arXiv subjects

W. Horiuchi

Publications and source records attributed to W. Horiuchi.

At least 19 recordsLinked to original sources

arXiv2026

Probing the Size of Neutron and Proton Single-Particle Orbitals from Nucleon Knockout Reactions

The size of neutron and proton single-particle orbitals of $^{52}$Ca, $^{53}$Ca, $^{54}$Ca, and $^{55}$Sc were investigated via nucleon knockout reactions at $\sim$ 230 MeV/nucleon. The determination method is based on the measured fragment momentum distributions in $(p,pn)$ and $(p,2p)$ reactions, which are shown to be sensitive to the spatial extension of the wave function of the knocked-out nucleon, interpreted within the distorted wave impulse approximation (DWIA) framework. A systematic sensitivity study is carried out for the $(p,pn)$ recoil-momentum distribution method and is presented in this work. The experimental momentum distributions are compared to state-of-the-art mean field and $ab$ $initio$ in-medium similarity renormalization group and self-consistent Green's function calculations in combination with DWIA reaction theory calculations. Based on this work, the 1$p$ neutron orbitals are consistently found $0.48-0.78$ fm larger than the $0f_{7/2}$ neutron orbitals in $^{52-54}$Ca, while the size evolution of the valence proton orbitals remains inconclusive due to the large associated statistical uncertainties.

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arXiv2026

Deformation, halo, and bubble structure: A paradigm shift of exotic phenomena in light to medium mass nuclei

The emergence of exotic nuclear structures, such as deformation, one- and two-neutron halos, and bubble configurations, marks a paradigm shift in our understanding of light- to medium-mass nuclei far from stability, particularly near and within the island of inversion extending across $N=20-28$. In this review, we integrate microscopic structure calculations using the antisymmetrized molecular dynamics method with reaction theories such as the Glauber model for high-energy collisions, and highlight the use of the fully quantum mechanical finite-range distorted wave Born approximation for calculating both inclusive and exclusive Coulomb breakup observables for these medium mass systems. These theoretical frameworks enable precise probing of nuclear density profiles through observables such as total reaction cross sections, neutron removal cross sections, relative energy spectra, parallel momentum distributions, and angular distributions. Applications to several nuclei in the island of inversion reveal enhanced halo extensions, neutron-neutron correlations in Borromean nuclei, and central density depletions in bubbles, challenging traditional shell-model paradigms. Furthermore, the sensitivity of astrophysical reaction rates to these exotic inputs is explored, demonstrating their role in the refinement of r-process nucleosynthesis models and elemental abundance predictions. This unified approach not only bridges nuclear structure and reactions, but also highlights the driplines as frontiers for unraveling nuclear matter under extreme conditions, with implications for rare-isotope beam experiments and beyond.

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arXiv2026

Is $^{40}$Mg a Borromean halo nucleus? A case built on the electric-dipole response

We investigate the low-energy electric-dipole response of $^{40}$Mg using a $^{38}$Mg$+n+n$ three-body model. This model is implemented using a three-body hyperspherical formalism with an analytical transformed harmonic oscillator basis. In this study, two different neutron-neutron interactions are considered: a scalar Gaussian density-dependent central potential and a more realistic finite-range potential which includes central, spin-orbit, and tensor components. We examine how electric-dipole response is affected by the choice of the interaction.

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arXiv2025

Glauber-theory calculations of high-energy nuclear scattering observables using variational Monte Carlo wave functions

Experiments using intermediate- to high-energy radioactive nuclear beams present numerous findings. Extracting important properties of physical observables relies on a firm theoretical analysis. Though Glauber theory is believed to work well, no convincing calculation has so far been done. We perform ab initio Glauber theory calculations of both elastic differential cross sections and total reaction cross sections for p+12C, 12C+12C, and 6He+12C systems. The wave functions of both 6He and 12C are generated by variational Monte Carlo calculations with spatial and spin-isospin correlations induced by realistic two- and three-nucleon potentials. Glauber's phase-shift function is computed by Monte Carlo integration up to all orders of nucleon-nucleon multiple scatterings. We show an excellent performance of the Glauber description to the selected data on the above systems. We also find that the cumulant expansion of the phase-shift function converges rapidly up to the second order for the above systems. This finding will open up interesting applications for the analysis of high-energy nuclear experiments.

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arXiv2025

Glauber-theory analysis of nuclear reactions on 12C target with variational Monte Carlo wave functions

The application of Glauber theory has been playing an increasingly important role with the study of unstable or exotic nuclei. Its adaptation to medium and high-energy nucleus-nucleus collisions is severely limited because one has to evaluate the matrix elements of multiple-scattering operators. The extraction of physical observables has been done using 'approximate' Glauber theory whose validity is hard to evaluate. We perform a full calculation of the matrix elements using Monte Carlo integration and analyze the elastic differential cross sections and the total reaction cross sections for p+12C, 4,6He+12C, and 12C+12C collisions. We use the variational Monte Carlo wave functions for 4,6He and 12C obtained by using realistic two- and three-nucleon potentials. We demonstrate the performance of the Glauber-theory calculations by comparing with available experimental data. We further discuss the accuracy of the conventional approximate methods in the light of the cumulant expansion for Glauber's phase-shift function.

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arXiv2025

Quantification of the evaporation process during fragmentation of space-relevant nuclei on elemental targets

This study examines charge-changing cross sections for 12C, 14N, 16O, and 20Ne projectiles on elemental targets (C, Al, Cu) at a beam energy of around 290 MeV/nucleon. The two-stage abrasion-ablation model is used, with the abrasion stage described via the Glauber model, incorporating validated single-nucleon density distributions from proton elastic scattering data. In the ablation stage, where particle evaporation occurs, the contribution to charge-changing cross sections is estimated using two approaches. First, a statistical decay model is employed to analyze the evaporation of protons following neutron removal in the abrasion stage. The second approach estimates evaporation contributions by subtracting the direct process component (abrasion) from the experimental charge-changing cross section data. A comparison between these estimated contributions from the experimental data and the predictions of the statistical model enables a systematic evaluation of the evaporation process. The key factors influencing evaporation, such as excitation energy distribution parameters, decay width of the emitted particles, and nucleon separation energies, are analyzed. A strong correlation is observed between the maximum excitation energy available for evaporation and the neutron separation energy of the projectiles across different targets, highlighting the role of evaporation dynamics in the charge-changing cross sections.

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arXiv2024

Investigating nuclear density profiles to reveal particle-hole configurations in the island of inversion

Background: In the mass regions with an abnormal shell structure, the so-called ``island of inversion," the spin-parity of odd-mass nuclei provides quantitative insights into the shell evolution. However, the experimental determination of the spin-parity is often challenging, leaving it undetermined in many nuclei. Purpose: We discuss how the shell structure affects the density profiles of nuclei in the island of inversion and investigate whether these can be probed from the total reaction and elastic scattering cross sections. Method: The antisymmetrized molecular dynamics (AMD) is employed to generate various particle-hole configurations and predict the energy levels of these nuclei. The obtained density distributions are used as inputs to the Glauber model, which is employed to calculate the total reaction and elastic scattering cross sections for revealing their relationship to the particle-hole configurations. Results: In addition to the well-known correlation between nuclear deformation and radius, we show the correlations between the particle-hole configurations and both central density and diffuseness. We show that different particle-hole configurations are well reflected in the total reaction and elastic scattering cross sections. Conclusion: The total reaction and elastic scattering cross sections are useful probes to identify the spin-parity of nuclei when different particle-hole configurations coexist.

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arXiv2024

Prediction of two-neutron halos in the $N=28$ isotones $^{40}$Mg and $^{39}$Na

The ground states of the nuclei $^{40}$Mg and $^{39}$Na are investigated using the hyperspherical formalism. Since they are located at the edge of the "big island of inversion", we concentrate on whether we are likely to find two-neutron Borromean halos in these nuclei. A three-body model with effective $n$-$n$ and $^{38}$Mg$+n$ interactions is built for $^{40}$Mg based on the available data. We also give predictions for the low-lying spectrum of $^{38}$Na$=^{37}$Na$+n$ and two-neutron separation energy of the $^{39}$Na nucleus. Depending on parameter choice, we report an increase in the matter radii in the range $0.1$-$0.5$ fm relative to those of the core nuclei. The results suggest a two-neutron halo structure in $^{40}$Mg for a subset of parameters, reinforcing the prediction of a Borromean halo nucleus. The calculations indicate that a two-neutron halo is even more likely for $^{39}$Na. As expected, the halo is linked to the disappearance of the shell gap in these nuclei due to the inversion of the $2p_{3/2}$ and $1f_{7/2}$ orbitals. We study the total cross section for scattering of these nuclei from a carbon target using a Glauber model and show that these provide a clear signal to assess the halo structure.

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arXiv2024

Shell-cluster transition in $^{48}$Ti

Background: Whether or not the $α$ ($^4$He nucleus) clustering exists in the medium-mass region of nuclear systems is a fundamental and intriguing question. However, the recent analysis of the $α$ knockout reaction on $^{48}$Ti [Phys. Rev. C 103, L031305 (2021)] poses a puzzle: The microscopic wave function gives an $α$ knockout cross section that is two orders of magnitude smaller than the experiment, while basic nuclear properties such as the charge radius and the electromagnetic transition probabilities are well explained. Purpose: The ground-state structure of $^{48}$Ti is investigated by using proton- and $α$-nucleus elastic scattering at a few to several hundred MeV, which offers different sensitivity to the region of the nuclear density profiles. Method: Four types of density distributions, the $jj$-coupling shell model and three cluster model configurations, are generated in a single scheme by the antisymmetrized quasi-cluster model (AQCM). The angular distribution of the proton- and $α$-$^{48}$Ti elastic scattering cross sections are obtained with a reliable high-energy reaction theory, the Glauber model. Results: The $jj$-coupling shell model configuration is found to best reproduce the proton-nucleus elastic scattering cross section. On the other hand, the trace of the $α$ cluster structure in the tail region of the wave function is embedded in the $α$-nucleus elastic scattering cross section. Conclusion: Our results suggest that the structure of the nucleus changes as a function of distance from the center, from the $jj$-coupling shell model structure in the surface region to the $α$+$^{44}$Ca cluster structure in the tail region. This picture is consistent with the finding of the $α$ knockout reaction on $^{48}$Ti.

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arXiv2024

Isospin-forbidden electric dipole transition of the 9.64 MeV state of $^{12}$C

The electric dipole transition of the 3$^-$ state at 9.64 MeV of $^{12}$C to the 2$^+$ state at 4.44 MeV is speculated to play a key role in the triple-$α$ reaction at high temperatures. A theoretical prediction of its transition width is a challenge to nuclear theory because it belongs to a class of isospin-forbidden transitions. We extend a microscopic 3$α$ cluster-model to include isospin 1 impurity components, and take into account both isovector and isoscalar electirc dipole operators. Several sets of $2^+$ and $3^-$ wave functions are generated by solving a radius-constrained equation of motion with the stochastic variational method, resulting in reproducing very well the electric quadrupole and octupole transition probabilities to the ground state. The electric dipole transition width is found to be 7--31 meV, 16 meV on the average, and more than half of the width is contributed by the isospin mixing of $α$ particles.

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arXiv2023

Global density-dependent $α$-nucleon interaction for $α$-nucleus elastic scattering

We provide a global density-dependent $^4$He-nucleon (DD-$αN$) interaction to construct the $α$-nucleus optical model potential (OMP) in a wide range of incident energies. The global parametrization for the DD-$αN$ interaction is obtained based on the proton-$^4$He OMP which reproduces the elastic scattering cross-section data very well in the incident energies of 12.04--500 MeV per nucleon. We derive the $α$-nucleus potential by a folding procedure with the point-nucleon density obtained by a microscopic mean-field model using the present DD-$αN$ interaction. The density dependence of the DD-$αN$ interaction is fixed phenomenologically to reproduce the $α$-nucleus elastic scattering cross-section data by the $^{16}$O, $^{40}$Ca, $^{58}$Ni, $^{90}$Zr, and $^{208}$Pb targets at $E/A =$ 10--342.5 MeV. We also show the total reaction cross sections, which are helpful in fixing one free parameter, the renormalization factor for the imaginary part of the $α$-nucleus potential. Lastly, we show some examples, which clearly demonstrate the validity and power of the present DD-$αN$ approach.

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arXiv2023

Novel approach to the removal of the Pauli-forbidden states in the orthogonality condition model: A case of multi-$α$ systems

We propose to use a basis function constructed based on the microscopic cluster model for an efficient description of multi-cluster systems with the orthogonality condition originating from the Pauli principle. The basis function is expressed analytically by a superposition of correlated Gaussian functions. We demonstrate the power of this approach by taking an example of a $3α$ system, $^{12}$C. A comparison with the conventional pseudopotential method using the projection operator is made. The present method offers efficient and numerically stable computations as the number of basis functions is significantly reduced compared to the conventional method. We show that the present basis function includes reasonably small components of the Pauli-forbidden states, allowing us to discuss simply the structure of the first excited $0^+$ state, Hoyle state.

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arXiv2023

Dineutron-dineutron correlation in $^8$He

Background: The four-neutron correlation has been attracting much attention for decades. In addition to the study on the tetra-neutron system, it is worthwhile to investigate the correlation in bound systems. Purpose: The $^8$He nucleus is a system where four neutrons are weakly bound around the $^4$He core. The dineutron ($2n$) correlation has been long discussed in various weakly-bound neutron-rich nuclei such as $^6$He and $^{11}$Li, whereas the $^8$He nucleus gives us an opportunity to investigate the $2n$-$2n$ type four-neutron correlation. Methods: We introduce a microscopic $^4{\rm He}+4n$ model and describe the ground-state structure of $^8$He. The mixing of the two-$2n$ component in the ground state is examined. The ground-state wave function is verified by investigating various observables including high-energy scattering cross sections. Results: Our model reasonably reproduces the available experimental data, the binding energy, charge radius, total reaction cross section, and proton-nucleus elastic scattering cross section data. We find that the significant mixing of the two-$2n$ cluster configurations around $^4$He in the ground state of $^8$He: The ground state has a squared overlap of about 45% with a $2n$-$^4$He-$2n$ configuration with the $^4$He-$2n$ distance of 3 fm and opening angle of 80$^\circ$. Conclusion: The ground state of $^8$He contains a certain amount of the two-$2n$ cluster component, indicating the strong nuclear deformation, which was experimentally observed recently.

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arXiv2023

Evidence of bicluster structure in the ground state of $^{20}$Ne

We explore the structure of the ground state of $^{20}$Ne by investigating various density profiles. Four candidates for the ground state configurations, (a) $j$-$j$ coupling and (b) SU(3) shell model and (c) $5α$ and (d) $^{16}{\rm O}+α$ cluster model configurations are generated by utilizing the antisymmetrized quasicluster model. A high-energy reaction theory, the Glauber model, relates these one-body density distributions and reaction observables. The angular distributions of the elastic scattering cross sections clearly distinguish these configurations and tell which is the most plausible one: The ground state of $^{20}$Ne favors a 16+4 nucleon bi-cluster structure. A comprehensive investigation of other electric observables also supports this conclusion.

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arXiv2023

Enlarged deformation region in neutron-rich Zr isotopes by the second intruder orbit

Nuclear deformations and density profiles of neutron-rich even-even Zr isotopes are investigated using the Skyrme-Hartree-Fock-Bogoliubov method. Large quadrupole and hexadecapole deformations are predicted along with large enhancement of the total reaction cross sections at the neutron number $N=60$-74. Strong nuclear deformation starting at $N=60$ is induced by the occupation of the intruder orbit with the asymptotic quantum number $[nn_zΛ]Ω$ = [550]1/2 originating from the spherical $0h_{11/2}$ orbit. The deformation region is further enlarged from $N=72$ to 74 owing to the occupation of the next intruder orbit with [530]1/2 originating from the spherical $1f_{7/2}$ orbit. This characteristic nuclear deformation is crucially reflected in the systematic behavior of the nuclear radii and the density profiles near the nuclear surface.

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arXiv2023

Electron wave functions in beta-decay formulas revisited (II): Completion including recoil-order and induced currents

We present complete formulas of the allowed and first-forbidden transitions of the nuclear beta decay taking into account the recoil-order and induced currents up to the next-to-leading order (NLO). The longitudinal part of the vector current is cleared away by the use of the conservation of the vector current for the multipole operators of the natural-parity transitions, which makes the effect of the meson exchange current for the vector current as small as possible. The formula is transparent enough to be applied to various beta-decay processes. As a numerical demonstration, we apply our formulas to the beta decay of a neutron-rich nucleus $^{160}$Sn. We find that the NLO corrections amount to 10--20\% of the total decay rate, whereas the induced currents alter the rate at most 5\%.

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arXiv2023

The $^{19}$N($n$,$γ)^{20}$N capture rate in light of the probable bubble nature of $^{20}$N

We aim to explore the bubble nature of the exotic nucleus $^{20}$N within the microscopic antisymmetrized molecular dynamics (AMD) approach. Constraining its structural parameters, we analyse its static properties. Subsequently, we use the AMD infused finite-range distorted-wave Born approximation theory to calculate the Coulomb breakup of $^{20}$N as an indirect approach to estimate the $^{19}$N$(n,γ)^{20}$N radiative capture rate.

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arXiv2023

Three-$α$ configurations of the second $J^π=2^+$ state in $^{12}$C

We investigate geometric configurations of $α$ ($^4$He nucleus) clusters in the second $J^π=2^+$ state of $^{12}$C, which has been discussed as a rotational band member of the second $0^+$ state, the Hoyle state. The ground and excited $0^+$ and $2^+$ states are described by a three-$α$ cluster model. The three-body Schrödinger equation with orthogonality conditions is accurately solved by the stochastic variational method with correlated Gaussian basis functions. To analyse the structure of these resonant states in a convenient form, we introduce a confining potential. The two-body density distributions together with the spectroscopic information clarify the structure of these states. We find that main configurations of both the second $0^+$ and $2^+$ states are acute-angled triangle shapes originating from the $^8$Be($0^+$)$+α$ configuration. However, the $^8$Be$+α$ components in the second $2^+$ state become approximately 2/3 because the 8Be subsystem is hard to excite, indicating that the state is not an ideal rigid rotational band member of the Hoyle state.

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