arXiv subjects

Hiroyuki Sagawa

Publications and source records attributed to Hiroyuki Sagawa.

At least 19 recordsLinked to original sources

arXiv2026

Growth of quartet correlations in neutron-rich Tellurium isotopes within quartet Bardeen-Cooper-Schrieffer theory

Quartet correlations in neutron-rich Te isotopes are investigated within the quartet Bardeen-Cooper-Schrieffer (BCS) framework. Taking $^{100}$Sn as an inert core, we consider two valence protons and valence neutrons occupying the $2d_{5/2} \oplus 1g_{7/2}$ model space, and solve the quartet BCS variational equations with a charge-independent isovector pairing interaction. The effective pairing strength is constrained from empirical neutron pairing gaps in the Te isotopic chain. We find that the valence quartet number increases as the valence neutron number is enlarged from $N_{\rm val}=2$ to $14$. The same increasing behavior is also found for the condensed quartet component. The proton occupation of the $1g_{7/2}$ orbit is strongly enhanced relative to the conventional like-particle BCS reference and is driven close to the degeneracy-weighted limit. These results suggest that additional valence neutrons enhance the quartet admixture in the correlated quartet BCS state, while redistributing the fixed proton weight from pair-like configurations to quartet configurations.

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arXiv2026

Microscopic mechanism of the Fayans pairing for the enhancement of charge radii

The Fayans energy density functional (EDF), and in particular its pairing sector, have been claimed to be able to reproduce the experimental data of charge radii in many instances. A particularly intriguing case is that of the $ \mathrm{Ca} $ isotopes between $ A = 40 $ and $ 48 $, where charge radii exhibit a "bell shape". In our work, we examine the microscopic origin of this behaviour. We prepare in total $ 25 $ paramerizations of the Fayans-like pairing interaction, that are equivalent in fulfilling the same criteria for the reproduction of empirical pairing gaps. We find that both the density and the density-gradient dependence of the pairing interaction are important to reproduce the well-known enhancement of charge radii in the open-shell nuclei, leading to the "bell shape" behaviour of $ \mathrm{Ca} $ isotopes. In particular, this originates from the repulsive nature of the rearrangement potential, and cannot simply be mocked up by a refit of the pairing strength. At the same time, we notice some drawbacks of the Fayans standard EDFs, that may call for investigating a more general form of it.

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arXiv2026

Deformed neutron halo nuclei and soft dipole excitations in the 40<A<90 mass region

We study deformed neutron halo nuclei in the mass region $40 < A < 90$ and their soft electric dipole ($E1$) excitations based on the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc). Three candidates, $^{43}$Si, $^{69}$Ti, and $^{75}$Cr, are selected for detailed analysis. Unique features are identified in the decoupled densities of possible $s$- and $p$-wave deformed halo nuclei in this mass region, which are influenced by large high-$l$ configurations. It is shown that the dipole response is a highly sensitive observable to detect the halo component of the single-particle wave function in deformed halo nucleus, and it helps identify the configuration and the magnitude of deformation for halo nuclei in the $40 < A < 90$ mass region. Experimental confirmation of the dipole strength in the low-energy region is highly desirable to explore possible deformed halo candidates in the medium-heavy mass region.

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arXiv2026

The quenching of the axial-vector coupling constant $g_A$ in $β$-decay: joint effects from chiral two-body currents and many-body correlations

In nuclear $β$-decay calculations, the axial-vector coupling constant $g_A \approx 1.27$ usually needs to be quenched phenomenologically by a factor $q~\approx$ 0.75 to reproduce {the Gamow-Teller (GT) transition strengths}. We propose a novel approach to quench the GT {strength} of $β$-decay within the microscopic random phase approximation (RPA) plus particle-vibration coupling (PVC) approach, incorporating the contributions of two-body currents (TBC) derived from chiral effective field theory ($χ$EFT). Self-consistent RPA+PVC calculations are performed in three doubly magic nuclei, $^{56}$Ni, $^{100}$Sn, and $^{132}$Sn, with various Skyrme energy density functionals, and the effect of TBC is evaluated by using the obtained many-body wavefunctions. A combined effects of the many-body correlations introduced by PVC and chiral TBC quench the GT strength and reproduce quantitatively experimental data without any additional adjustments. The extracted quenching factors $q$ by the present microscopic model lie in the range $\approx$ 0.73--0.80, which is quite close to the commonly adopted empirical value of $q \approx 0.75$.

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arXiv2026

Novel microscopic approaches for Spin-Isospin excitations and Beta-decay

We explore unsolved nuclear structure problems related with the spin and isospin degree of freedom by using microscopic models which accommodate realistic isoscalar and isovector pairing interactions, and also tensor correlations. For the attempt of universal theoretical framework for both nuclear and astrophysical phenomena, we adopt a self-consistent Hartree-Fock (HF)+random phase approximation (RPA) models, and a state-of-the-art beyond mean field model, Subtracted Second RPA (SSRPA), including the couplings to two-particle two-hole states. The quenching problems of magnetic dipole and Gamow-Teller transitions are discussed in terms of the coupling to 2p-2h configurations and also the tensor correlations. The $β$ decay life time of semi-magic and magic nuclei are discussed in RPA and SSRPA models.

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arXiv2026

A Halo: The Trigger to a New Era of Nuclear Correlations

In this contribution to the Halo-40 Proceedings, we discuss two topics regarding halo phenomena: The first is the pairing anti-halo effect on the neutron radius of halo nuclei and its restoration due to the coupling to the continuum; the second is the soft dipole excitation of deformed halo nuclei. We demonstrate the importance of Hartree-Fock-Bogoliubov and the relativistic Hartree-Bogoliubov theory in continuum for properly taking into account the halo nature of extended wave functions in calculations of neutron radii, as well as the soft dipole excitations of halo nuclei. It was shown that the anti-halo effect is very sensitive to the continuum coupling induced by Bogoliubov-type quasi-particles, which largely cancels the anti-halo effect on the neutron radius. The soft dipole excitations of deformed halo nuclei Ne-31 and Mg-37 are discussed within the deformed Woods-Saxon model. We point out that the sharp peak just above the threshold in the dipole response is created by the halo effect, and its strength can be used to identify the magnitude of deformation and the halo configuration in the Nilsson level scheme.

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arXiv2025

Medium-scale anisotropies measured by Telescope Array surface detectors

The Telescope Array (TA) experiment, the largest observatory for ultra-high energy cosmic rays in the Northern Hemisphere, has identified two medium-scale anisotropies: the TA Hotspot near the constellation Ursa Major and an excess in the direction of the Perseus-Pisces supercluster. Studying these medium-scale anisotropies may provide insights into the origins of ultra-high energy cosmic rays. This presentation will explore an oversampling analysis of TA surface detector data to evaluate these medium-scale event excesses and will present the latest findings on the TA Hotspot and the Perseus-Pisces supercluster excess.

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arXiv2025

A Bayesian Framework for UHECR Source Association and Parameter Inference

The identification of potential sources of ultra-high-energy cosmic rays (UHECRs) remains challenging due to magnetic deflections and propagation losses, which are particularly strong for nuclei. In previous iterations of this work, we proposed an approach for UHECR astronomy based on Bayesian inference through explicit modelling of propagation and magnetic deflection effects. The event-by-event mass information is expected to provide tighter constraints on these parameters and to help identify unknown sources. However, the measurements of the average mass through observations from the surface detectors at the Pierre Auger Observatory already indicate that the UHECR masses are well represented through its statistical average. In this contribution, we present our framework which uses energy and mass moments of $\ln A$ to infer the source parameters of UHECRs, including the mass composition at the source. We demonstrate the performance of our model using simulated datasets based on the Pierre Auger Observatory and Telescope Array Project. Our model can be readily applied to currently available data, and we discuss the implications of our results for UHECR source identification.

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arXiv2025

Mirror-skin thickness: a possible observable sensitive to the charge symmetry breaking energy density functional

We propose a new observable, named the mirror-skin thickness, in order to extract the strength of the charge symmetry breaking (CSB) term in an energy density functional (EDF). The mirror-skin thickness of $ N = 20 $ isotones and $ Z = 20 $ isotopes is studied by using Hartree-Fock-Bogoliubov (HFB) calculations with various Skyrme EDFs and adding CSB and charge independence breaking (CIB) terms. It is shown that the mirror-skin thickness is sensitive only to the CSB EDF, but hardly depends on either the isospin symmetric part of the nuclear EDF or the CIB term. Therefore, this observable can be used to extract the magnitude of the CSB term in the EDF quantitatively, either from experimental data or ab initio calculations. We have studied the accuracy in the mirror-skin thickness that is needed to extract sensible information. Our study may also help to understand the inconsistency between the strength of the phenomenological CSB and that extracted from ab initio calculations [Naito et al. Nuovo. Cim. C 47, 52 (2024)]. Among possible mirror pairs for experimental study, we propose the mirror-skin thickness between $ {}^{42} \mathrm{Ca} $ and $ {}^{42} \mathrm{Ti} $, which could be accessed in future experiments in RIBF and/or FRIB.

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arXiv2025

Charge symmetry breaking effects of $ω$-$ρ^0$ mixing in relativistic mean-field model

We present a relativistic mean-field model that incorporates charge symmetry breaking (CSB) of nuclear force via $ ω$-$ ρ^0 $ meson mixing, along with corrections to the electromagnetic interaction including the nucleon form factors, first-order vacuum polarization, and Coulomb exchange and pairing terms. The model parameters are refitted using the mass differences of $ T = 1/2 $ mirror nuclei and ground-state properties of magic nuclei, yielding DD-ME-CSB parameter set. The DD-ME-CSB parameter set reproduces the mass differences of mirror nuclei reasonably well up to $ T = 2 $, demonstrating the importance of $ ω$-$ ρ^0 $ mixing. A connection of the present model to a Skyrme-type CSB interaction is also established through a gradient expansion of the energy density functional.

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arXiv2025

Charge symmetry breaking in hypernuclei within RMF model

We study the charge symmetry breaking (CSB) effect in the binding energy of mirror hypernuclei in the mass region $A=7\sim 48$ in relativistic mean field (RMF) models introducing $NN$ and $ΛN$ interactions. The phenomenological $ΛN$ CSB interaction is introduced and the strength parameter is fitted to reproduce the experimental binding energy difference between the mirror hypernuclei $^{12}_Λ$B and $^{12}_Λ$C. This model is applied to calculate the CSB energy anomaly in mirror hypernuclei with the mass $A=7\sim48$. The model is further applied to predict the binding energy difference of mirror hypernuclei of $A$=40 with the isospin $T=1/2$, $3/2$ and $5/2$ nuclei together with various hyper Ca isotopes and their mirror hypernuclei. Finally the binding energy systematics of $A=$48 hypernuclei are predicted with/without the CSB effect by the PK1 and TM2 energy density functionals (EDFs).

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arXiv2025

Octupole correlations in superdeformed bands of $^{56}$Ni

The projected multi-dimensionally-constrained relativistic Hartree-Bogoliubov model was employed to calculate the potential energy surface of the high-spin states in $^{56}\text{Ni}$. It is pointed out for the first time that possible octupole deformations exist for the positive and negative parity superdeformed bands in $^{56}\text{Ni}$, with deformations $β_{30}\sim0.14$ and $β_{30}\sim0.24$, respectively, along with a large prolate deformation of $β_{20}\sim 0.42$. These octupole deformations are induced by the coupling between $2p_{3/2}$ and $1g_{9/2}$ orbits at the deformation $β_{20}\sim 0.4$. The calculated excitation energies of the two rotational bands are consistent with the observed superdeformed bands of $^{56}\text{Ni}$. In addition, two rotational bands are predicted, consisting of one superdeformed band with negative parity and one hyperdeformed bands with positive parity.

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arXiv2024

Dipole response of deformed halo nuclei $^{31}$Ne and $^{37}$Mg

We study the soft electric dipole ($E1$) response of deformed halo nuclei $^{31}$Ne and $^{37}$Mg using a deformed Woods-Saxon potential, with the potential depth adjusted to reproduce empirical separation energy of last neutron orbit, i.e., 150 keV for $^{31}$Ne and 220 keV for $^{37}$Mg. The configuration dependence of the $E1$ strength near the neutron threshold is pointed out. The halo configurations $[321]3/2$ at $β_2=0.5$ and $[330]1/2$ at $β_2=0.24$ in $^{31}$Ne contain large amplitudes of halo $p$-shell orbits, which significantly enhance the threshold strength by several times compared to the non-halo configuration $[202]5/2$ at $β_2=0.32$. In $^{37}$Mg, the last neutron configuration is assigned as $[321]1/2$ at a large deformation of $β_2=0.46$, which involves a halo $p$-shell configuration that significantly enhances the soft dipole strength. This enhancement is about 60\% larger than that of the $[321]3/2$ configuration in $^{31}$Ne because of large $p$-shell probability in $^{37}$Mg. Experimental confirmation of the soft dipole strength is highly desired to determine the deformation and the configuration of the last neutron orbits both in $^{31}$Ne and $^{37}$Mg.

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arXiv2024

QCD sum rule approach to Okamoto-Nolen-Schiffer anomaly

A new framework is introduced to connect between a charge symmetry breaking (CSB) energy density functional (EDF) and the low-energy constants derived from quantum chromodynamics (QCD). By constructing a QCD-based CSB EDF, this method provides new insights into the Okamoto-Nolen-Schiffer anomaly, a long-standing puzzle in the energy differences of mirror nuclei that lacks a robust microscopic explanation. Using examples such as $ {}^{17} \mathrm{F} $-$ {}^{17} \mathrm{O} $, $ {}^{15} \mathrm{O} $-$ {}^{15} \mathrm{N} $, $ {}^{41} \mathrm{Sc} $-$ {}^{41} \mathrm{Ca} $, and $ {}^{39} \mathrm{Ca} $-$ {}^{39} \mathrm{K} $, we demonstrate that the proposed interaction effectively resolves the anomaly within the range of theoretical uncertainties.

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arXiv2024

Nuclear Pairing Energy vs Mean Field Energy: Do They Talk To Each Other For Searching The Energy Minimum?

We study the evolution of the total binding energy (TBE) and pairing energy of Pb, Hg and Ar isotopes, as a function of the nuclear deformation. As for the nuclear model, we exploit a deformed relativistic Hartree-Bogoliubov theory in the continuum (DRHBc), and a deformed Skyrme Hartree-Fock plus BCS model. It is found that the dependence of pairing energy on the deformation is strongly correlated to that of the mean field energy, which is obtained by subtracting the pairing energy from the TBE; in other words, the energy minimum characterized by a large negative mean field energy has a smaller negative pairing energy or, equivalently, a smaller positive pairing gap, while a stronger pairing energy is found in the region away from the minimum of the total energy. Consequently, the two energies show an anti-symmetric feature in their deformation dependence, although the energy scales are very different. Moreover, since the pairing energy has a negative sign with respect to to the pairing gap, the evolution of mean field energy follows closely that of the pairing gap. This implies that the pairing energy (or pairing gap) and the mean field energy talk to each other and work together along the potential energy curve to determine the energy minimum and/or the local minimum.

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arXiv2024

Shape coexistence in Ne isotopes and hyperon impurity effect on low-lying states

Based on the beyond-mean-field Skyrme-Hartree-Fock model, we investigate the shape coexistence in Ne isotopes and the effect of $\la$ hyperon on the energy level structure in the nuclei. The up-to-date Skyrme-type $N\la$ interaction SLL4 and the $NN$ interaction SGII are employed. Low-lying energy spectra of $^{20,22,24,26,28,30,32,34}$Ne, including the low-lying states with $J\leq 6$, are predicted, discussed in detail, and found in good agreement with experimental results. The electric quadrupole transition rate is also examined. The coexistences of a ground state rotational band and a $\be$ vibrational band are revealed in $^{20,22,24}$Ne. Unlike the previously discovered shrinkage effect of $\la_{s}$ on the ground state nuclei, it is found that the $\la_{s}$ may alter the excitation mode of the second band by affecting the distribution of the collective wave function, thereby causing the $\be$ vibrational band transitions to a vibrational band with equidistant energy levels.

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arXiv2024

QCD-based charge symmetry breaking interaction and the Okamoto-Nolen-Schiffer anomaly

An approach is proposed to link the charge symmetry breaking (CSB) nuclear interaction and the low-energy constants in quantum chromodynamics (QCD) by matching the CSB effect in nuclear matter. The resulting CSB interaction is applied to study the Okamoto-Nolen-Schiffer anomaly, still lacking a satisfactory microscopic understanding, on the energy differences of mirror nuclei by taking $ {}^{17} \mathrm{F} $-$ {}^{17} \mathrm{O} $, $ {}^{15} \mathrm{O} $-$ {}^{15} \mathrm{N} $, $ {}^{41} \mathrm{Sc} $-$ {}^{41} \mathrm{Ca} $, and $ {}^{39} \mathrm{Ca} $-$ {}^{39} \mathrm{K} $ as typical examples. The magnitude and sign of the QCD-based CSB interactions are found to resolve the anomaly successfully within theoretical uncertainties.

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arXiv2023

Microscopic study of $M$1 resonances in Sn isotopes

The magnetic dipole ($M$1) resonances of even-even $^{112-120, 124}$Sn isotopes are investigated in the framework of the self-consistent Skyrme Hartree-Fock (HF) + BCS and Quasiparticle Random Phase Approximation (QRPA). The Skyrme energy density functionals SLy5 and T11 with and without tensor terms are adopted in our calculations. The mixed type pairing interaction is used to take care of the pairing effect for open-shell nuclei both in the ground and excited states calculations. The calculated magnetic dipole strengths are compared with available experimental data. The QRPA results calculated by SLy5 and T11 with tensor force show a better agreement with the experimental data than those without the tensor force. By analyzing the HF and QRPA strength distributions of $^{112}$Sn and $^{124}$Sn, we discuss the effect of tensor force on the $M$1 resonances in detail. It is found that the $M$1 resonance is sensitive to the tensor interaction, and favors especially a negative triplet-odd tensor one. Depending on the nucleus, a quenching factor of the $M$1 operator of about 0.71-0.95 is needed to reproduce the total observed transition strength. In our calculations, we also find some low-lying, pygmy-type magnetic dipole states distributed below 6.0 MeV, and they are formed mainly from the neutron configuration $ν$2$d_{5/2}$$\rightarrow$$ν$2$d_{3/2}$.

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