters, i.e., the symmetry energy ($J$) and slope ($L$) at saturation density, each within a broadly accepted range, as well as $K_{\rm sym}$. All of the above EoS parameters are varied independently of each other. The skewness parameter ($Q_{\rm sym}$) is presently kept fixed at 650 MeV. For all EoS parameter sets which describe the selected nuclear data within better than $0.3\%$, we calculate the neutron-star equation of state and mass-radius relation and analyze the results in terms of Pearson correlation coefficients $r$. We find that the value of $K_{\rm sym}$ is strongly correlated with the radius of both a canonical and a massive star ($|r|>0.9$). If we impose that all known astronomical constraints on the neutron star radii must be satisfied, we deduce $-150 < K_{\rm sym}<0$. As a result, the symmetry energy as a function of the density is consistently found to have an inflection point at $ρ_0<ρ<2ρ_0$. We take the opportunity to report that the neutron skin thickness of $^{208}$Pb shows no correlation at all with the neutron star radii ($|r|<0.1$), in contrast with studies which focus on the role of $L$ only.

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arXiv2021

Daejeon16 interaction with contact-term corrections for heavy nuclear systems

The Daejeon16 two-nucleon interaction is employed in many-body approaches based on the mean-field approximation. The perturbative character of Daejeon16 is verified by comparing results for 16O from the Hartree-Fock (HF) approximation and from the no-core shell model and by examining the magnitude of perturbative corrections to the HF energy in light and heavy nuclei. In order to approximately describe energies and radii across the nuclear chart, a phenomenological correction in the form of a two-plus-three-nucleon contact interaction is introduced. With fitted parameters we achieve a very good description of medium-mass nuclei in terms of energy and size and also in terms of the centroid energy of the giant monopole resonance and the dipole polarizability calculated within the random-phase approximation (RPA). Our results provide further justification for the use of Daejeon16 augmented with phenomenological corrections as an effective interaction of perturbative character in a variety of applications.

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arXiv2021

Constraining the density dependence of the symmetry energy with nuclear data and astronomical observations in the KIDS framework

The KIDS framework for the nuclear equation of state (EoS) and energy density functional (EDF) offers the possibility to explore symmetry-energy (SE) parameters such as J (value at saturation density), L (slope), Ksym (curvature) and so on independently of each other and of assumptions about the effective mass. Here we examine the performance of EoSs with different SE parameters in reproducing nuclear properties and astronomical observations in an effort to constrain especially L and Ksym or the droplet-model counterpart Ktau. Assuming a standard EoS for symmetric matter, we explore several points on the hyperplane of (J,L,Ksym or Ktau) values. For each point, the corresponding EDF parameters and a pairing parameter are obtained for applications in spherical even-even nuclei. This is the first application of KIDS EDFs with pairing correlations. The EoSs are tested successively on properties of closed-shell nuclei, along the Sn isotopic chain, and on astronomical observations, in a step-by-step process of elimination and correction. A small regime of best-performing parameters is determined. The results strongly suggest that Ksym is negative and no lower than -200MeV, that Ktau lies between roughly -400 and -300MeV and that L lies between 40 and 65MeV with L<55MeV more likely. Correlations between symmetry-energy parameters are critically discussed. Predictions for the position of the neutron drip line and the neutron skin thickness of selected nuclei are reported. They are only weakly affected by the choice of effective mass values. Parts of the drip line can be sensitive to the SE parameters. The results underscore the role of Ktau and of precise astronomical input. Better constraints are possible with precise fits to nuclear energies and, in the future, more-precise input from astronomy.

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arXiv2020

Deformed relativistic Hartree-Bogoliubov theory in continuum with point coupling functional: examples of even-even Nd isotopes

The aim of this work is to develop the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) theory based on the point-coupling density functionals and extend it to provide a unified description for all even-even nuclei in the nuclear chart by overcoming all possible challenges. The nuclear superfluidity is considered via Bogoliubov transformation. Densities and potentials are expanded in terms of Legendre polynomials to include the axial deformation degrees of freedom. Sophisticated relativistic Hartree-Bogoliubov equations in coordinate space are solved in the DiracWoods-Saxon basis to consider the continuum effects. Numerical checks are performed from light nuclei to heavy nuclei. The techniques to construct the DRHBc mass table for even-even nuclei are explored. The DRHBc theory is extended to study heavier nuclei beyond magnesium isotopes. Taking Nd isotopes as examples, the experimental binding energies, two-neutron separation energies, quadrupole deformations, and charge radii are reproduced rather well. The deformation and continuum play essential roles in the description of nuclear masses and prediction of drip-line nuclei. By examining the single-particle levels in the canonical basis and their contributions to the total density, the thickness of the neutron skin, the particles number in continuum, and the Coulomb barrier, the exotic structures including the neutron skin and the proton radioactivity are predicted.

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arXiv2020

Neutron drip line in the deformed relativistic Hartree-Bogoliubov theory in continuum: Oxygen to Calcium

The location of the neutron drip line, currently known for only the lightest elements, remains a fundamental question in nuclear physics. Its description is a challenge for microscopic nuclear energy density functionals, as it must take into account in a realistic way not only the nuclear potential, but also pairing correlations, deformation effects and coupling to the continuum. The recently developed deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) aims to provide a unified description of even-even nuclei throughout the nuclear chart. Here, the DRHBc with the successful density functional PC-PK1 is used to investigate whether and how deformation influences the prediction for the neutron drip-line location for even-even nuclei with 8<=Z<=20, where many isotopes are predicted deformed. The results are compared with those based on the spherical relativistic continuum Hartree-Bogoliubov (RCHB) theory and discussed in terms of shape evolution and the variational principle. It is found that the Ne and Ar drip-line nuclei are different after the deformation effect is included. The direction of the change is not necessarily towards an extended drip line, but rather depends on the evolution of the degree of deformation towards the drip line. Deformation effects as well as pairing and continuum effects treated in a consistent way can affect critically the theoretical description of the neutron drip-line location.

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arXiv2019

From homogeneous matter to finite nuclei: Role of the effective mass

Recent astronomical observations, nuclear-reaction experiments, and microscopic calculations have placed new constraints on the nuclear equation of state (EoS) and revealed that most nuclear structure models fail to satisfy those constraints upon extrapolation to infinite matter. A reverse procedure for imposing EoS constraints on nuclear structure has been elusive. Here we present for the first time a method to generate a microscopic energy density functional (EDF) for nuclei from a given immutable EoS. The method takes advantage of a natural Ansatz for homogeneous nuclear matter, the Kohn-Sham framework, and the Skyrme formalism. We apply it to the realistic nuclear EoS of Akmal-Pandharipande-Ravenhall and describe successfully closed-(sub)shell nuclei. In the process, we provide predictions for the neutron skin thickness of nuclei based directly on the given EoS. Crucially, bulk and static nuclear properties are found practically independent of the assumed effective mass value - a unique result in bridging EDF of finite and homogeneous systems in general.

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arXiv2019

Analysis of nuclear structure in a converging expansion scheme

In the framework of the KIDS generalized energy density functional (EDF), the nuclear equation of state (EoS) is expressed as an expansion in powers of the Fermi momentum or the cubic root of the density ($ρ^{1/3}$). Although an optimal number of converging terms was obtained in specific cases of fits to empirical data and pseudodata, the degree of convergence remains to be examined not only for homogeneous matter but also for finite nuclei. One goal of the present work is to validate the minimal and optimal number of EoS parameters required for the description of homogeneous nuclear matter over a wide range of densities relevant for astrophysical applications. The major goal is to examine the validity of the adopted expansion scheme for an accurate description of finite nuclei. To this end we vary the values of the high-order derivatives of the EoS, namely the skewness of the energy of symmetric nuclear matter and the kurtosis of the symmetry energy, at saturation and examine the relative importance of each term in $ρ^{1/3}$ expansion for homogeneous matter. For given sets of EoS parameters determined in this way, we define equivalent Skyrme-type functionals and examine the convergence in the description of finite nuclei focusing on the masses and charge radii of closed-shell nuclei. The EoS of symmetric nuclear matter is found to be efficiently parameterized with only 3 parameters and the symmetry energy (or the energy of pure neutron matter) with 4 parameters when the EoS is expanded in the power series of the Fermi momentum. Higher-order EoS parameters do not produce any improvement, in practice, in the description of nuclear ground-state energies and charge radii, which means that they cannot be constrained by bulk properties of nuclei.

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arXiv2018

Nuclear structure and the nucleon effective mass: explorations with the versatile KIDS functional

The connection from the structure and dynamics of atomic nuclei (finite nuclear system) to the nuclear equation of state (thermodynamic limit) is primarily made through nuclear energy-density functional (EDF) theory. Failure to describe both entities simultaneously within existing EDF frameworks means that we have either seriously misjudged the scope of EDF or not fully taken advantage of it. Enter the versatile KIDS Ansatz, which is based on controlled, order-by-order extensions of the nuclear EDF with respect to the Fermi momentum and allows a direct mapping from a given, immutable equation of state to a convenient Skyrme pseudopotential for applications in finite nuclei. A recent proof-of-principle study of nuclear ground-states revealed the subversive role of the effective mass. Here we summarize the formalism and previous results and present further explorations related to giant resonances. As examples we consider the electric dipole polarizability of 68Ni and the giant monopole resonance (GMR) of heavy nuclei, particularly the "fluffiness" of 120Sn. We find that the choice of the effective mass parameters and that of the compression modulus affect the centroid energy of the GMR to comparable degrees.

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arXiv2017

Density dependence of the nuclear energy-density functional

The explicit density (rho) dependence in the coupling coefficients of the non-relativistic nuclear energy-density functional (EDF) encodes effects of three-nucleon forces and dynamical correlations. The necessity for a coupling coefficient in the form of a small fractional power of rho is empirical and the power often chosen arbitrarily. Consequently, precision-oriented parameterisations risk overfitting and loss of predictive power. Observing that the Fermi momentum kF~rho^1/3 is a key variable in Fermi systems, we examine if a power hierarchy in kF can be inferred from the properties of homogeneous matter in a domain of densities which is relevant for nuclear structure and neutron stars. For later applications we want to determine an EDF that is of good quality but not overtrained. We fit polynomial and other functions of rho^1/3 to existing microscopic calculations of the energy of symmetric and pure neutron matter and analyze the fits. We select a form and parameter set which we found robust and examine the parameters' naturalness and the resulting extrapolations. A statistical analysis confirms that low-order terms like rho^1/3 and rho^2/3 are the most relevant ones. It also hints at a different power hierarchy for symmetric vs. pure neutron matter, supporting the need for more than one rho^a terms in non-relativistic EDFs. The EDF we propose accommodates adopted properties of nuclear matter near saturation. Importantly, its extrapolation to dilute or asymmetric matter reproduces a range of existing microscopic results, to which it has not been fitted. It also predicts neutron-star properties consistent with observations. The coefficients display naturalness. Once determined for homogeneous matter, EDFs of the present form can be mapped onto Skyrme-type ones for use in nuclei. The statistical analysis can be extended to higher orders and for different ab initio calculations.

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arXiv2017

Nuclear Energy Density Functional for KIDS

The density functional theory (DFT) is based on the existence and uniqueness of a universal functional $E[ρ]$, which determines the dependence of the total energy on single-particle density distributions. However, DFT says nothing about the form of the functional. Our strategy is to first look at what we know, from independent considerations, about the analytical density dependence of the energy of nuclear matter and then, for practical applications, to obtain an appropriate density-dependent effective interaction by reverse engineering. In a previous work on homogeneous matter, we identified the most essential terms to include in our "KIDS" functional, named after the early-stage participating institutes. We now present first results for finite nuclei, namely the energies and radii of $^{16,28}$O, $^{40,60}$Ca.

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