arXiv subjects

Koichi Saito

Publications and source records attributed to Koichi Saito.

At least 55 records · Page 3Linked to original sources

arXiv2017

Equation of state for hybrid stars with strangeness

Considering the mass constraint from the resent pulsar observations, we study the properties of neutron stars including hyperons and quarks explicitly. Using the chiral quark-meson coupling model with relativistic Hartree-Fock approximation, the equation of state (EoS) for hadronic matter is calculated by taking into account the strange ($σ^{\ast}$ and $ϕ$) mesons as well as the light non-strange ($σ$, $ω$, $\vecρ$, and $\vecπ$) mesons in SU(3) flavor symmetry. On the other hand, the EoS for quark matter is constructed with the simple MIT bag or the flavor-SU(3) Nambu-Jona-Lasinio model, and we investigate the effect of the hadron-quark coexistence on the neutron-star properties, imposing smooth crossover or Gibbs criterion for chemical equilibrium. The mass-radius relation of a neutron star, as well as physical quantities such as EoSs, particle fractions, and the speed of sound in matter are presented. We find that, in order to prevent the quark appearance at very low densities, the stiff hadronic EoS should be required under both of the hadron-quark crossover and the first-order phase transition.

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arXiv2016

Equation of state for neutron star matter with NJL model and Dirac-Brueckner-Hartree-Fock approximation

As the interior density of a neutron star can become very high, it has been expected and discussed that quark matter may exist inside it. To describe the transition from hadron to quark phases (and vice versa), there are mainly two methods; one is the first-order phase transition, and the other is the crossover phenomenon. In the present study, using the flavor-SU (3) NJL model with the vector coupling interaction, we have calculated the equation of state for the quark phase at high density. Furthermore, for the hadron phase at low density, we have used two kinds of the equations of state; one is a relatively soft one by the QHD model, and the other is a stiff one calculated with relativistic Brueckner-Hartree-Fock approximation. Using those equations of state for the two phases, we have investigated the influence of various choices of parameters concerning the crossover region on the mass and radius of a neutron star.

astro-ph.HE↗

arXiv2015

Equation of state for neutron stars with hyperons and quarks in relativistic Hartree-Fock approximation

We construct the equation of state (EoS) for neutron stars explicitly including hyperons and quarks. Using the quark-meson coupling model with relativistic Hartree-Fock approximation, the EoS for hadronic matter is derived by taking into account the strange ($σ^{\ast}$ and $ϕ$) mesons as well as the light non-strange ($σ$, $ω$, $\vecπ$ and $\vecρ$) mesons. Relevant coupling constants are determined to reproduce the experimental data of nuclear matter and hypernuclei in SU(3) flavor symmetry. For quark matter, we employ the MIT bag model with one-gluon-exchange interaction, and Gibbs criteria for chemical equilibrium in the phase transition from hadrons to quarks. We find that the strange vector ($ϕ$) meson and the Fock contribution make the hadronic EoS stiff, and that the maximum mass of a neutron star can be consistent with the observed mass of heavy neutron stars even if the coexistence of hadrons and quarks takes place in the core. However, in the present calculation the transition to pure quark matter does not occur in stable neutron stars. Furthermore, the lower bound of the critical chemical potential of the quark-hadron transition at zero temperature turns out to be around 1.5 GeV in order to be consistent with the recent observed neutron star data.

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arXiv2015

Hyperons in neutron stars

Using the Dirac-Brueckner-Hartree-Fock approach, the properties of neutron-star matter including hyperons are investigated. In the calculation, we consider both time and space components of the vector self-energies of baryons as well as the scalar ones. Furthermore, the effect of negative-energy states of baryons is partly taken into account. We obtain the maximum neutron-star mass of $2.08\,M_{\odot}$, which is consistent with the recently observed, massive neutron stars. We discuss a universal, repulsive three-body force for hyperons in matter.

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arXiv2014

Neutron stars with Hyperons in Dirac-Brueckner-Hartree-Fock approach

Using the Dirac-Brueckner-Hartree-Fock (DBHF) approach including the hyperon degrees of freedom, we investigate the properties of neutron-star matter. To handle the hyperons in matter, we first examine the importance of the space part of baryon self-energies at high densities, and secondly study the effect of negative-energy states of baryons, which can provide an unambiguous relationship between the in-medium reaction matrices for baryon-baryon scattering and the baryon self-energies. We solve the coupled, Bethe-Salpeter equations in the nuclear-matter rest frame by using the Bonn potentials. We assume that eight kinds of nonstrange and strange mesons ($σ,\,δ,\,ω,\,ρ,\,η,\,π,\,K,\,K^{\ast}$) take part in the interactions between two baryons. Then, we calculate the baryon self-energies, the energy density and pressure of matter. The present calculation provides a hard equation of state in neutron-star matter at high densities, which is generated by the effect of Pauli exclusion, the short-range correlations between two baryons, etc. We finally predict the maximum neutron-star mass of $2.02\,M_{\odot}$, which is consistent with both the recently observed masses, $1.97\pm0.04M_{\odot}$ (J1614-2230) and $2.01\pm0.04M_{\odot}$ (J0348+0432).

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arXiv2014

Equation of state for neutron stars: Hyperon mixing in SU(3) flavor symmetry

Using various kinds of relativistic mean-field models as well as the quark-meson coupling model, we study in detail the properties of neutron stars. We find that the equation of state in SU(3) flavor symmetry can support a neutron star with mass of $(1.8 \sim 2.1) M_{\odot}$ even if hyperons exist inside the core of a neutron star.

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arXiv2013

Properties of dense, asymmetric nuclear matter in Dirac-Brueckner-Hartree-Fock approach

Within the Dirac-Brueckner-Hartree-Fock approach, using the Bonn potentials, we investigate the properties of dense, asymmetric nuclear matter and apply it to neutron stars. In the actual calculations of the nucleon self-energies and the energy density of matter, we study in detail the validity of an angle-averaged approximation and an averaging of the total momentum squared of interacting two-nucleons in nuclear matter. For practical use, we provide convenient parametrizations for the equation of state for symmetric nuclear matter and pure neutron matter. We also parametrize the nucleon self-energies in terms of polynomials of nucleon momenta. Those parametrizations can accurately reproduce the numerical results up to high densities.

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arXiv2013

Equation of state for neutron stars in SU(3) flavor symmetry

Using several relativistic mean field models (such as GM1, GM3, NL3, TM1, FSUGold and IU-FSU) as well as the quark-meson coupling model, we calculate the particle fractions, the equation of state, the maximum mass and radius of a neutron star within relativistic Hartree approximation. In determining the couplings of the isoscalar, vector mesons to the octet baryons, we examine the extension of SU(6) spin-flavor symmetry to SU(3) flavor symmetry. Furthermore, we consider the strange ($σ^{\ast}$ and $ϕ$) mesons, and study how they affect the equation of state. We find that the equation of state in SU(3) symmetry can sustain a neutron star with mass of $(1.8 \sim 2.1) M_{\odot}$ even if hyperons exist inside the core. In addition, the strange vector ($ϕ$) meson and the variation of baryon structure in matter also play important roles in supporting a massive neutron star.

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arXiv2013

Effects of the density-dependent weak form factors on the neutrino reaction via neutral current for the nucleon in nuclear medium and $^{12}$C

The nucleon form factors in free space are usually thought to be modified when a nucleon is bound in a nucleus or immersed in a nuclear medium. We investigate effects of the density-dependent axial and weak-vector form factors on the electro-neutrino ($ν_e$) and anti-electro-neutrino $({\bar ν_e})$ reactions via neutral current (NC) for a nucleon in nuclear medium or $^{12}$C. For the density-dependent form factors, we exploit the quark-meson-coupling (QMC) model, and apply them to the $ν_e$ and ${\bar ν_e}$ induced reactions by NC. About 12% decrease of the total cross section by $ν_e$ reaction on the nucleon is obtained at normal density, $ρ= ρ_0 \sim 0.15 {fm}^{-3} $, as well as about 18% reduction of total $ν_e$ cross section on $^{12}$C, by the modification of the weak form factors of the bound nucleon. However, similarly to the charged current reaction, effects of the nucleon property change in the ${\bar ν}_e$ reaction reduce significantly the cross sections about 30% for the nucleon in matter and $^{12}$C cases. Such a large asymmetry in the ${\bar ν}_e$ cross sections is addressed to originate from the different helicities of ${\bar ν}_e$ and $ν_e$.

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arXiv2013

Asymmetry in the neutrino and anti-neutrino reactions in a nuclear medium

We study the effect of the density-dependent axial and vector form factors on the electro-neutrino ($ν_e$) and anti-neutrino $({\bar ν}_e)$ reactions for a nucleon in nuclear matter or in $^{12}$C. The nucleon form factors in free space are presumed to be modified for a bound nucleon in a nuclear medium. We adopt the density-dependent form factors calculated by the quark-meson coupling (QMC) model, and apply them to the $ν_e$ and ${\bar ν}_e$ induced reactions with the initial energy $E = $ 8 $\sim$ 80 MeV. We find that the total $ν_e$ cross sections on $^{12}$C as well as a nucleon in nuclear matter are reduced by about 5% at the nuclear saturation density, $ρ_0$. This reduction is caused by the modification of the nucleon structure in matter. Although the density effect for both cases is relatively small, it is comparable with the effect of Coulomb distortion on the outgoing lepton in the $ν$-reaction. In contrast, the density effect on the ${\bar ν}_e$ reaction reduces the cross section significantly in both nuclear matter and $^{12}$C cases, and the amount maximally becomes of about 35% around $ρ_0$. Such large asymmetry in the $ν_e$ and ${\bar ν}_e$ cross sections, which seems to be nearly independent of the target, is originated from the difference in the helicities of ${\bar ν}_e$ and $ν_e$. It is expected that the asymmetry influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae.

nucl-th↗

arXiv2012

Neutron Star Properties in the Chiral Quark-Meson Coupling Model

We study the properties of neutron star using the chiral quark-meson coupling model, in which the quark-quark hyperfine interaction due to the exchanges of gluon and pion based on chiral symmetry is considered. We also examine the effects of hyperons and $Δ$-isobars in a neutron star. Extending the SU(6) spin-flavor symmetry to more general SU(3) flavor symmetry in the vector-meson couplings to baryons, the maximum mass of neutron star can reach the recently observed, massive pulsar mass, $1.97 \pm 0.04 M_{\odot}$. In this calculation, $Λ$ and $Ξ$ are generated in a neutron star, while $Σ$ and $Δ$-isobars do not appear.

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arXiv2012

EoS for massive neutron stars

Using relativistic Hartree-Fock approximation, we investigate the properties of the neutron-star matter in detail. In the present calculation, we consider not only the tensor coupling of vector mesons to octet baryons and the form factors at interaction vertexes but also the internal (quark) structure change of baryons in dense matter. The relativistic Hartree-Fock calculations are performed in two ways: one is the calculation with the coupling constants determined by SU(6) (quark model) symmetry, the other is with the coupling constants based on SU(3) (flavor) symmetry. For the latter case, we use the latest Nijmegen (ESC08) model. Then, it is very remarkable that the particle composition of the core matter in SU(3) symmetry is completely different from that in SU(6) symmetry. In SU(6) symmetry, all octet baryons appear in the density region below $\sim 1.2$ fm$^{-3}$, while, in the ESC08 model, only the Ξ^- hyperon is produced. Furthermore, the medium modification of the internal baryon structure hardens the equation of state for the core matter. Taking all these effects into account, we can obtain the maximum neutron-star mass which is consistent with the recently observed mass, 1.97 \pm 0.04 M_\sun (PSR J1614-2230). We therefore conclude that the extension from SU(6) symmetry to SU(3) symmetry in the meson-baryon couplings and the internal baryon-structure variation in matter certainly enhance the mass of neutron star. Furthermore, the effects of the form factor at vertex and the Fock contribution including the tensor coupling due to the vector mesons are indispensable to describe the core matter. In particular, the Fock term is very vital in reproducing the preferable value of symmetry energy, a_4 (\simeq 30 - 40 MeV), in nuclear matter.

astro-ph.SR↗

arXiv2011

Effects of Fock term, tensor coupling and baryon structure variation on a neutron star

The equation of state for neutron matter is calculated within relativistic Hartree-Fock approximation. The tensor couplings of vector mesons to baryons are included, and the change of baryon internal structure in matter is also considered using the quark-meson coupling model. We obtain the maximum neutron-star mass of $\sim 2.0 M_\odot$, which is consistent with the recently observed, precise mass, $1.97\pm0.04 M_{\odot}$. The Fock contribution is very important and, in particular, the inclusion of tensor coupling is vital to obtain such large mass. The baryon structure variation in matter also enhances the mass of a neutron star.

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arXiv2010

The quark-meson coupling model and chiral symmetry

We extend the quark-meson coupling (QMC) model to incorporate chiral symmetry. The relationship between the QMC model and chiral perturbation theory is also discussed. The nuclear central potential is modified by the effect of internal structure of nucleon.

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arXiv2007

Relativistic mean-field model with density-dependent meson-nucleon couplings

Within the relativistic mean-field approach, we extend the Miyazaki model, where the NN$σ$ and NN$ω$ interactions are modified to suppress the couplings between positive- and negative-energy states of a nucleon in matter. Assuming appropriate density-dependence of the meson-nucleon couplings, we study nuclear matter and finite nuclei. The model can reproduce the observed properties of $^{16}$O and $^{40}$Ca well. We also examine if the model is natural.

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arXiv2003

Effect of different choices of the Boltzmannized flux operator on thermal exchange and recombination reactions

The rate constants for recombination and exchange reactions are calculated using the flux correlation approach with a general form of the Boltzmannized flux operator, which can simultaneously describe the Kubo and traditional half-split forms. First, we consider an exactly solvable model, i.e., the free particle case, in terms of a new scaling function. Next, as a non-trivial case, we study the recombination and exchange reactions at very high pressure. Since the rate constant is calculated by Laplace transform of the flux correlation function, the result depends on how the Boltzmannized flux operator is chosen. We find that a choice of the flux operator affects the rate constant considerably. For the recombination reaction, the ratio of the rate constant in the half-split form to that in the Kubo form approaches zero in the high pressure limit.

physics.chem-ph↗

arXiv2002

Flux Correlation Approach to Thermal Reactions and Recombination Rate

The rate constants for recombination and exchange processes are studied in terms of two different flux correlation approaches: one is the Yamamoto approach, which is based on the linear response theory, and the other is the Miller one. Using those approaches we consider two exactly solvable cases, i.e., the free particle and the parabolic potential models. Since the rate constants for recombination and exchange processes are calculated by Laplace transforms of the flux correlation functions, the two approaches give different results. In the present calculation, we find that the rate constant in the Yamamoto approach is larger than that in the Miller approach by about 40% at low temperature ($\sim 100$ K) and high pressure ($\sim 1$ GPa). The difference is considerable in the region where quantum effects dominate.

physics.chem-ph↗

arXiv2002

Meson mixing amplitudes in asymmetric nuclear matter

Using a purely hadronic model, we study the charge-symmetry-breaking $ρ$-$ω$, $σ$-$δ$, $σ$-$ρ$ and $δ$-$ω$ mixing amplitudes in isospin asymmetric nuclear matter. The basic assumption of the model is that the mixing amplitude is generated by nucleon and anti-nucleon loops and hence driven entirely by the difference between proton and neutron Fermi momenta and the proton-neutron mass difference. We find that the behavior of the mixing amplitude is very complicated in the spacelike region and quite sensitive to the proton fraction of nuclear medium and nuclear density. In particular, in neutron rich nuclei (like Pb) and/or neutron stars the mixing amplitudes become about 10 $\sim$ 100 times as large as those in typical mirror nuclei.

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