arXiv subjects

Kouji Kashiwa

Publications and source records attributed to Kouji Kashiwa.

At least 73 records · Page 4Linked to original sources

arXiv2014

Phase structure of $SU(3)$ gauge-Higgs unification models at finite temperature

Five-dimensional $SU(3)$ gauge-Higgs unification models are studied at finite temperature in the warped extra dimension $S^1/\mathbb{Z}_2$. In order to investigate the phase structure, we developed a technique to compute the one-loop effective potential with the nontrivial Polyakov loop phase and with the nontrivial Wilson line phase along the extra dimension. Effective potentials as functions of two gauge-field condensations are shown for several simple matter contents, including fundamental, sextet, and adjoint representational Dirac fermions. Possible extensions and applications of our formalism are also briefly discussed.

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arXiv2014

Phase structure of pure SU(3) lattice gauge theory in 5 dimensions

We investigate the nonperturbative phase structure of five-dimensional SU(3) pure Yang-Mills theory on the lattice. We perform numerical simulations using the Wilson plaquette gauge action on an anisotropic lattice with a four-dimensional lattice spacing (a4) and with an independent value in the fifth dimension (a5). We investigate both cases of a4 > a5 and a4 < a5. The Polyakov loops in the fourth and the fifth directions are observed, and we find that there are four possible phases for the anisotropic five-dimensional quenched QCD theory on the lattice. We determine the critical values of the lattice bare coupling and the anisotropy parameter for each phase transition. Furthermore, we find that the two center domains where the Polyakov loop have locally different charges of the center symmetry appears in single configuration in the specific region of the lattice parameters.

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arXiv2014

Effective model approach to meson screening masses at finite temperature

Temperature dependence of pion and sigma-meson screening masses is evaluated by the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model with the entanglement vertex. We propose a practical way of calculating meson screening masses in the NJL-type effective models. The method based on the Pauli-Villars regularization solves the well-known difficulty that the evaluaton of screening masses is not easy in the NJL-type effective models.The PNJL model with the entanglement vertex and the Pauli-Villars regularization well reproduces lattice QCD results on temperature dependence of the chiral condensate and the Polyakov loop. The method is applied to analyze temperature dependence of pion screening mass calculated with state-of-the-art lattice simulations with success in reproducing the lattice QCD results.

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arXiv2013

Phase structure and Hosotani mechanism in QCD-like theory with compact dimensions

We investigated the phase diagram of $SU(3)$ gauge theory in four dimension with one compact dimension by using the perturbative one-loop effective potential. Effects of the adjoint and fundamental fermions are investigated and then the rich phase structure in the quark-mass and compact-size scale is realized. Our results are qualitatively consistent with the recent lattice calculation and clearly show that the lattice calculation can be understood from the Hosotani mechanism. Moreover, we show the result obtained by using the flavor twisted boundary condition for fundamental fermion which does not break the $Z_3$ symmetry, explicitly.

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arXiv2013

Quark contribution for center domain in heavy ion collisions

The center domain structure is revisited with introduction of quark contribution to understand phenomenology in high-energy heavy ion collisions. We show the quark contribution may allow metastable states that would lead to a natural and consistent explanation for the temperature dependences of color opacity and viscosity. We also argue the possibility of indirect observations of the center domain structure in experiments due to a topological critical temperature.

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arXiv2013

Phase structure and Hosotani mechanism in gauge theories with compact dimensions revisited

We investigate the phase structure of SU(3) gauge theory in four and five dimensions with one compact dimension by using perturbative one-loop and PNJL-model-based effective potentials, with emphasis on spontaneous gauge symmetry breaking. When adjoint matter with the periodic boundary condition is introduced, we have rich phase structure in the quark-mass and compact-size space with gauge-symmetry-broken phases, called the $SU(2)\times U(1)$ split and the $U(1)\times U(1)$ re-confined phases. Our result is qualitatively consistent with the recent lattice calculations. When fundamental quarks are introduced in addition to adjoint quarks, the split phase becomes more dominant and larger as a result of explicit center symmetry breaking. We also show that another $U(1)\times U(1)$ phase (pseudo-reconfined phase) with negative vacuum expectation value of Polyakov loop exists in this case. We study chiral properties in these theories and show that chiral condensate gradually decreases and chiral symmetry is slowly restored as the size of the compact dimension is decreased.

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arXiv2013

Quasi-particle and matrix models of the semi Quark Gluon Plasma

We make a simple observation about two models used to treat the region near the critical temperature of QCD, quasiparticle and matrix models. While they appear very different, we show how these two models might be related. We also present results for the temperature dependence of the ratio of the shear viscosity to the entropy in a matrix model, and suggest that quasi-particle models may behave similarly.

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arXiv2013

The Roberge-Weiss transition and 't Hooft loops

Roberge and Weiss showed that for SU(N) gauge theories, phase transitions occur in the presence of an imaginary quark chemical potential. We show that at asymptotically high temperature, where the phase transition is of first order, that even with dynamical quarks 't Hooft loops of arbitrary Z(N) charge are well defined at the phase boundary. To leading order in weak coupling, the 't Hooft loop satisfies Casimir scaling in the pure glue theory, but not with quarks. Because the chemical potential is imaginary, typically the interaction measure is negative on one side of the phase transition. Using a matrix model to model the deconfining phase transition, we compute the phase diagram for heavy quarks, in the plane of temperature and imaginary chemical potential. In general we find intersecting lines of first order transitions. Using a modified Polyakov loop which is Roberge-Weiss symmetric, we suggest that the interface tension is related to the 't Hooft loop only at high temperature, where the imaginary part of this Polyakov loop, and not the real part, is discontinuous across the phase boundary.

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arXiv2013

Differences and similarities between fundamental and adjoint matters in SU(N) gauge theories

We investigate differences and similarities between fundamental fermions and adjoint fermions in SU(N) gauge theories. The gauge theory with fundamental fermions possesses ZN symmetry only in the limit of infinite fermion mass, whereas the gauge theory with adjoint fermions does have the symmetry for any fermion mass. The flavor-dependent twisted boundary condition (FTBC) is then imposed on fundamental fermions so that the theory with fundamental fermions can possess ZN symmetry for any fermion mass. We show similarities between FTBC fundamental fermions and adjoint fermions, using the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model. In the mean-field level, the PNJL model with FTBC fundamental fermions has dynamics similar to the PNJL model with adjoint fermions for the confinement/deconfinement transition related to ZN symmetry. The chiral property is somewhat different between the two models, but there is a simple relation between chiral condensates in the two models. As an interesting high-energy phenomenon, a possibility of the gauge symmetry breaking is studied for FTBC fundamental fermions.

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arXiv2013

Polyakov loop and QCD thermodynamics from the gluon and ghost propagators

We investigate quark deconfinement by calculating the effective potential of the Polyakov loop using the non-perturbative propagators in the Landau gauge measured in the finite-temperature lattice simulation. With the leading term in the 2-particle-irreducible formalism the resultant effective potential exhibits a first-order phase transitions for the pure SU(3) Yang-Mills theory at the critical temperature consistent with the empirical value. We also estimate the thermodynamic quantities to confirm qualitative agreement with the lattice data near the critical temperature. We then apply our effective potential to the chiral model study and calculate the order parameters and the thermodynamic quantities. Unlike the case in the pure Yang-Mills theory the thermodynamic quantities are sensitive to the temperature dependence of the non-perturbative propagators, while the behavior of the order parameters is less sensitive, which implies the importance of the precise determination of the temperature-dependent propagators.

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arXiv2013

Two-color QCD at imaginary chemical potential and its impact on real chemical potential

We study properties of two-color QCD at imaginary chemical potential ($μ$) from the viewpoint of the Roberge-Weiss (RW) periodicity, the charge conjugation and the pseudo-reality. At $μ=\pm iπT/2$, where $T$ is temperature, the system is symmetric under the combination of the charge conjugation ${\cal C}$ and the ${\mathbb Z}_{2}$ transformation. The symmetry, called ${\cal C} {\mathbb Z}_{2}$ symmetry, is preserved at lower $T$ but broken at higher $T$. The Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model has the same properties as two-color QCD for ${\cal C} {\mathbb Z}_{2}$ symmetry and the pseudo-reality. The nontrivial correlation between the chiral restoration and the deconfinement are investigated by introducing the entanglement vertex in the PNJL model. The order of ${\cal C} {\mathbb Z}_{2}$ symmetry breaking at the RW endpoint is second-order when the correlation is weak, but becomes first-order when the correlation is strong. We also investigate the impact of the correlation on the phase diagram at real $μ$.

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arXiv2012

Impact of vector-current interactions on the QCD phase diagram

Using a nonlocal version of the Polyakov-loop-extended Nambu--Jona-Lasinio model, we investi- gate effects of a nonderivative vector-current interaction (relating to the quark-number density) at both real and imaginary chemical potentials. This repulsive vector interaction between quarks has the following impact on the chiral first-order phase transition: at imaginary chemical potential it sharpens the transition at the Roberge-Weiss (RW) end point and moves this critical point toward lower temperatures; at real chemical potential, the critical end point moves on a trajectory towards larger chemical potentials and lower temperatures with increasing vector coupling strength. The conditions are discussed at which the first-order phase transition disappears and turns into a smooth crossover.

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arXiv2012

Quark back reaction to deconfinement transition via gluon propagators

Contribution of the quark back reaction to the deconfinement phase transition is studied in the thermodynamical potential of the gluonic sector which consists of the gluon and ghost propagators calculated in the lattice QCD simulations. Starting from QCD thermodynamic potential, we define the gluonic potential in the leading-order of the 2PI formalism, which can describe the deconfinement phase transition. Then the gluonic potential can be written by using the microscopic characters; the gluon and ghost propagators in Landau gauge fixing. To include effects of the quark back reaction, we calculate the gluon propagators in lattice QCD simulations with two-flavored dynamical quarks. Fitting the lattice data by the Gribov-Stingl form and investigating the phase transition of the gluonic potential, we find that enhancement of the quark back reaction reduces the critical temperature of the deconfinement phase transition.

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arXiv2012

Critical endpoint for deconfinement in matrix and other effective models

We consider the position of the deconfining critical endpoint, where the first order transition for deconfinement is washed out by the presence of massive, dynamical quarks. We use an effective matrix model, employed previously to analyze the transition in the pure glue theory. If the param- eters of the pure glue theory are unaffected by the presence of dynamical quarks, and if the quarks only contribute perturbatively, then for three colors and three degenerate quark flavors this quark mass is very heavy, m_de \sim 2.5 GeV, while the critical temperature, T_de, barely changes, \sim 1% below that in the pure glue theory. The location of the deconfining critical endpoint is a sensitive test to differentiate between effective models. For example, models with a logarithmic potential for the Polyakov loop give much smaller values of the quark mass, m_de \sim 1 GeV, and a large shift in T_de \sim 10% lower than that in the pure glue theory.

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arXiv2011

Nonlocal Polyakov-Nambu-Jona-Lasinio model and imaginary chemical potential

With the aim of setting constraints for the modeling of the QCD phase diagram, the phase structure of the two-flavor Polyakov-loop extended Nambu and Jona-Lasinio (PNJL) model is investigated in the range of imaginary chemical potentials ($μ_\mathrm{I}$) and compared with available $N_f=2$ lattice QCD results. The calculations are performed using the advanced nonlocal version of the PNJL model with the inclusion of vector-type quasiparticle interactions between quarks, and with wave-function-renormalization corrections. It is demonstrated that the nonlocal PNJL model reproduces important features of QCD at finite $μ_\mathrm{I}$, such as the Roberge-Weiss (RW) periodicity and the RW transition. Chiral and deconfinement transition temperatures for $N_f=2$ turn out to coincide both at zero chemical potential and at finite $μ_\mathrm{I}$. Detailed studies are performed concerning the RW endpoint and its neighborhood where a first-order transition occurs.

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arXiv2011

Entanglement between chiral and deconfinement transitions under strong uniform magnetic background field

Effects of a uniform (electro-) magnetic background field are investigated in the two-flavor nonlocal Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model. Temperature-dependences of the chiral order-parameter and the Polyakov-loop are investigated under the magnetic field. In the nonlocal PNJL model with four-dimensional momentum dependent distribution function, pseudo-critical temperatures of the chiral and deconfinement transitions are coincident each other even if the magnetic field is strong. It means that we can check the reliability of the distribution function under the strong magnetic field by comparing with lattice QCD data, particularly from the viewpoint of the entanglement between the chiral and deconfinement transitions.

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arXiv2011

Extensions and further applications of the nonlocal Polyakov--Nambu--Jona-Lasinio model

The nonlocal Polyakov-loop-extended Nambu--Jona-Lasinio (PNJL) model is further improved by including momentum-dependent wave-function renormalization in the quark quasiparticle propagator. Both two- and three-flavor versions of this improved PNJL model are discussed, the latter with inclusion of the (nonlocal) 't Hooft-Kobayashi-Maskawa determinant interaction in order to account for the axial U(1) anomaly. Thermodynamics and phases are investigated and compared with recent lattice-QCD results.

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arXiv2011

Violations of parity and charge conjugation in the theta vacuum with imaginary chemical potential

Charge conjugation (C) and Parity (P) are exact symmetries at theta =pi and Theta = mu/(iT)=pi, where theta is the parameter of the so-called theta-vacuum, mu is the imaginary quark-number chemical potential and T is the temperature. Spontaneous breakings of these discrete symmetries are investigated by the the Polyakov-loop extended Nambu--Jona-Lasinio (PNJL) model. At zero T, P symmetry is spontaneously broken while C symmetry is conserved. As T increases, P symmetry is restored just after C symmetry is spontaneously broken, so that either P or C symmetry or both the symmetries are spontaneously broken for any T. The chiral-symmetry restoration and the deconfinement transition at theta =Theta =0 are remnants of the P restoration and the C breaking at theta =Theta =pi, respectively.

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