Hiroaki Abuki

Chiral crossover, deconfinement, and quarkyonic matter within a Nambu-Jona-Lasinio model with the Polyakov loop

We study the interplay between the chiral and the deconfinement transitions, both at high temperature and high quark chemical potential, by a nonlocal Nambu-Jona-Lasinio model with the Polyakov loop in the mean field approximation and requiring neutrality of the ground state. We consider three forms of the effective potential of the Polyakov loop: two of them with a fixed deconfinement scale, cases I and II, and the third one with a {mu} dependent scale, case III. In cases I and II, at high chemical potential {mu} and low temperature T, the main contribution to the free energy is due to the Z(3)-neutral three-quark states, mimicking the quarkyonic phase of the large N{sub c} phase diagram. On the other hand, in case III the quarkyonic window is shrunk to a small region. Finally we comment on the relations of these results to lattice studies and on possible common prospects. We also briefly comment on the coexistence of quarkyonic and color superconductive phases.

Structural change of Cooper pairs and momentum dependent gap in color superconductivity

The two-flavor color superconductivity is studied over a wide range of baryon density with a single model. We pay a special attention to the spatial-momentum dependence of the gap and to the spatial-structure of Cooper pairs. At extremely high baryon density (O(10^{10} rho_0) with rho_0 being the normal nuclear matter density), our model becomes equivalent to the usual perturbative QCD treatment and the gap is shown to have a sharp peak near the Fermi surface due to the weak-coupling nature of QCD. On the other hand, the gap is a smooth function of the momentum at lower densities (O(10 rho_0)) due to strong color magnetic and electric interactions. To study the structural change of Cooper pairs from high density to lower density, quark correlation in the color superconductor is studied both in the momentum space and in the coordinate space. The size of the Cooper pair is shown to become comparable to the averaged inter-quark distance at low densities. Also, effects of the momentum-dependent running coupling and the antiquark pairing, which are both small at high density, are shown to be non-negligible at low densities. These features are highly contrasted to the standard BCS superconductivity in metals.

Extensive study of phase diagram for charge neutral homogeneous quark matter affected by dynamical chiral condensation: Unified picture for thermal unpairing transitions from weak to strong coupling

Abstract We study the phase structures of charge-neutral quark matter under the β -equilibrium for a wide range of the quark–quark coupling strength within a four-fermion model. A comprehensive and unified picture for the phase transitions from weak to strong coupling is presented. We first develop a technique to deal with the gap equation and neutrality constraints without recourse to numerical derivatives, and show that the off-diagonal color densities automatically vanish with the standard assumption for the diquark condensates. The following are shown by the numerical analyses: (i) The thermally-robustest pairing phase is the two-flavor pairing (2SC) in any coupling case, while the second one for relatively low density is the up-quark pairing (uSC) phase or the color–flavor-locked (CFL) phase depending on the coupling strength and the value of strange quark mass. (ii) If the diquark coupling strength is large enough, the phase diagram is much simplified and is free from the instability problems associated with imaginary Meissner masses in the gapless phases. (iii) The interplay between the chiral and diquark dynamics may bring a non-trivial first order transition even in the pairing phases at high density. We confirm (i) also by using the Ginzburg–Landau analysis expanding the pair susceptibilities up to quartic order in the strange quark mass. We obtain the analytic expression for the doubly critical density where the two lines for the second order phase transitions merge, and below which the down-quark pairing (dSC) phase is taken over by the uSC phase. Also we study how the phase transitions from fully gapped states to partially ungapped states are smeared at finite temperature by introducing the order parameters for these transitions.

BCS-BEC crossover in a relativistic superfluid and its significance to quark matter

The character change of a superfluid state due to the variation of the attractive force is investigated in the relativistic framework with a massive fermion. Two crossovers are found. One is a crossover from the usual BCS state to the Bose-Einstein condensation (BEC) of bound fermion pairs. The other is from the BEC to the relativistic Bose-Einstein condensation (RBEC) of nearly massless bound pairs where antiparticles as well as particles dominate the thermodynamics. Possible realization of the BEC and RBEC states in the quark matter is also pointed out.

BCS/BEC crossover in quark matter and evolution of its static and dynamic properties from the atomic unitary gas to color superconductivity

Abstract We study the evolution of dynamic properties of the BCS/BEC (Bose–Einstein condensate) crossover in a relativistic superfluid as well as its thermodynamics. We put particular focus on the change in the soft mode dynamics throughout the crossover, and find that three different effective theories describe it; these are, the time-dependent Ginzburg–Landau (TDGL) theory in the BCS regime, the Gross–Pitaevskii (GP) theory in the BEC regime, and the relativistic Gross–Pitaevskii (RGP) equation in the relativistic BEC (RBEC) regime. Based on these effective theories, we discuss how the physical nature of soft mode changes in the crossover. We also discuss some fluid-dynamic aspects of the crossover using these effective theories with particular focus on the shear viscosity. In addition to the study of soft modes, we show that the “quantum fluctuation” is present in the relativistic fermion system, which is in contrast to the usual Nozieres–Schmitt-Rink (NSR) theory. We clarify the physical meaning of the quantum fluctuation, and find that it drastically increases the critical temperature in the weak coupling BCS regime.

Crystalline chiral condensates off the tricritical point in a generalized Ginzburg-Landau approach

We present an extensive study on inhomogeneous chiral condensates in QCD at finite density in the chiral limit using a generalized Ginzburg-Landau (GL) approach. Performing analyses on higher harmonics of one-dimensionally (1D) modulated condensates, we numerically confirm the previous claim that the solitonic chiral condensate characterized by Jacobis elliptic function is the most favorable structure in 1D modulations. We then investigate the possibility of realization of several multidimensional modulations within the same framework. We also study the phase structure far away from the tricritical point by extending the GL functional expanded up to the eighth order in the order parameter and its spatial derivative. On the same basis, we explore a new regime in the extended GL parameter space and find that the Lifshitz point is the point where five critical lines meet at once. In particular, the existence of an intriguing triple point is demonstrated, and its trajectory consists of one of those critical lines.

Nambu-Jona-Lasinio model of dense three-flavor matter with axial anomaly: The low temperature critical point and BEC-BCS diquark crossover

We study the QCD phase structure in the three-flavor Nambu-Jona-Lasinio model, incorporating the interplay between the chiral and diquark condensates induced by the axial anomaly. We demonstrate that for an appropriate range of parameters of the model, the interplay leads to the low temperature critical point in the phase structure predicted by a previous Ginzburg-Landau analysis. We also show that a Bose-Einstein condensate (BEC) of diquark molecules emerges in the intermediate density region, and as a result, a BEC-BCS crossover is realized with increasing quark chemical potential.

Fate of pion condensation in quark matter : From the chiral limit to the physical pion mass

We study aspects of the pion condensation in two-flavor neutral quark matter using the Nambu-Jona-Lasinio model of QCD at finite density. We investigate the role of electric charge neutrality, and explicit symmetry breaking via quark mass, both of which control the onset of the charged pion ({pi}{sup c}) condensation. We show that the equality between the electric chemical potential and the in-medium pion mass, {mu}{sub e}=M{sub {pi}{sup -}}, as a threshold, persists even for a composite pion system in the medium, provided the transition to the pion condensed phase is of the second order. Moreover, we find that the pion condensate in neutral quark matter is extremely fragile with respect to the symmetry breaking effect via a current quark mass m, and is ruled out for m larger than the order of 10 keV.

Electrical neutrality and pion modes in the two flavor Polyakov-Nambu-Jona-Lasinio model

We study the phase diagram and the pion modes in the electrically neutral two flavor Polyakov\char21{}Nambu\char21{}Jona-Lasinio model. One of the main results of this paper is that in the model with massive quarks, when electrical neutrality is required, pions do not condense in the ground state of the model: the isospin chemical potential

How do chiral condensates affect color superconducting quark matter under charge neutrality constraints

{\ensuremath{\mu}}_{I}=\ensuremath{-}{\ensuremath{\mu}}_{e}/2