arXiv subjects

Hiroaki Kouno

Publications and source records attributed to Hiroaki Kouno.

89 records · Page 5Linked to original sources

arXiv2007

Effects of a multi-quark interaction on color superconducting phase transition in an extended NJL model

We study the interplay of the chiral and the color superconducting phase transitions in an extended Nambu--Jona-Lasinio model with a multi-quark interaction that produces the nonlinear chiral-diquark coupling. We observe that this nonlinear coupling adds up coherently with the ω^2 interaction to produce the chiral-color superconductivity coexistence phase or cancel each other depending on its sign. We discuss that large coexistence region in the phase diagram is consistent with the quark-diquark picture for the nucleon whereas its smallness is the prerequisite for the applicability of the Ginzburg-Landau approach.

hep-ph↗

arXiv2006

Repulsive force by vector mesons and quark-hadron phase transition

Using a phenomenological model with vector-type interactions, we discuss a role of repulsive force in the quark-hadron phase transition at high density. For realization of the quark phase at high density, strong vector coupling is needed in the hadron phase, while it is forbidden in the quark phase. For the quark-phase, a NJL-type model with a multi-quark interaction is investigated. We show that, in this model, the restoration of chiral symmetry decreases effective vector coupling and the quark phase is realized at high density, even if we have strong vector coupling at low density. In this model, the strong coupling in the hadron phase is induced by the chiral symmetry breaking.

nucl-th↗

arXiv2006

Quark propagators at finite temperature with the clover action

We study properties of the finite temperature quark propagator by using the SU(3) quenched lattice simulation in the Landau gauge and report numerical results of the standard Wilson quark case as well as the improved clover one. The mass function in the deconfinement phase is different from that of the confinement phase, especially at low momentum regions.

hep-lat↗

arXiv2006

Chiral phase transition in an extended NJL model with higher-order multi-quark interactions

The chiral phase transition is studied in an extended Nambu--Jona-Lasinio model with eight-quark interactions. Equations for scalar and vector quark densities, derived in the mean field approximation, are nonlinear and mutually coupled. The scalar-type nonlinear term hastens the restoration of chiral symmetry, while the scalar-vector mixing term makes the transition sharper. The scalar-type nonlinear term shifts the critical endpoint toward the values predicted by lattice QCD simulations and the QCD-like theory.

nucl-th↗

arXiv2004

Auxiliary field method at finite temperature and/or finite density

We discuss the relation between the effective vector meson mass and equation of state (EOS) for nuclear matter. In the mean field approximation, the EOS becomes softer due to the reduction of the effective omega-meson mass, if we assume that the omega-meson mean field is proportional to the baryon density. We examine the assumption by using the auxiliary field method at finite temperature and /or density.

nucl-th↗

arXiv2004

Effective hadron masses and effective interactions in nuclear matter

Relations among the effective hadron masses and the effective interactions in nuclear matter are studied by using the generalized relativistic mean field theory. We found that we can make the effective omega-meson mass smaller and the equation of state softer if we require that the omega-meson mean field is proportional to the baryon density. In this case, there is a simple and exact relation between the effective omega-meson mass and the effective omega-nucleon coupling. Because of this relation, the effective omega-nucleon coupling automatically decreases if the effective omega-meson mass decreases. Consequently, the equation of state becomes softer. A trial to embed the QCD sum-rule results into the hadron field theory is also shown.

nucl-th↗

arXiv2004

Effective meson masses, effective meson-nucleon couplings and neutron star radii

Using the generalized mean field theory, we have studied the relation among the effective meson masses, the effective meson-nucleon couplings and the equation of state (EOS) in asymmetric nuclear matter. If the effective omega-meson mass becomes smaller at high density, the EOS becomes stiffer. However, if we require that the omega-meson mean field is proportional to the baryon density, the effective omega-nucleon coupling automatically becomes smaller at the same time as the effective omega-meson mass becomes smaller. Consequently, the EOS becomes softer. A similar relation is found for the effective rho-meson mass and the effective rho-nucleon coupling. We have also studied the relation among the effective meson masses, the effective meson-nucleon couplings and a radius R of a neutron star. The R depends somewhat on the value of the effective omega-meson mass and the effective omega-nucleon coupling.

nucl-th↗

arXiv2001

Effective hadron masses and couplings in nuclear matter and incompressibility

The role of effective hadron masses and effective couplings in nuclear matter is studied using a generalized effective Lagrangian for sigma-omega model. A simple relation among the effective masses, the effective couplings and the incompressibility K is derived. Using the relation, it is found that the effective repulsive and the effective attractive forces are almost canceled to each other at the normal density. Inversely, if this cancellation is almost complete, K should be 250-350MeV.

nucl-th↗

arXiv2000

The effective ω-meson mass with a vector field self-interaction

We calculate the effective mass m_ω^* of the ω-meson in nuclear medium by the σ-ωmodel with a vector field self-interaction (VFSI). We found that the direct and the indirect effects of the VFSI almost cancel each other and the result is changed only slightly at the normal density. The VFSI makes m^*_ωsmaller at lower density and makes m_ω^* larger at higher density. However, both effects are not large.

nucl-th↗

arXiv2000

A relation among the effective nucleon mass, the incompressibility and the effective $σ$-meson mass in nuclear matter

A relation among the effective nucleon mass $M^*$, the incompressibility $K$ and the effective $σ$-meson mass $m_{\rm s}^*$ in nuclear matter is studied by using the relativistic nuclear model. We found that there is a strong correlation between $M^*$ and $m_{\rm s}^*$, while there is only a weak correlation between $K$ and $m_{\rm s}^*$. At the normal density, $m_{\rm s}^*$ is smaller than the one at zero density, if $M^*$ is smaller than 0.8 times of the nucleon mass at zero density. It is also found that the off-shell effective mass $μ_{\rm s}^*$ is related directly to $K$ and $M^*$ at the normal density.

nucl-th↗

arXiv1999

Nonlinear effects in Schwinger-Dyson Equation

We study nonlinear effects in the QED ladder Schwinger-Dyson(SD) equation. Without further approximations, we show that all nonlinear effects in the ladder SD equation can be included in the effective couplings and how a linear approximation works well. The analyses is generalized in the case of the improved ladder calculation with the Higashijima-Miransky approximation.

hep-ph↗

arXiv1998

Non-perturbative renormalization group equations and approximate discrete gamma_5-symmetry in quantum hadrodynamics

The quantum hadrodynamics are studied by using non-perturbative renormalization group equations. Approximate discrete gamma_5-symmetry is studied. It is shown that the contributions of one-loop diagrams are important for effective potential. A relation between the local-potential approximation and the usual Hartree approximation is discussed. The local-potential approximation includes contribution of the Fock term to nucleon self-energy, contribution in the usual random phase approximation to meson self-energy and vertex corrections in part as well as the Hartree contribution to the effective potential. Approximate discrete gamma_5-symmetry is studied phenomenologically in the Hartree approximation. This approximate symmetry may exist in nuclear matter.

nucl-th↗

arXiv1997

Meson-nucleon vertex corrections to the vacuum polarization in the cutoff field theory and tensor coupling

At zero-density, meson-nucleon vertex corrections to vacuum polarization are studied in the $σ-ω$ model with the cutoff. It is shown that the properties of the vertex corrections to vacuum polarization are somewhat different from those described by the ordinary renormalization procedures when the cutoff is small (< 5 GeV). The low-energy effective Lagrangian is constructed in the framework of the renormalization group method. The weak tensor and derivative couplings of meson-nucleon interactions may be needed in the low-energy effective theory of mesons and nucleons.

nucl-th↗

arXiv1997

Vacuum Effects and Compressional Properties of Nuclear Matter in Cutoff Field Theory

Including the vacuum effects, the compressional properties of nuclear matter are studied in the cutoff field theory. Under the Hartree approximation, the low-energy effective Lagrangian is derived in the framework of the renormalization group methods. The coefficients are determined in a way where the physical results hardly depend on the value of the cutoff which is conveniently introduced into the theory. It is shown that, to reproduce the empirical data of the nucleus incompressibility, the compressibility of the nuclear matter is favorable to be 250$\sim$350MeV.

nucl-th↗

arXiv1996

Quark condensate in nuclear matter based on Nuclear Schwinger-Dyson formalism

The effects of higher order corrections of ring diagrams for the quark condensate are studied by using the bare vertex Nuclear Schwinger Dyson formalism based on $σ$-$ω$ model. At the high density the quark condensate is reduced by the higher order contribution of ring diagrams more than the mean field theory or the Hartree-Fock .

nucl-th↗

arXiv1995

Nucleon mean free path in nuclear matter based on nuclear Schwinger-Dyson formalism

A mean free path of nucleon moving through nuclear matter with kinetic energy of more than 100MeV is formulated based on the bare vertex nuclear Schwinger-Dyson (BNSD) method in the Walecka model. The self-energy which is derived from the higher order diagrams more than the forth order includes the Feynman part of propagator of energetic nucleon and grows up rapidly as an increase of kinetic energy. To avoid too large growth of these diagrams, meson propagators are modified by introducing some form factors to take account of a internal structure of hadron. It is confirmed that the mean free path calculated by the BNSD method agrees good with experimental data if a reasonable form factor is chosen, i.e., a dipole (quadrupole) type of form factor with a cut-off parameter about 750 MeV $\sim$ 1000 MeV (1200 MeV $\sim$ 1500 MeV).

nucl-th↗

arXiv1995

Bulk properties of nuclear matter in the relativistic Hartree approximation with cut-off regularization

A method of cut-off regularization is proposed to evaluate vacuum corrections in nuclear matter in the framework of the Hartree approximation. Bulk properties of nuclear matter calculated by this method are a good agreement with results analyzed by empirical values. The vacuum effect is quantitatively evaluated through a cut-off parameter and its role for saturation property and compressional properties is clarified.

nucl-th↗