Searcharxiv⌕ Search

arXiv subjects

Satoshi Chiba

Publications and source records attributed to Satoshi Chiba.

At least 55 records · Page 3Linked to original sources
arXiv2005

In-medium Effects in the Eta-Photoproduction through the $S_{11}$ Resonance with the Relativistic Approach

In-medium effects in photon excitation of the S$_{11}$ resonance from nucleon and its decay to $η$-meson using the RMF approach is studied. We examine the in-medium effects by varing the Dirac fields of the nucleon and the S$_{11}$ resonance separately. We obtained a conclusion that the Dirac fields of the nucleon, which reduces the nucleon effective mass, enlarge the width of S$_{11}$ resonance, and small Dirac-fields of the resonance shifts the peak position, and these combined effects can explain the in-medium properties observed in the experimental data for $^{12}$C($γ$,$η$) reaction.

nucl-th↗
arXiv2005

Nuclear Electromagnetic Current in the Relativistic Approach with the Momentum-Dependent Selfenergies

We define the new description of the electromagnetic current to hold the current conservation in the momentum-dependent Dirac fields from the Ward Takahashi identity. To describe the momentum-dependence we solve the relativistic Hartree-Fock approximation by using the one-pion exchange. In addition we discuss on contribution from the one-pion exchange current and the core polarization. It is shown that this current can reduce the convection current in the isovector case, whose value has been too big due to the small effective mass in the usual relativistic Hartree approximation.

nucl-th↗
arXiv2005

Nuclear pasta structures and the charge screening effect

Non uniform structures of the nucleon matter at subnuclear densities are numerically studied by means of the density functional theory with relativistic mean-fields coupled with the electric field. A particular role of the charge screening effects is demonstrated.

nucl-th↗
arXiv2005

Hadron-quark matter phase transition in neutron stars

A structured mixed phase consisting of quark and hadron phases is numerically studied with the Coulomb screening effect and the surface effect. We carefully introduced the Coulomb potential, so that a geometrical structure becomes mechanically unstable when the surface tension is large. Charge densities are largely rearranged by the screening effect, and thereby the equation of state shows the similar behavior to that given by the Maxwell construction. Therefore, although bulk calculations with the Gibbs conditions show that the mixed phase may exist in a wide density region, we can see it is restricted to a narrow density region by the surface effect and the Coulomb screening effect.

hep-ph↗
arXiv2005

Nuclear pasta structures in neutron stars and the charge screening

Non-uniform structures of the nucleon matter are expected at subnuclear densities and above the nuclear density: they are called nuclear pastas and kaon pastas, respectively. We numerically study these phases by means of the density functional theory with relativistic mean-fields and the electric field; the electric field is properly taken into account. Our results demonstrate a particular role of the charge screening effects on these non-uniform structures.

nucl-th↗
arXiv2004

^{1}S_{0} proton superfluidity in neutron star matter: Impact of bulk properties

We study the ^{1}S_{0} proton pairing gap in neutron star matter putting emphasis on influence of the Dirac effective mass and the proton fraction on the gap within the relativistic Hartree-Bogoliubov model. The gap equation is solved using the Bonn-B potential as a particle-particle channel interaction. It is found that the maximal pairing gap Δ_{max} is 1-2 MeV, which has a strong correlation with the Dirac effective mass. Hence we suggest that it serves as a guide to narrow down parameter sets of the relativistic effective field theory. Furthermore, the more slowly protons increase with density in the core region of neutron stars, the wider the superfluid range and the slightly lower the peak of the gap become.

nucl-th↗
arXiv2004

Stability and structure of quark matter in a molecular dynamics framework

We study stability and structure of quark matters as a function of density in a framework of molecular dynamics (MD). Using appropriate effective interactions and the frictional cooling method, we search for the minimum energy of the system. Transition from confined to deconfined phase is observed at densities of 2 -- 3$ρ_0$, where $ρ_0$ is the nuclear matter saturation density. The $uds$ matter becomes more stable than the charge-neutral $ud$ matter at 3$ρ_0$, but the $udd$ matter is the most stable even at high density.

nucl-th↗
arXiv2004

Numerical study of the hadron-quark mixed phase

The Coulomb screening effect and the finite-size effect such as surface tension are figured out in the hadron-quark deconfinement phase transition. We study the mixed phase of the quark droplets immersed in hadron matter. We see that the droplet phase is mechanically unstable if the surface tension is strong enough. Once the Coulomb potential is properly taken into account, we could effectively satisfy the condition for charge chemical equilibrium in the Maxwell construction. As a result, we suggest the Maxwell construction revives the physical meaning effectively.

nucl-th↗
arXiv2004

Coulomb screening effect on the nuclear-pasta structure

Using the density functional theory (DFT) with the relativistic mean field (RMF) model, we study the non-uniform state of nuclear matter, ``nuclear pasta''. We self-consistently include the Coulomb interaction together with other interactions. It is found that the Coulomb screening effect is significant for each pasta structure but not for the bulk equation of state (EOS) of the nuclear pasta phase.

nucl-th↗
arXiv2003

Kaon Condensation and the Non-Uniform Nuclear Matter

Non-uniform structures of nuclear matter are studied in a wide density-range. Using the density functional theory with a relativistic mean-field model, we examine non-uniform structures at sub-nuclear densities (nuclear ``pastas'') and at high densities, where kaon condensate is expected. We try to give a unified view about the change of the matter structure as density increases, carefully taking into account the Coulomb screening effects from the viewpoint of first-order phase transition.

nucl-th↗
arXiv2003

Possibility of ΛΛpairing and its dependence on background density in relativistic Hartree-Bogoliubov model

We calculate a ΛΛpairing gap in binary mixed matter of nucleons and Λhyperons within the relativistic Hartree-Bogoliubov model. Lambda hyperons to be paired up are immersed in background nucleons in a normal state. The gap is calculated with a one-boson-exchange interaction obtained from a relativistic Lagrangian. It is found that at background density ρ_{N}=2.5ρ_{0} the ΛΛpairing gap is very small, and that denser background makes it rapidly suppressed. This result suggests a mechanism, specific to mixed matter dealt with relativistic models, of its dependence on the nucleon density. An effect of weaker ΛΛattraction on the gap is also examined in connection with revised information of the ΛΛinteraction.

nucl-th↗
arXiv2001

Vlasov calculations of nuclear ground states

We describe some properties of nuclear ground states using a microscopic Vlasov approach. We propose a new method to find the energy minimum of the system enforcing the fermionic nature of the system. Calculations are performed for two different parametrization of the mean field: a simple momentum independent Skyrme potential and a momentum dependent one as proposed by Gale, Bertsch and Das Gupta. We show that the binding energies and radii of nuclei from $^{16}$O to $^{208}$Pb are reasonably reproduced by opportunely tuning the surface term. We also made some calculations for neutron rich/poor nuclei.

nucl-th↗
arXiv2001

Formation and decay of super heavy systems

We investigate the formation and the decay of heavy systems which are above the fission barrier. By using a microscopic simulation of constraint molecular dynamics (CoMD) on Au+Au collision, we observe composite states stay for very long time before decaying by fission.

nucl-th↗
arXiv2001

Nuclear fragmentation by tunneling

Fragmentation of nuclear system by tunneling is discussed in a molecular dynamics simulation coupled with imaginary time method. In this way we obtain informations on the fragmenting systems at low densities and temperatures. These conditions cannot be reached normally (i.e. above the barrier) in nucleus-nucleus or nucleon-nucleus collisions. The price to pay is the small probability of fragmentation by tunneling but we obtain observables which can be a clear signature of such phenomena.

nucl-th↗
arXiv1999

Isoscalar Giant Quadrupole Resonance State in the Relativistic Approach with the Momentum-Dependent Self-Energies

We study the excited energy of the isoscalar giant quadrupole resonance with the scaling method in the relativistic many-body framework. In this calculation we introduce the momentum-dependent parts of the Dirac self-energies arising from the one-pion exchange on the assumption of the pseudo-vector coupling with nucleon field. It is shown that this momentum-dependence enhances the Landau mass significantly and thus suppresses the quadrupole resonance energy even giving the small Dirac effective mass which causes a problem in the momentum-independent mean-field theory.

nucl-th↗
arXiv1996

Analysis of proton-induced fragment production cross sections by the Quantum Molecular Dynamics plus Statistical Decay Model

The production cross sections of various fragments from proton-induced reactions on $^{56}$Fe and $^{27}$Al have been analyzed by the Quantum Molecular Dynamics (QMD) plus Statistical Decay Model (SDM). It was found that the mass and charge distributions calculated with and without the statistical decay have very different shapes. These results also depend strongly on the impact parameter, showing an importance of the dynamical treatment as realized by the QMD approach. The calculated results were compared with experimental data in the energy region from 50 MeV to 5 GeV. The QMD+SDM calculation could reproduce the production cross sections of the light clusters and intermediate-mass to heavy fragments in a good accuracy. The production cross section of $^{7}$Be was, however, underpredicted by approximately 2 orders of magnitude, showing the necessity of another reaction mechanism not taken into account in the present model.

nucl-th↗