International Journal of Modern Physics A

In order to explore the 4α-particle condensate state in ¹⁶O, we solve a full four-body equation of motion based on the 4α OCM (Orthogonality Condition Model) in a large 4α model space spanned by Gaussian basis functions. A full spectrum up to the state is reproduced consistently with the lowest six 0⁺ states of experimental spectrum. The state is obtained at 2 MeV above the 4α breakup threshold and has a dilute density structure, where the rms radius is more than 5 fm. The state has an appreciably large α condensate fraction 61 %, and a highly amount of components, both of which are strong pieces of evidence of the state being the 4α condensate state.

We explain how to treat a microscopic wave function of α-condensation named THSR taking 3α-condensation as a typical example. The microscopic model, which fully takes into account the Pauli principle between all the constituent nucleons and effective inter-nucleon forces simultaneously, can play an important role in reproducing an α-gas-like nature thanks to α-condensation. We study its typical features by giving numerical results of the norm kernel for 3α-condensation.

The condensed states of α-clusters or di-neutrons are studied by introducing a Monte Carlo technique for the description of the THSR (Tohsaki Horiuchi Schuck Röpke) wave function. The condensed state of five alpha particles is shown to be stable around the threshold energy even after taking into account the coupling effect between normal cluster states with ¹⁶O+α configurations. Next, the mixing of di-neutron components in ⁸He is studied. The ground state of ⁸He corresponds to the sub-closed-shell configuration of the spin-orbit favored orbits (p_3/2) for the neutrons, and the shell-model-like configuration is considered to be dominant. However, ⁸He is a drip-line nucleus with a two-neutron separation energy of 2.1 MeV, and the di-neutron configuration, which is a characteristic feature of the weakly bound systems, mixes in the ground state. Finally, characteristic mechanism for the binding of clusters in neutron-rich nuclei (s² bond) is discussed.

A selfconsistent many body approach for the description of gases with quartets, trions, and pairs is presented. Applications to 3D Fermi systems at low density are discussed.

From the viewpoint of a unified description of cluster structure and scattering in the A = 90 region, α scattering from ⁸⁹Y is investigated. α clustering and weak coupling in ⁹³Nb is discussed.

Strengths of monopole excitation to cluster states are discussed in light nuclei. They are in general so strong to be comparable to the single particle strength and shares an appreciable portion of the sum rule value, in spite of large difference of the structure between the cluster state and the shell-model-like ground state. We point out that the essential reasons of the large strengths are twofold: One is the fact that the clustering degree of freedom is possessed even in simple shell model wave functions (this fact is indicated by Bayman-Bohr theorem), and the other is due to the ground-state correlation induced by the activation of the cluster degree of freedom originally involved in the ground state. They are demonstrated in the case of ¹⁶O as a typical light nucleus. The present results imply that the measurement of strong monopole transitions or excitations is generally very useful for the study of cluster states.

We analyze the α inelastic scattering on ¹²C by the coupled-channel calculation based on the folding model potential. We show that the ordinary folding model calculation tends to overestimate the inelastic angular distribution, and that this discrepancy is not due to the 3α breakup effect. In order to resolve this problem, we propose a folding procedure considering the well-developed 3α cluster structure of .

The self-consistent Green's functions method is employed to study the spectroscopic factors of quasiparticle states around ^16,28O and ^40,60Ca. The Faddeev random phase approximation (FRPA) is used to account for the coupling of particles with collective excitation modes. Results for ¹⁶O are reviewed first. The same approach is applied to isotopes with large proton-neutron asymmetry to estimate its effect on spectroscopic factors. The results, based on the chiral N3LO force, exhibit an asymmetry dependence similar to that observed in heavy-ion knockout experiments but weaker in magnitude.

Positive-parity states of ²⁸Si are studied with antisymmetrized molecular dynamics and multi-configuration mixing focusing on clustering and deformations. By superposition of wave functions obtained by energy variation imposing constraints on quadrupole deformation parameter β and distance between α and ²⁴Mg, and ¹²C and ¹⁶O clusters, two oblate bands, two prolate bands and developed α-²⁴Mg band are obtained. It is found that cluster structure components are contained in those bands.

Three-body cluster-model calculations are performed for the new types of big-bang nucleosynthesis (BBN) reactions that are calalyzed by a supersymmetric (SUSY) particle stau, a scalar partner of the tau lepton. If a stau has a lifetime ≳ 10³s, it would capture a light element previously synthesized in standard BBN and form a Coulombic bound state. The bound state, an exotic atom, is expected to induce various reactions, such as (αX^-) + d → ⁶Li + X^-, in which a negatively charged stau (denoted as X^-) works as a catalyzer. Recent literature papers have claimed that some of these stau-catalyzed reactions have significantly large cross sections so that inclusion of the reactions into the BBN network calculation can change drastically abundances of some elements, giving not only a solution to the ⁶Li-⁷Li problem (calculated underproduction of ⁶Li by ~ 1000 times and overproduction of ⁷Li+⁷Be by ~ 3 times) but also a constraint on the lifetime and the primordial abundance of the elementary particle stau. However, most of these literature calculations of the reaction cross sections were made assuming too naive models or approximations that are unsuitable for those complicated low-energy nuclear reactions. We use a few-body calculational method developed by the authors, and provides precise cross sections and rates of the stau-catalyzed BBN reactions for the use in the BBN network calculation.

We study the structure of vortices and their interaction with nuclei in the inner crust of neutron stars. Vortices carry most of the angular momentum of the crust and according to some models are expected to play a key role in the phenomenon of glitches. The quantities of interest (the energy, the pairing gap, the density and the velocity field) are calculated in the framework of the Hartree-Fock-Bogoliubov (HFB) quantum mean field theory. We solve the HFB equations testing four different Skyrme functionals in the particle hole-channel and using a density-dependent contact interaction in the particle-particle channel. We find that the structure of a vortex is strongly affected by the presence of the nucleus, a fact which reflects itself in the density dependence of the pinning energy we calculate.

Prospects of interesting hypernuclear structures are discussed for typical light p-shell systems with the cluster model application and also for the medium-heavy systems with shell-model applications. Corresponding to the recent experimental progress, importance of production reaction spectroscopy are emphasized.

In recent years, great progress has been made in hypernuclear physics through new experimental techniques as well as theoretical development. Precision γ-ray spectroscopy of hypernuclei has revealed fine structure of various light Λ hypernuclei. Introduction of the (π^-, K⁺) reaction has opened a possibility of studying neutron-rich Λ hypernuclei. The J-PARC facility will allow us to accelerate these experimental studies of Λ hypernuclei and extend hypernuclear physics to double strangess nuclear systems.

We study the hyperon-nucleon (YN) forces by using quenched lattice QCD. The Bethe-Salpeter amplitudes are calculated for the lowest scattering state of the systems so as to obtain the YN potentials. The numerical calculation is twofold: (i) The pΞ⁰ potentials and scattering lengths are obtained by using lattice QCD with β = 5.7, the lattice spacing of a = 0.1416(9) fm, on the 32³ × 32 lattice. Two kinds of ud quark mass are used, corresponding to m_π ≃ 0.37 GeV and 0.51 GeV. The spatial lattice volume is (4.5 fm)³. The scattering lengths obtained from Lüscher's formula show that the pΞ⁰ interactions are both attractive at ¹S₀ and ³S₁ channels, and the interaction in the ³S₁ is more attractive than in the ¹S₀. These attractive forces become stronger as the u, d quark mass decreases. (ii) The pΛ potentials are calculated. The lattice setup is almost same as the former calculation except for the temporal part. The calculation is performed on 32³ × 48 lattice. Two kinds of ud quark mass are used, corresponding to m_π ≃ 0.47 GeV and 0.51 GeV. The lowest scattering energies in the finite lattice volume are calculated.

We have studied the structure of K^-pp by solving this system in a variational method, starting from the ansatz that Λ* = Λ(1405) is a K^-p quasi-bound state. The structure of K^-pp reveals a molecular feature, namely, the K^- in Λ* as an "atomic center" plays a key role in producing strong covalent bonding with the other proton. Deeply bound nuclear systems are formed by this "super-strong" nuclear force due to migrating real bosons a la Heitler-London-Heisenberg, which overcompensates the stiff nuclear incompressibility. Theoretical background of our phenomenological interaction is discussed, in connection with "decaying-state" description of kaonic nuclear clusters, concerning the effective interaction based on chiral SU(3) dynamics. It is of virtual importance to determine whether the Λ* mass is 1405 MeV or 1420 MeV. It is found that a K^- absorption at rest in ³He provides a testing ground better than that in ⁴He.

The formation of a deeply-bound K^-pp state by the ³He(in-flight K^-, n) reaction is investigated theoretically in a distorted-wave impulse approximation using the Green's function method. The expected inclusive and semi-exclusive spectra at p_K^- = 1.0 GeV/c and θ_n = 0∘ are calculated for the forthcoming J-PARC E15 experiment. We discuss these spectra with some K^--"pp" optical potentials, taking into account the energy-dependence of the imaginary part of the potentials by a phase space suppression factor.

Four-body calculations with explicitly correlated Gaussian bases are performed for excited states of ⁴He. All the levels below E_x=26 MeV are reproduced reasonably well using realistic potentials. An analysis is made to show how the state becomes a resonance but those having almost the same structure as this state in different spin-isospin channels are not observed as resonances. The calculation of spectroscopic amplitudes, nucleon decay widths, and spin-dipole transition strengths demonstrates that the state and the three lowest-lying negative-parity states with 0^- and 2^- have ³H+p and ³He+n cluster configurations, leading to the interpretation that these negative-parity states are the inversion-doublet partners of the state.

We propose a new approach of the Brueckner-AMD that makes the antisymmetrized molecular dynamics (AMD) calculations possible with realistic nuclear forces. In this method, we solve the Bethe-Goldstone equation and calculate the G-matrix for every nucleon pair described by wave packets of AMD correctly. We report the results of applications to light nuclei with the Brueckner-AMD.

We show the importance of the tensor correlation on the structures of He and Li isotopes. We develop the tensor optimized shell model (TOSM) to describe the strong tensor correlation of the nuclear force in nuclei. We investigate the exotic structures of ^9,10,11Li based on the ⁹Li+n+n model where TOSM is used for ⁹Li. The coupling between the tensor correlation in ⁹Li and the extra two neutrons emerges Pauli-blocking, which increases the s² component and develops the halo structure in ¹¹Li.

We further treat the short range repulsion of the nuclear force by the unitary correlation operator method (UCOM), while the tensor correlation is described by TOSM. We propose a new approach of TOSM+UCOM to describe nuclei from the nuclear force.

We describe the principle of the time-dependent density-functional theory (TDDFT), as well as that of the density functional theory for a localized density and for a deformed density profile. The real-time real-space TDDFT is utilized to calculate photoabsorption cross sections for even-even nuclei. Results for prolate and oblate nuclei, ²⁴Mg and ²⁸Si, reproduce basic features of experimental data.

Lifetime measurement of the first excited 2⁺ state in ¹⁸C was performed using an upgraded recoil shadow method to determine the electric quadrupole transition. The measured mean lifetime is 18.9 ± 0.9 (stat) ± 4.4 (syst) ps, which corresponds to a value of 4.3 ± 0.2 ± 1.0 e²fm⁴, or about 1.5 Weisskopf units. The mean lifetime of the first 2⁺ state in ¹⁶C was remeasured to be about 18 ps, about four times shorter than the value reported previously. By incorporating the γ-ray angular distribution measured in this work into the previous measurement, this discrepancy has been explained. The observed transition strengths in ^16,18C are hindered compared to the empirical transition strengths, indicating that the anomalous hindered E2 strength observed in ¹⁶C persists in ¹⁸C.

The generalized two-center cluster model (GTCM), which can treat various single particle configurations in general two center systems, is applied to the light neutron-rich system, ¹²Be= α+α+4N. We discuss the change of the neutrons' configuration around two α-cores as a variation of an excitation energy. We found that the covalent, ionic and atomic configurations appear with a prominent degenerating feature above the α+⁸He_g.s. particle-decay threshold.

Structure of excited states in ¹⁴C were investigated with a method of the antisymmetrized molecular dynamics model. Variation with constraints on the quadrupole deformations (β, γ) was performed, and the configurations were superposed within the AMD framework. Above the ground band, three excited bands with the developed 3α-cluster core structures were suggested. Characteristic structures such as a triaxial structure and a linear-chain structure appear in the excited bands. It was found that the ¹⁰Be correlation exists in the linear-chain structure.

We present analyses of breakup effects of ⁶He on the elastic scattering by the continuum-discretized coupled-channels method, in which the reaction system is described as a four-body model, n+n+⁴He+target. In this analysis, three-body breakup continuum of ⁶He is discretized by daiagonalizing the internal Hamiltonian of ⁶He in a space spanned by the Gaussian basis functions. The calculated elastic cross sections are in good agreement with the experimental data, which shows that nuclear and Coulomb breakup effects are significant.

We present a brief history of the structure study of ¹⁶O as a typical example in the development of the cluster model theory over 50 years.