arXiv subjects

K. Hagino

Publications and source records attributed to K. Hagino.

At least 145 records · Page 8Linked to original sources

arXiv2011

Relation between shrinkage effect and compression of rotational spectrum in $^7_Λ$Li hypernucleus

It has been shown experimentally that the $^6$Li nucleus shrinks by adding a $Λ$ particle while its spectrum is also compressed. We discuss the compatibility of these two effects, contrasting also with a relation between the shrinkage effect and the stability of the spectrum in the $^9_Λ$Be hypernucleus. To this end, we employ two-body $d$-$^5_Λ$He and $α$-$^5_Λ$He cluster models for the $^7_Λ$Li and $^9_Λ$Be hypernuclei, respectively. We first argue that a Gaussian-like interaction between two clusters leads to a stabilization of the spectrum against an addition of a $Λ$ particle, even though the intercluster distance is reduced. In the case of $^7_Λ$Li, the spin-orbit interaction between the intercluster motion and the deuteron spin has to be considered also. We show that the shrinkage effect makes the expectation value of the spin-orbit potential larger, lowering the excitation energy for the $^6$Li($3^+)\otimes Λ(1/2^+)$ level.

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arXiv2011

Description of single-Lambda hypernuclei with relativistic point coupling model

We extend the relativistic point coupling model to single-$Λ$ hypernuclei. For this purpose, we add $N$-$Λ$ effective contact couplings to the model Lagrangian, and determine the parameters by fitting to the experimental data for $Λ$ binding energies. Our model well reproduces the data over a wide range of mass region although some of our interactions yield the reverse ordering of the spin-orbit partners from that of nucleons for heavy hypernuclei. The consistency of the interaction with the quark model predictions is also discussed.

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arXiv2011

Impurity effect of $Λ$ hyperon on shape-coexistence nucleus $^{44}$S in the energy functional based collective Hamiltonian

The non-relativistic Skyrme energy density functional (EDF) based collective Hamiltonian, that takes into account dynamical correlations related to the restoration of broken symmetries and fluctuations of quadrupole collective variables, is applied to quantitatively study the impurity effect of $Λ$ hyperon on the collectivity of $^{44}$S. Several Skyrme forces for both the nucleon-nucleon ($NN$) and $Λ$-nucleon ($ΛN$) interactions are used. The influence of pairing strengths on the polarization effect of $Λ$ hyperon is also examined. It is found that although these Skyrme forces with different pairing strengths give somewhat different low-lying spectra for $^{44}$S, all of them give similar and generally small size of $Λ$ reduction effect (within 5%) on the collective properties.

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arXiv2011

Hyperon effects in covariant density functional theory with recent astrophysical observations

Motivated by recent observational data, the equations of state with the inclusion of strangeness-bearing $Λ$-hyperons and the corresponding properties of neutron stars are studied, based on the covariant density functional (CDF) theory. To this end, we specifically employ the density dependent relativistic Hartree-Fock (DDRHF) theory and the relativistic mean field theory (RMF). The inclusion of $Λ$-hyperons in neutron stars shows substantial effects in softening the equation of state. Because of the extra suppression effect originated from the Fock channel, large reductions on both the star mass and radius are predicted by the DDRHF calculations. It is also found that the mass-radius relations of neutron stars with $Λ$-hyperons determined by DDRHF with the PKA1 parameter set are in fairly good agreement with the observational data where a relatively small neutron stars radius is required. Therefore, it is expected that the exotic degrees of freedom such as the strangeness-bearing structure may appear and play significant roles inside the neutron stars, which is supported further by the systematical investigations on the consistency between the maximum neutron star mass and $Λ$-coupling strength.

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arXiv2011

Effect of proton-proton Coulomb repulsion on soft dipole excitations of light proton-rich nuclei

We perform three-body model calculations for soft dipole excitations of the proton-rich Borromean nucleus $^{17}$Ne. To this end, we assume that $^{17}$Ne takes the $^{15}$O+p+p structure, in which the two valence protons are excited from the $0^+$ ground state configuration to $1^-$ continuum states. We employ a density-dependent contact force for the nuclear part of the pairing interaction, and discretize the continuum states with the box boundary condition. We show by explicitly including the Coulomb interaction between the valence protons that the Coulomb repulsion does not significantly alter the E1 strength distribution. We point out that the effect of the Coulomb repulsion in fact can be well simulated by renormalizing the nuclear pairing interaction.

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arXiv2011

Unveiling pairing anti-halo effect in odd-even staggering in reaction cross sections of weakly bound nuclei

We investigate the spatial extension of weakly bound Ne and C isotopes by taking into account the pairing correlation with the Hartree-Fock-Bogoliubov (HFB) method and a 3-body model, respectively. We show that the odd-even staggering in the reaction cross sections of $^{30,31,32}$Ne and $^{14,15,16}$C are successfully reproduced, and thus the staggering can be attributed to the pairing anti-halo effect. A correlation between a one-neutron separation energy and the anti-halo effect is demonstrated for $s$- and p-waves using the HFB wave functions.

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arXiv2011

Impurity effect of Lambda hyperon on collective excitations of atomic nuclei

Taking the ground state rotational band in $^{24}$Mg as an example, we investigate the impurity effect of $Λ$ hyperon on collective excitations of atomic nuclei in the framework of non-relativistic energy density functional theory. To this end, we take into account correlations related to the restoration of broken symmetries and fluctuations of collective variables by solving the eigenvalue problem of a five-dimensional collective Hamiltonian for quadrupole vibrational and rotational degrees of freedom. The parameters of the collective Hamiltonian are determined with constrained mean-field calculations for triaxial shapes using the SGII Skyrme force. We compare the low-spin spectrum for $^{24}$Mg with the spectrum for the same nucleus inside $^{25}_Λ$Mg. It is found that the $Λ$ hyperon stretches the ground state band and reduces the $B(E2:2^+_1 \rightarrow 0^+_1)$ value by $\sim 9%$, mainly by softening the potential energy surface towards the spherical shape, even though the shrinkage effect on the average proton radius is only $\sim0.5%$.

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arXiv2011

Coulomb dissociation of the deformed halo nucleus 31Ne

The recently observed large cross sections for the Coulomb dissociation of $^{31}$Ne nucleus indicate that this nucleus takes a halo structure in the ground state. We analyse these experimental data using the particle-rotor model, that takes into account the rotational excitation of the core nucleus $^{30}$Ne. We show that the experimental data can be well reproduced when the quadrupole deformation parameter of $^{30}$Ne is around $β_2=0.2\sim0.3$, at which the ground state takes the spin and parity of $I^π$=3/2$^-$. This state corresponds to the Nilsson level [330 1/2] in the adiabatic limit of the particle-rotor model. On the other hand, the state corresponding to the Nilsson level [321 3/2] with $β_2\sim 0.55$ can be excluded when the finite excitation energy of the core is taken into account, even though this configuration is a candidate for the ground state of $^{31}$Ne in the Nilsson model analysis. We discuss briefly also a possibility of the $I^π$=1/2$^+$ configuration with $β_2\sim 1$ and $β_2\sim -0.4$.

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arXiv2010

Two-neutron halo nuclei in one dimension: dineutron correlation and breakup reaction

We propose a simple schematic model for two-neutron halo nuclei. In this model, the two valence neutrons move in a one-dimensional mean field, interacting with each other via a density-dependent contact interaction. We first investigate the ground state properties, and demonstrate that the dineutron correlation can be realized with this simple model due to the admixture of even- and odd-parity single-particle states. We then solve the time-dependent two-particle Schrödinger equation under the influence of a time-dependent one-body external field, in order to discuss the effect of dineutron correlation on nuclear breakup processes. The time evolution of two-particle density shows that the dineutron correlation enhances the total breakup probability, especially for the two-neutron breakup process, in which both the valence neutrons are promoted to continuum scattering states. We find that the interaction between the two particles definitely favours a spatial correlation of the two outgoing particles, which are mainly emitted in the same direction.

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arXiv2010

Shape of Lambda hypernuclei in (beta,gamma) deformation plane

We study the shape of $Λ$ hypernuclei in the full ($β,γ$) deformation plane, including both axially symmetric and triaxial quadrupole deformations. To this end, we use the constrained Skyrme Hartree-Fock+BCS method on the three-dimensional Cartesian mesh. The potential energy surface is analyzed for carbon hypernuclei as well as for sd-shell hypernuclei such as $^{27,29}_Λ$Si and $^{25,27}_Λ$Mg. We show that the potential energy surface in the ($β,γ$) plane is similar to each other between the hypernuclei and the corresponding core nuclei, although the addition of $Λ$ hyperon makes the energy surface somewhat softer along the $γ$ direction. Our calculation implies that the energy of the $γ$ vibration for $^{25,27}_Λ$Mg nuclei is lowered by about 0.15 MeV with respect to that of $^{24,26}$Mg nuclei.

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arXiv2010

Effect of pairing correlations on incompressibility and symmetry energy in nuclear matter and finite nuclei

The role of superfluidity in the incompressibility and in the symmetry energy is studied in nuclear matter and finite nuclei. Several pairing interactions are used: surface, mixed and isovector dependent. Pairing has a small effect on the nuclear matter incompressibility at saturation density, but the effects are significant at lower densities. The pairing effect on the centroid energy of the isoscalar Giant Monopole Resonance (GMR) is also evaluated for Pb and Sn isotopes by using a microscopic constrained-HFB approach, and found to change at most by 10% the nucleus incompressibility $K_A$. It is shown by using the Local Density Approximation (LDA) that most of the pairing effect on the GMR centroid come from the low-density nuclear surface.

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arXiv2010

Iterative solution of a Dirac equation with inverse Hamiltonian method

We solve a singe-particle Dirac equation with Woods-Saxon potentials using an iterative method in the coordinate space representation. By maximizing the expectation value of the inverse of the Dirac Hamiltonian, this method avoids the variational collapse, in which an iterative solution dives into the Dirac sea. We demonstrate that this method works efficiently, reproducing the exact solutions of the Dirac equation.

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arXiv2010

Role of non-collective excitations in low-energy heavy-ion reactions

We investigate the effect of single-particle excitations on heavy-ion reactions at energies near the Coulomb barrier. To this end, we describe single-particle degrees of freedom with the random matrix theory and solve the coupled-channels equations for one-dimensional systems. We find that the single-particle excitations hinder the penetrability at energies above the barrier, leading to a smeared barrier distribution. This indicates that the single-particle excitations provide a promising way to explain the difference in a quasi-elastic barrier distribution recently observed in $^{20}$Ne + $^{90,92}$Zr systems.

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arXiv2010

Diproton correlation in a proton-rich Borromean nucleus 17Ne

We carry out three-body-model calculations for a proton-rich Borromean nucleus $^{17}$Ne by assuming a $^{15}$O + $p$ + $p$ structure. To this end, we use a density-dependent contact interaction between the valence protons, explicitly treating also the Coulomb interaction. We find that the two-particle density distribution for $^{17}$Ne is similar to that for $^{16}$C, which has two valence neutrons outside the $N$ = 8 core. That is, the two protons take a spatially compact configuration, while the Coulomb repulsion plays a minor role. This indicates that there is a strong diproton correlation in the ground state of the $^{17}$Ne nucleus. We also show that the Coulomb interaction reduces the expectation value of the proton-proton interaction by about 14\%.

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arXiv2009

Cooper pair sizes in 11Li and in superfluid nuclei: a puzzle?

We point out a strong influence of the pairing force on the size of the two neutron Cooper pair in $^{11}$Li, and to a lesser extent also in $^6$He. It seems that these are quite unique situations, since Cooper pair sizes of stable superfluid nuclei are very little influenced by the intensity of pairing, as recently reported. We explore the difference between $^{11}$Li and heavier superfulid nuclei, and discuss reasons for the exceptional situation in $^{11}$Li.

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arXiv2009

Reaction mechanisms for weakly-bound, stable nuclei and unstable, halo nuclei on medium-mass targets

An experimental overview of reactions induced by the stable, but weakly-bound nuclei 6Li, 7Li and 9Be, and by the exotic, halo nuclei 6He, 8B, 11Be and 17F on medium-mass targets, such as 58Ni, 59Co or 64Zn, is presented. Existing data on elastic scattering, total reaction cross sections, fusion processes, breakup and transfer channels are discussed in the framework of a CDCC approach taking into account the breakup degree of freedom.

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arXiv2009

Subbarrier fusion reactions with dissipative couplings

Using the random matrix model, we discuss the effect of couplings to non-collective states on the penetrability of a one dimensional potential barrier. We show that these non-collective excitations hinder the penetrability and thus smear the barrier distribution at energies above the barrier, while they do not affect significantly the penetrability at deep subbarrier energies. The energy dependence of the Q-value distribution obtained with this model is also discussed.

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arXiv2009

$β$-decay half-lives at finite temperatures for N=82 isotones

Using the finite temperature quasi-particle random phase approximation (FTQRPA) on the basis of finite temperature Skyrme-Hartree-Fock + BCS method, we study $β^-$-decay half-lives for even-even neutron magic nuclei with N=82 in a finite temperature environment. We find that the $β^-$-decay half-life first decreases as the temperature increases for all the nuclei we study, although the thermal effect is found to be small at temperatures relevant to r-process nucleosynthesis. Our calculations indicate that the half-life begins to increase at high temperatures for open shell nuclei. We discuss this behavior in connection to the pairing phase transition.

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