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Teiji Kunihiro

Publications and source records attributed to Teiji Kunihiro.

At least 73 records · Page 4Linked to original sources
arXiv2010

Spectral Function of Fermion Coupled with Massive Vector Boson at Finite Temperature in Gauge Invariant Formalism

We investigate spectral properties of a fermion coupled with a massive gauge boson with a mass m at finite temperature (T) in the perturbation theory. The massive gauge boson is introduced as a U(1) gauge boson in the Stueckelberg formalism with a gauge parameter α. We find that the fermion spectral function has a three-peak structure for T \sim m irrespective of the choice of the gauge parameter, while it tends to have one faint peak at the origin and two peaks corresponding to the normal fermion and anti-plasmino excitations familiar in QED in the hard thermal loop approximation for T \gg m. We show that our formalism successfully describe the fermion spectral function in the whole T region with the correct high-T limit except for the faint peak at the origin, although some care is needed for choice of the gauge parameter for T \gg m. We clarify that for T \sim m, the fermion pole is almost independent of the gauge parameter in the one-loop order, while for T \gg m, the one-loop analysis is valid only for α\ll 1/g where g is the fermion-boson coupling constant, implying that the one-loop analysis can not be valid for large gauge parameters as in the unitary gauge.

hep-ph↗
arXiv2010

Nature of the sigma meson as revealed by its softening process

The pi pi scattering is studied in two-flavor chiral models with a finite pion mass to investigate the nature of the sigma meson which is observed as the lowest scalar-isoscalar resonance. We compare several models with different origins of the sigma meson, such as the chiral partner of the pion and the dynamically generated pi pi molecule. We find that the dynamically generated sigma meson exhibits a novel pattern of the threshold enhancement reflecting the s-wave nature of the resonance, which is qualitatively different from the softening of the chiral partner introduced as a bare field. This behavior around the threshold energy region is universal as far as the bare sigma pole stays away from the threshold throughout the symmetry restoration process. On the other hand, for m_pi=0, the dynamically generated sigma behaves similarly to the chiral partner in the symmetry restoration limit, implying the possibility of the dynamically generated chiral partner.

hep-ph↗
arXiv2010

Chiral Symmetry and Axial Anomaly in Hadron and Nuclear Physics --- a review --

The important role played by chiral symmetry and axial anomaly in QCD in nuclear physics is reviewed. Some recent topics on possible chiral restoration in hot and/or dense matter are picked up. We also discuss so called {\em effective} restoration of chiral anomaly hot and/or dense matter, as may be seen in a character change of $η'$ meson.

nucl-th↗
arXiv2010

Second-order Relativistic Hydrodynamic Equations for Viscous Systems; how does the dissipation affect the internal energy?

We derive the second-order dissipative relativistic hydrodynamic equations in a generic frame with a continuous parameter from the relativistic Boltzmann equation. We present explicitly the relaxation terms in the energy and particle frames. Our results show that the viscosities are frame-independent but the relaxation times are generically frame-dependent. We confirm that the dissipative part of the energy-momentum tensor in the particle frame satisfies $δT^μ_μ= 0$ obtained for the first-order equation before, in contrast to the Eckart choice $u_μδT^{μν} u_ν= 0$ adopted as a matching condition in the literature. We emphasize that the new constraint $δT^μ_μ= 0$ can be compatible with the phenomenological derivation of hydrodynamics based on the second law of thermodynamics.

hep-ph↗
arXiv2009

Toward Identifying the QCD Critical Point: attenuation of the sound mode around the critical point

Motivated by the fact that QCD critical point (CP) belongs to the same universality class as the liquid-gas transition, the dynamical density fluctuations around the CP is analyzed using relativistic fluid dynamics for a viscous fluid. It is shown that relativistic effects on the dynamical structure factor of the density fluctuation appears only in the width of the sound and thermal modes through a modification of the transport/kinetic coefficients. It is found that the mechanical density fluctuation which is enhanced by the relativistic effects are attenuated, whereas the entropy fluctuation in turn becomes more prominent around the QCD CP. This attenuation of the density fluctuation around the QCD CP may imply that if the suppression or even total disappearance of Mach cone is observed as the incident energy of the relativistic heavy-ion collisionsis lowered, it can be a signal of the existence of the QCD critical point.

nucl-th↗
arXiv2009

Two-flavor lattice QCD study of the axial charges of N(1535) and N(1650)

We show the first lattice QCD results on the axial charges $g_A^{N^*N^*}$ of $N^*(1535)$ and $N^*(1650)$. The measurements are performed with two flavors of dynamical quarks employing the renormalization-group improved gauge action at $β$=1.95 and the mean-field improved clover quark action with the hopping parameters, $κ$=0.1375, 0.1390 and 0.1400. We construct 2$\times$2 correlation matrices and diagonalize them so that the signals of $N^*(1535)$ and $N^*(1650)$ are properly separated. Wraparound contributions in the correlator, which can be another source of signal contaminations, are eliminated by imposing the Dirichlet boundary condition in the temporal direction. The axial charge of $N^*(1535)$ is found to take small values as $g_A^{N^*N^*}\sim {\mathcal O}(0.1)$, whereas that of $N^*(1650)$ is approximately 0.5, which is almost independent of quark masses and consistent with the predictions by the naive nonrelativistic quark model.

hep-lat↗
arXiv2009

Critical Opalescence around the QCD Critical Point and Second-order Relativistic Hydrodynamic Equations Compatible with Boltzmann Equation

The dynamical density fluctuations around QCD critical point (CP) are analyzed using relativistic dissipative fluid dynamics, and we show that the sound mode around the QCD CP is strongly attenuated whereas the thermal fluctuation stands out there. We speculate that if possible suppression or disappearance of a Mach cone, which seems to be created by the partonic jets at RHIC, is observed as the incident energy of the heavyion collisions is decreased, it can be a signal of the existence of the QCD CP. We have presented the Israel-Stewart type fluid dynamic equations that are derived rigorously on the basis of the (dynamical) renormalization group method in the second part of the talk, which we omit here because of a lack of space.

hep-ph↗
arXiv2009

$X$ Meson aka $η'$ and Kobayashi-Maskawa-'t Hooft Six-quark Vertex -- $U(1)_A$ Anomaly and Generalized Nambu-Jona-Lasinio Model --

In 1970, Kobayashi and Maskawa concluded that an effective six-quark vertex with a determinantal form is necessary in the chiral effective models to account for the large mass of $X$ meson, which is now called the $η'$. The determinantal interaction has an $SU(3)_L \otimes SU(3)_R$ symmetry but not $U(3)_L \otimes U(3)_R$, and, hence accounts for the explicit breaking of $U(1)_A$ symmetry in quantum chromodynamics (QCD); the vertex was later derived by 't Hooft as an instanton-induced quark interaction. The vertex, which may be called the Kobayashi-Maskawa-'t Hooft (KMT) term, is widely used in quantitative analyses of hadron physics and QCD phase transitions at finite temperature and density. An account is made for the KMT term with recent extensive applications. Described are also personal experiences with Professor Maskawa and Professor Kobayashi,including an encounter with Professor Maskawa when the author first presented his work on the KMT term.

hep-ph↗
arXiv2009

Vector interaction, charge neutrality and multiple chiral critical point structures

We investigate the combined effect of the repulsive vector interaction and the positive electric chemical potential on the chiral phase transition by considering neutral color superconductivity (CSC). The chiral condensate, diquark condensate and quark number densities are solved in both two-flavor and two-plus-one-flavor Nambu-Jona-Lasinio(NJL) models with the so called Kobayashi-Maskawa-'t Hooft term under the charge neutrality constraint. We demonstrate that multiple chiral critical-point structures always exist in the NJL model within the self-consistent mean-field approximation and the number of chiral critical points can vary from zero to four, which is dependent on the magnitudes of vector interaction and the diquark coupling. The difference between the dynamical chemical potentials induced by vector interaction for u and d quarks can effectively reduce the Fermi sphere disparity between the two flavors of diquark paring. Thus the vector interaction works to significantly suppress the unstable region associated with chromomagnetic instability in the phase of neutral asymmetric homogenous CSC.

hep-ph↗
arXiv2009

Towards a Theory of Entropy Production in the Little and Big Bang

We propose a broadly applicable formalism for the description of coarse grained entropy production in quantum mechanical processes. Our formalism is based on the Husimi transform of the quantum state, which encodes the notion that information about any quantum state is limited by the experimental resolution. We show in two analytically tractable cases (the decay of an unstable vacuum state and reheating after cosmic inflation) that the growth rate of the Wehrl entropy associated with the Husimi function approaches the classical Kolmogorov-Sinai entropy. We also discuss various possible applications of our formalism, including the production of entropy in the early stages of a relativistic heavy ion collision.

hep-ph↗
arXiv2008

Axial charges of N(1535) and N(1650) in lattice QCD with two flavors of dynamical quarks

We show the first lattice QCD results on the axial charge $g_A^{N^*N^*}$ of $N^*(1535)$ and $N^*(1650)$. The measurements are performed with two flavors of dynamical quarks employing the renormalization-group improved gauge action at $β$=1.95 and the mean-field improved clover quark action with the hopping parameters, $κ$=0.1375, 0.1390 and 0.1400. In order to properly separate signals of $N^*(1535)$ and $N^*(1650)$, we construct 2$\times$2 correlation matrices and diagonalize them. Wraparound contributions in the correlator, which can be another source of signal contaminations, are eliminated by imposing the Dirichlet boundary condition in the temporal direction. We find that the axial charge of $N^*(1535)$ takes small values as $g_A^{N^*N^*}\sim {\mathcal O}(0.1)$, whereas that of $N^*(1650)$ is about 0.5, which is found independent of quark masses and consistent with the predictions by the naive nonrelativistic quark model.

hep-lat↗
arXiv2008

Spectral properties of massless and massive quarks coupled with massive boson at finite temperature

We study the properties of massless and massive quarks coupled with a scalar and pseudoscalar boson at finite temperature in Yukawa models at the one-loop order. The behavior of the spectral function and the pole structure of the propagator are analyzed as functions of temperature $T$ and the quark mass $m_f$. It is shown that the three-peak structure of the spectral function found in a previous work for massless quarks is formed at temperatures comparable to the boson mass even for finite $m_f$, but gradually ceases to exist as $m_f$ becomes larger. We identify the three poles of the quark propagator corresponding to the collective excitations of the quark in the complex energy plane. It is shown that the three trajectories made by the poles along with a variation of $T$ undergo a structural rearrangement at a critical quark mass when $m_f$ is increased. This suggests that the physics content of the collective quark excitations is changed in a drastic way at this point. The results are nicely accounted for with the notion of the level mixing induced by a resonant scattering of the massive boson with quarks and holes of thermally excited anti-quarks.

hep-ph↗
arXiv2008

Stable First-order Particle-frame Relativistic Hydrodynamics for Dissipative Systems

We propose a stable first-order relativistic dissipative hydrodynamic equation in the particle frame (Eckart frame) for the first time. The equation to be proposed was in fact previously derived by the authors and a collaborator from the relativistic Boltzmann equation. We demonstrate that the equilibrium state is stable with respect to the time evolution described by our hydrodynamic equation in the particle frame. Our equation may be a proper starting point for constructing second-order causal relativistic hydrodynamics, to replace Eckart's particle-flow theory.

nucl-th↗
arXiv2007

How do diquark fluctuations and chiral soft modes affect di-lepton production in the deconfined phase?

We examine diquark fluctuations and chiral soft modes existing in the precritical region of color superconductivity and chiral transition, respectively, at finite temperature and density. We evaluate how they contribute to anomalous di-lepton production; although there appear peaks in the spectral function owing to the existence of the soft modes, the enhancement of the production rate may not be so prominent to be a clear signal of the phsase transitions.

hep-ph↗
arXiv2007

Lattice Study of Low-lying Nonet Scalar Mesons in Quenched Approximation

Using lattice QCD simulation in the quenched approximation, we study the $κ$ meson, which is ^3P_0 in the quark model, and compare experimental and other lattice data. The $κ$ is the lowest scalar meson with strangeness and constitutes the scalar nonet. The obtained mass is much higher than the recent experimental value, and therefore the $κ(800)$ is difficult to consider as a simple two-body constituent-quark structure, and may have another unconventional structure.

hep-lat↗
arXiv2007

Fluctuations of the chiral condensate and quasi-particle spectra near phase transition

We investigate the quark spectrum near but above the critical temperature of the chiral transition, taking into account the precursory soft modes. It is found that there appear novel excitation spectra of quasi-quarks and quasi-antiquarks with a three-peak structure. By a detailed analysis on the formation of the three-peak structure using Yukawa models, it is shown that the new quark spectra originate from the mixing between a quark (anti-quark) and an antiquark hole (quark hole) caused by a resonant scattering of the quasi-quark with the soft modes which have a small but finite excitation energy with a small width near the critical temperature.

hep-ph↗
arXiv2007

Low Mass Scalar Mesons and Related Topics

We give a brief survey on the physical significance of the low-mass scalar mesons in QCD, and also report on recent lattice studies on the sigma and kappa mesons. The importance to explore the in-medium properties of the hadrons is mentioned.

hep-ph↗