arXiv subjects

P. Papakonstantinou

Publications and source records attributed to P. Papakonstantinou.

At least 19 recordsLinked to original sources

arXiv2026

CREX and PREX-II reconciled within energy-density functional theory

The CREX and PREX-II measurements of the neutron-skin thickness of 48Ca and 208Pb challenge standard nuclear energy-density functional (EDF) descriptions of nuclei and nuclear matter. We show that the apparent tension arises from an implicit constraint in EDF theory, which ties the density dependence of the functional at the dilute nuclear surface to that of uniform matter near saturation. Relaxing this surface-bulk coupling and independently constraining the dilute-density sector, while preserving realistic saturation and high-density behavior, yields EDFs that simultaneously reproduce the neutron skins of both nuclei, their electric dipole polarizabilities, and neutron-star mass-radius relations. The established correlation between the neutron-skin thickness and the symmetry-energy slope parameter L at saturation is retained but becomes substantially broader. The results show that current neutron-skin data do not require extreme values of L and highlight an underconstrained degree of freedom associated with low-density nuclear matter.

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arXiv2026

Prediction of deformed halo nuclei $^{43,45}$Si from multiple criteria based on structure and reaction analyses

Possible deformed neutron halos in silicon isotopes are investigated from both structure and reaction perspectives using the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) combined with the Glauber model. The experimental neutron separation energies of silicon isotopes are well reproduced by the DRHBc theory. Multiple halo criteria are examined, including the global ones based on root-mean-square radii and density profiles, as well as the microscopic ones based on single-particle orbitals and their spatial distributions. Calculations employing different density functionals and pairing strengths consistently indicate the emergence of $p$-wave neutron halos in $^{43,45}$Si, accompanied by pronounced shape decoupling between the halo and the core. Moreover, the enhanced reaction cross sections and the narrow longitudinal momentum distributions of one-neutron removal residues provide additional evidence supporting the halo structures in $^{43,45}$Si.

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arXiv2023

Variational and parquet-diagram calculations for neutron matter. V. Triplet pairing

We apply a large-scale summation of Feynman diagrams, including the class of parquet-diagrams {\em plus} important contributions outside the parquet class, for calculating effective pairing interactions and subsequently the superfluid gap in P-wave pairing in neutron matter. We employ realistic nucleon-nucleon interactions of the $v_8$ type and perform calculations up to a Fermi momentum of $1.8\,$fm$^{-1}$. We find that many-body correlations lead to a strong reduction of the spin-orbit interaction, and, therefore, to an almost complete suppression of the $^3$P$_2$ and $^3$P$_2$-$^3$F$_2$ gaps. We also find pairing in $^3$P$_0$ states; the strength of the pairing gap depends sensitively on the potential model employed. Our results for triplet pairing are relevant for assessing superfluidity in neutron star interiors, whose presence can affect the cooling of neutron stars.

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arXiv2023

Missed prediction of the neutron halo in $^{37}$Mg

Halo phenomena have long been an important frontier in both experimental and theoretical nuclear physics. $^{37}$Mg was identified as a halo nucleus in 2014 and remains the heaviest nuclear halo system to date. While the halo phenomenon in $^{37}$Mg was not predicted before the discovery, its description has been still challenging afterwards. In this Letter, we report a microscopic and self-consistent description of the neutron halo in $^{37}$Mg using the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc) that was developed in 2010. The experimental neutron separation energies and empirical matter radii of neutron-rich magnesium isotopes as well as the deformed $p$-wave halo characteristics of $^{37}$Mg are well reproduced without any free parameters. In particular, the orbital occupied by the halo neutron in $^{37}$Mg, exhibiting $p$-wave components comparable to those suggested in experiments, remains consistent across various employed density functionals including PC-F1, PC-PK1, NL3*, and PK1. The DRHBc theory investigated only even-even magnesium isotopes in previous works and for that reason missed predicting $^{37}$Mg as a halo nucleus before 2014. Although the core and the halo of $^{37}$Mg are both prolate, higher-order shape decoupling on the hexadecapole and hexacontatetrapole levels is predicted.

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arXiv2023

Collapse of the $N=28$ shell closure in the newly discovered $^{39}$Na and the development of deformed halos towards the neutron dripline

Halos and changes of nuclear magicities have been extensively investigated in exotic nuclei during past decades. The newly discovered $^{39}$Na with the neutron number $N=28$ provides a new platform to explore such novel phenomena near the neutron dripline of the sodium isotopic chain. We study the shell property and the possible halo structure in $^{39}$Na within the deformed relativistic Hartree-Bogoliubov theory in continuum. It is found that the lowering of $2p$ orbitals in the spherical limit results in the collapse of the $N=28$ shell closure in $^{39}$Na, and a well deformed ground state is established. The pairing correlations and the mixing of $pf$ components driven by deformation lead to the occupation of weakly bound or continuum $p$-wave neutron orbitals. An oblate halo is therefore formed around the prolate core in $^{39,41}$Na, making $^{39}$Na a single nucleus with the coexistence of several exotic structures, including the quenched $N=28$ shell closure, Borromean structure, deformed halo, and shape decoupling. The microscopic mechanisms behind the shape decoupling phenomenon and the development of halos towards dripline are revealed.

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arXiv2022

Damping of the isovector giant dipole resonance in $^{40,48}$Ca

The fine structure of the IsoVector Giant Dipole Resonance (IVGDR) in the doubly-magic nuclei $^{40,48}$Ca observed in inelastic proton scattering experiments under $0^\circ$ is used to investigate the role of different mechanisms contributing to the IVGDR decay width. Characteristic energy scales are extracted from the fine structure by means of wavelet analysis. The experimental scales are compared to different theoretical approaches allowing for the inclusion of complex configurations beyond the mean-field level. Calculations are performed in the framework of RPA and beyond-RPA in a relativistic approach based on an effective meson-exchange interaction, with the UCOM effective interaction and, for the first time, with realistic two- plus three-nucleon interactions from chiral effective field theory employing the in-medium similarity renormalization group. All models highlight the role of Landau fragmentation for the damping of the IVGDR, while the differences in the coupling strength between one particle-one hole (1p-1h) and two particle-two hole (2p-2h) correlated (relativistic) and non-correlated (non-relativistic) configurations lead to very different pictures of the importance of the spreading width resulting in wavelet scales being a sensitive measure of their interplay. The relativistic approach with particle-vibration coupling, in particular, shows impressive agreement with the number and absolute values of the scales extracted from the experimental data.

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arXiv2018

Women Scientists Who Made Nuclear Astrophysics

Female role models reduce the impact on women of stereotype threat, i.e., of being at risk of conforming to a negative stereotype about one's social, gender, or racial group. This can lead women scientists to underperform or to leave their scientific career because of negative stereotypes such as, not being as talented or as interested in science as men. Sadly, history rarely provides role models for women scientists; instead, it often renders these women invisible. In response to this situation, we present a selection of twelve outstanding women who helped to develop nuclear astrophysics.

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arXiv2017

Structures in the nuclear electric-dipole spectrum

I undertake an anatomy of the nuclear electric-dipole spectrum and discuss selected observations. First, using data and calculations on 208Pb as a representative case, I identify five conspicuous structures in the spectra of heavy nuclei: (1) the giant dipole resonance (GDR), (2) the dipole compression mode, (3) the isoscalar low-energy dipole mode (IS-LED),(4) the isoscalar mode in the region of the GDR, and (5) a concentration of "pygmy" dipole strength (PDS) between the latter two. Next, compiling the data from Texas A&M I show that the energy of (4) follows roughly a A^{-1/6} dependence on the mass number. Finally, I summarize recent theoretical studies on the PDS in exotic nuclei, which suggest a strong influence of shell effects and loose binding.

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arXiv2017

Three-nucleon forces and superfluidity in neutron matter

The existence of superfluidity of the neutron component in the core of a neutron star, associated specifically with triplet $P-$wave pairing, is currently an open question that is central to interpretation of the observed cooling curves and other neutron-star observables. Ab initio theoretical calculations aimed at resolving this issue face unique challenges in the relevant high-density domain, which reaches beyond the saturation density of symmetrical nuclear matter. These issues include uncertainties in the three-nucleon (3N) interaction and in the effects of strong short-range correlations -- and more generally of in-medium modification of nucleonic self-energies and interactions. A survey of existing solutions to the gap equations in the triplet channel shows that the separate or combined impacts of 3N forces, coupled channels, and mass renormalization range from moderate to strong to devastating, thus motivating a detailed analysis of the competing effects. In the present work we track the effects of the 3N force and in-medium modifications in the representative case of the $^3P_2$ channel, based on the Argonne V18 two-nucleon (2N) interaction supplemented by 3N interactions of the Urbana IX family. Sensitivity of the results to the input interaction is clearly demonstrated, while consistency issues arise with respect to the simultaneous treatment of 3N forces and in-medium effects. We consider this pilot study as the first step towards a systematic and comprehensive exploration of coupled-channel $^3P F_2$ pairing using a broad range of 2N and 3N interactions from the current generation of refined semi-phenomenological models and models derived from chiral effective field theory.

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arXiv2017

From chiral NN(N) interactions to giant and pygmy resonances via extended RPA

The properties of giant and pygmy resonances are calculated starting from chiral two-and three-nucleon interactions. The aim is to assess the predictive power of modern Hamiltonians and especially the role of the three-nucleon force. Methods based on the random-phase approximation (RPA) provide an optimal description of the modes of interest with minimal computational requirements. Here we discuss the giant resonances (GRs) of 40,48Ca isotopes and their low-energy dipole response. A comparison with previous results obtained with a transformed Argonne V18 two-nucleon potential points to certain improvements.

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arXiv2016

Proton pygmy resonances: predictions for N=20 isotones

We study theoretically the low-energy electric-dipole response of N=20 isotones. We present results from a quasiparticle random-phase approximation (QRPA) and a continuum random-phase approximation (CRPA), and we compare them with results for the mirror Z=20 nuclei. According to our analysis, enhanced E1 strength is expected energetically well below the giant dipole resonance in the proton-rich isotones. Large amounts of E1 strength in the asymmetric N=20 isotones are predicted, unlike their equally asymmetric Z=20 mirror nuclei, pointing unambiguously to the role of structural effects such as loose binding. A proton-skin oscillation could develop especially in 46Fe. The proper description of non localized threshold transitions and the nucleon effective mass in mean-field treatments may affect theoretical predictions. We call for systematic theoretical investigations to quantify the role bulk-matter properties, in anticipation of measurements of E1 transitions in proton-rich nuclei.

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arXiv2015

Isoscalar and neutron modes in the E1 spectra of Ni isotopes and the relevance of shell effects and the continuum

We study theoretically the electric dipole transitions of even Ni isotopes at low energies, using the self-consistent quasi-particle random-phase approximation (RPA) with the D1S Gogny interaction and a continuum-RPA model with the SLy4 Skyrme force. We analyze isoscalar states, isovector states, and the dipole polarizability. We define a reference value for the polarizability, to remove a trivial dependence on the mass number. We compare our results with data and other calculations, with a focus on collective states, shell effects, and threshold transitions. Our results support the presence of a strong isoscalar transition, with little or moderate E1 strength, as a universal feature of ordinary nuclei. In moderately neutron-rich Ni isotopes, namely 68Ni and neighboring isotopes, this transition is found bimodal due to couplings with surface neutrons. An adequate treatment of the continuum states appears essential for describing suprathreshold E1 strength, especially beyond 68Ni. Very exotic isotopes (N>50) are found highly polarizable, with practically all their E1 strength in the continuum. The dipole polarizability and the neutron skin thickness are influenced by shell structure in different ways, so they can appear anticorrelated. A comparison with existing results for lighter (Ca) and heavier (Sn) nuclei suggests that the so-called pygmy dipole strength is influenced strongly by shell effects and that, partly for that reason, its isospin structure depends on the mass region.

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arXiv2014

Giant Resonances based on Unitarily Transformed Two-Nucleon plus Phenomenological Three-Nucleon Interactions

We investigate giant resonances of spherical nuclei on the basis of the Argonne V18 potential after unitary transformation within the Similarity Renormalization Group or the Unitary Correlation Operator Method supplemented by a phenomenological three-body contact interaction. Such Hamiltonians can provide a good description of ground-state energies and radii within Hartree-Fock plus low-order many-body perturbation theory. The standard Random Phase Approximation is applied here to calculate the isoscalar monopole, isovector dipole, and isoscalar quadrupole excitation modes of the 40Ca, 90Zr, and 208Pb nuclei. Thanks to the inclusion of the three-nucleon interaction and despite the minimal optimization effort, a reasonable agreement with experimental centroid energies of all three modes has been achieved. The role and scope of the Hartree-Fock reference state in RPA methods are discussed.

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arXiv2014

Properties and significance of the surface dipole mode

A strong isoscalar dipole resonance is known to be excited in a variety of nuclei, including isospin symmetric ones, at approximately 6-7 MeV. A series of theoretical studies and accumulating experimental evidence support an interpretation of the above dipole resonance as an elementary surface vibration. Obviously, such a mode is potentially as interesting as any collective excitation for a variety of reasons. In addition, though, it is found to account for the observed isoscalar segment of pygmy dipole strength. As discussed here, this has important implications for pygmy-strength interpretations and searches for genuine neutron-skin oscillations.

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arXiv2014

Second random-phase approximation, Thouless' theorem and the stability condition reexamined and clarified

It has been revealed through numerical calculations that the Second Random Phase Approximation (SRPA) with the Hartree-Fock solution as its reference state results in 1) spurious states at genuinely finite energy, contrary to common expectation, and 2) unstable solutions, which within the first-order Random Phase Approximation correspond to real low-energy collective vibrations. In the present work, these shortcomings of SRPA are shown to not contradict Thouless' theorem about the energy-weighted sum rule, and their origin is traced to the violation of the stability condition. A more general theorem is proven. Formal arguments are elucidated through numerical examples. Implications for the validity of SRPA are discussed.

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arXiv2014

Low-energy electric dipole response of Sn isotopes

We study the low-energy dipole (LED) strength distribution along the Sn isotopic chain in both the isoscalar (IS) and the isovector (IV, or E1) electric channels, to provide testable predictions and guidance for new experiments with stable targets and radioactive beams. We use the self-consistent Quasi-particle Random-Phase Approximation (QRPA) with finite-range interactions and mainly the Gogny D1S force. We analyze also the performance of a realistic two-body interaction supplemented by a phenomenological three-body contact term. We find that from N=50 and up to the N=82 shell closure (132Sn) the lowest-energy part of the IS-LED spectrum is dominated by a collective transition whose properties vary smoothly with neutron number and which cannot be interpreted as a neutron-skin oscillation. For the neutron-rich species this state contributes to the E1 strength below particle threshold, but much more E1 strength is carried by other, weak but numerous transitions around or above threshold. We find that strong structural changes in the spectrum take effect beyond N=82, namely increased LED strength and lower excitation energies. Our results with the Gogny interaction are compatible with existing data. On this basis we predict that a) the summed IS strength below particle threshold shall be of the same order of magnitude for N=50-82, b) the summed E1 strength up to approximately 12 MeV shall be similar for N=50-82 MeV, while c) the summed E1 strength below threshold shall be of the same order of magnitude for N ~ 64 - 82 and much weaker for the lighter, more-symmetric isotopes. We point out a general agreement of our results with other non-relativistic studies, the absence of a collective IS mode in some of those studies, and a possibly radical disagreement with relativistic models.

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arXiv2014

Isospin properties of electric dipole excitations in 48Ca

Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and nearly mono-energetic linearly polarized photons with energies between 6.6 and 9.51 MeV provided strength distribution and parities, and an (α,α'γ) experiment at E_α=136 MeV gave cross sections for an isoscalar probe. The unexpected difference observed in the dipole response is compared to calculations using the first-order random-phase approximation and points to an energy-dependent isospin character. A strong isoscalar state at 7.6 MeV was identified for the first time supporting a recent theoretical prediction.

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arXiv2013

Densities and energies of nuclei in dilute matter

We explore the ground-state properties of nuclear clusters embedded in a gas of nucleons with the help of Skyrme-Hartree-Fock microscopic calculations. Two alternative representations of clusters are introduced, namely coordinate-space and energy-space clusters. We parameterize their density profiles in spherical symmetry in terms of basic properties of the energy density functionals used and propose an analytical, Woods-Saxon density profile whose parameters depend, not only on the composition of the cluster, but also of the nucleon gas. We study the clusters' energies with the help of the local-density approximation, validated through our microscopic results. We find that the volume energies of coordinate-space clusters are determined by the saturation properties of matter, while the surface energies are strongly affected by the presence of the gas. We conclude that both the density profiles and the cluster energies are strongly affected by the gas and discuss implications for the nuclear EoS and related perspectives. Our study provides a simple, but microscopically motivated modeling of the energetics of clusterized matter at subsaturation densities, for direct use in consequential applications of astrophysical interest.

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