T. Koide

Chiral and color-superconducting phase transitions with vector interaction in a simple model

We investigate effects of the vector interaction on chiral and color superconducting (CSC) phase transitions at finite density and temperature in a simple Nambu-Jona-Lasinio model. It is shown that the repulsive density-density interaction coming from the vector term, which is present in the effective chiral models but has been omitted, enhances the competition between the chiral symmerty breaking (χSB) and CSC phase transition, and thereby makes the thermodynamic potential have a shallow minimum over a wide range of values of the correlated chiral and CSC order parameters. We find that when the vector coupling is increased, the first order transition between the χSB and CSC phases becomes weaker, and the coexisting phase in which both the chiral and color-gauge symmetry are dynamically broken comes to exisit over a wider range of the density and temperature. We also show that there can exist two endpoints, which are tricritical points in the chiral limit, along the critical line of the first order transition in some range of values of the vector coupling. Although our analysis is based on a simple model, the nontrivial interplay between the χSB and CSC phases induced by the vector interaction is expected to be a universal phenomenon and might give a clue to understanding results obtained with two-color QCD on the lattice.

Stability and Causality in relativistic dissipative hydrodynamics

The stability and causality of the Landau–Lifshitz theory and the Israel–Stewart-type causal dissipative hydrodynamics are discussed. We show that the problems of acausality and instability are correlated in relativistic dissipative hydrodynamics and instability is induced by acausality. We further discuss the stability of the scaling solution. The scaling solution of the causal dissipative hydrodynamics can be unstable against inhomogeneous perturbations.

Relativistic dissipative hydrodynamics: A minimal causal theory

We present a new formalism for the theory of relativistic dissipative hydrodynamics. Here, we look for the minimal structure of such a theory which satisfies the covariance and causality by introducing the memory effect in irreversible currents. Our theory has a much simpler structure and thus has several advantages for practical purposes compared to the Israel-Stewart theory (IS). It can readily be applied to the full three-dimensional hydrodynamical calculations. We apply our formalism to the Bjorken model and the results are shown to be analogous to the IS.

Precursor of color superconductivity in hot quark matter

We investigate possible precursory phenomena of color superconductivity in quark matter at finite temperature T with use of a simple Nambu-Jona-Lasinio model. It is found that the fluctuating pair field exists with a prominent strength even well above the critical temperature T_c. We show that the collective pair field has a complex energy located in the second Riemann sheet, which approaches the origin as T is lowered to T_c. We discuss the possible relevance of the precursor to the observables to be detected in heavy ion collisions.

Incorporating memory effects in phase separation processes

We consider the modification of the Cahn–Hilliard equation when a time delay process through a memory function is taken into account. We then study the process of spinodal decomposition in fast phase transitions associated with a conserved order parameter. Finite-time memory effects are seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the order parameter that follows a quench into the spinodal region. These effects are important in several systems characterized by fast processes, like non-equilibrium dynamics in the early universe and in relativistic heavy-ion collisions.

Pre-Critical Phenomena of Two-Flavor Color Superconductivity in Heated Quark Matter Diquark-Pair Fluctuations and Non-Fermi Liquid Behavior

We investigate the fluctuations of the diquark-pair field and their effects on observables above the critical temperature Tc in two-flavor color superconductivity (CSC) at moderate density using a Nambu-Jona-Lasinio-type effective model of QCD. Because of the strongcoupling nature of the dynamics, the fluctuations of the pair field develop a collective mode, which has a prominent strength even well above Tc. We show that the collective mode is actually the soft mode of CSC. We examine the effects of the pair fluctuations on the specific heat and the quark spectrum for T above but close to Tc. We find that the specific heat exhibits singular behavior because of the pair fluctuations, in accordance with the general theory of second-order phase transitions. The quarks display a typical non-Fermi liquid behavior, owing to the coupling with the soft mode, leading to a pseudo-gap in the density of states of the quarks in the vicinity of the critical point. Some experimental implications of the precursory phenomena are also discussed.

Pseudogap of color superconductivity in heated quark matter

We show that the pseudogap of the quark density of states is formed in hot quark matter as a precursory phenomenon of the color superconductivity on the basis of a low-energy effective theory. We clarify that the decaying process of quarks near Fermi surface to a hole and the diquark soft mode (qq){sub soft} is responsible for the formation of the pseudogap. Our result suggests that the pseudogap is a universal phenomenon in strong coupling superconductors.

Extensivity of irreversible current and stability in causal dissipative hydrodynamics

We extended our formulation of causal dissipative hydrodynamics (Koide et al 2007 Phys. Rev. C 75 034909) to be applicable to the ultra-relativistic regime by considering the extensiveness of irreversible currents. The new equation has a nonlinear term which suppresses the effect of viscosity. We found that such a term is essential to guarantee the positive definiteness of the inertia term and to stabilize numerical calculations in ultra-relativistic initial conditions. Because of the suppression of the viscosity, the behavior of the fluid is more close to that of the ideal fluid. Our result is essentially the same as that from the extended irreversible thermodynamics, but is different from the Israel–Stewart theory. A possible origin of the difference is discussed.

Shock propagation and stability in causal dissipative hydrodynamics

We studied shock propagation and its stability with causal dissipative hydrodynamics in (

Enhancement of critical slowing down in chiral phase transition—Langevin dynamics approach

1+1