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H. Abuki

Publications and source records attributed to H. Abuki.

15 recordsLinked to original sources
arXiv2016

Chiral spiral induced by a strong magnetic field

We study the modification of the chiral phase structure of QCD due to an external magnetic field. We first demonstrate how the effect of magnetic field can systematically be incorporated into a generalized Ginzburg-Landau framework. We then analyze the phase structure in the vicinity of the chiral critical point. In the chiral limit, the effect is found to be so drastic that it totally washes the tricritical point out of the phase diagram, bringing the continent for the chiral spiral. This is the case no matter how small is the intensity of the magnetic field. On the other hand, the current quark mass protects the chiral critical point from a weak magnetic field. However the critical point will eventually be covered by the chiral spiral phase as the magnetic field grows.

hep-ph↗
arXiv2013

Ginzburg-Landau phase diagram of QCD near chiral critical point - chiral defect lattice and solitonic pion condensate -

We investigate the influence of the isospin asymmetry on the phase structure of quark matter near the chiral critical point systematically using a generalized version of Ginzburg-Landau approach. The effect has proven to be so profound that it brings about not only a shift of the critical point but also a rich variety of phases in its neighborhood. In particular, there shows up a phase with spatially varying charged pion condensate which we name the "solitonic pion condensate" in addition to the "chiral defect lattice" where the chiral condensate is partially destructed by periodic placements of two-dimensional wall-like defects. Our results suggest that there may be an island of solitonic pion condensate in the low temperature and high density side of QCD phase diagram.

hep-ph↗
arXiv2009

Neutral quark matter in a Nambu-Jona Lasinio model with vector interaction

We investigate the three flavor Nambu-Jona Lasinio model of neutral quark matter at zero temperature and finite density, keeping into account the scalar, the pseudoscalar and the Kobayashi-Maskawa-'t Hooft interactions as well as the repulsive vector plus axial-vector interaction terms (vector extended NJL, VENJL in the following). We focus on the effect of the vector interaction on the chiral restoration at finite density in neutral matter. We also study the evolution of the charged pseudoscalar meson energies as a function of the quark chemical potential.

hep-ph↗
arXiv2009

The 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 $(π^c)$ condensation. We show that the equality between the electric chemical potential and the in-medium pion mass, $μ_{e}=M_{π^-}$, 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 to the symmetry breaking effect via a current quark mass $m$, and is ruled out for $m$ larger than the order of 10 keV.

hep-ph↗
arXiv2009

On the neutrality issue in the Polyakov-loop NJL model

We elucidate how the color neutrality is harmed in the Polyakov Nambu-Jona Lasinio (PNJL) model at finite density within the adopted mean field approximation. Also we point out how usual assumption about the diagonal form of the Wilson loop may fail in the presence of the diquark condensate on several grounds.

hep-ph↗
arXiv2008

Electrical neutrality and pion modes in the two flavor PNJL model

We study the phase diagram and the pion modes in the electrically neutral two flavor PNJL model. One of the main result of this paper is that when electrical neutrality is required, pions do not condense in the ground state of the model: the isospin chemical potential $μ_I = -μ_e/2$ is always smaller than the value required for pion condensation to occur. Moreover we investigate on the pions and $σ$ mass spectra. We find that the qualitative behavior of the masses resembles that obtained in the NJL model. We close this paper by studying the intriguing possibility that a bound state with the quantum numbers of the pions can be formed above the chiral phase transition.

hep-ph↗
arXiv2008

Polyakov-Nambu-Jona Lasinio model and Color-Flavor-Locked phase of QCD

The effect of Polyakov loop on the QCD phase diagram at high density is studied within the Nambu-Jona Lasinio model with Polyakov loop (PNJL model). We point out that the color neutrality is missing in the standard PNJL model at finite density. Moreover, we discuss how the color-flavor locked (CFL) phase is to be distorted by the inclusion of Polyakov loop.

hep-ph↗
arXiv2008

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 non local 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 $μ$ dependent scale, case III. In the cases I and II, at high chemical potential $μ$ 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 $N_c$ phase diagram. On the other hand in the 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.

hep-ph↗
arXiv2008

Enforced neutrality and color-flavor unlocking in the three-flavor Polyakov-loop NJL model

We study how the charge neutrality affects the phase structure of three-flavor PNJL model. We point out that, within the conventional PNJL model at finite density the color neutrality is missing because the Wilson line serves as an external ``colored'' field coupled to dynamical quarks. In this paper we heuristically assume that the model may still be applicable. To get color neutrality one has then to allow non vanishing color chemical potentials. We study how the quark matter phase diagram in $(T,m_s^2/μ)$-plane is affected by imposing neutrality and by including the Polyakov loop dynamics. Although these two effects are correlated in a nonlinear way, the impact of the Polyakov loop turns out to be significant in the $T$ direction, while imposing neutrality brings a remarkable effect in the $m_s^2/μ$ direction. In particular, we find a novel unlocking transition, when the temperature is increased, even in the chiral SU(3) limit. We clarify how and why this is possible once the dynamics of the colored Polyakov loop is taken into account. Also we succeed in giving an analytic expression for $T_c$ for the transition from two-flavor pairing (2SC) to unpaired quark matter in the presence of the Polyakov loop.

hep-ph↗
arXiv2007

BCS/BEC crossover in Quark Matter and Evolution of its Static and Dynamic properties

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 Nozi`eres--Schmit-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.

hep-ph↗
arXiv2006

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 --

We study the phase structures of charge neutral quark matter under the beta-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.

hep-ph↗
arXiv2005

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

We investigate the effects of the dynamical formation of the chiral condensates on color superconducting phases under the electric and color neutrality constraints at vanishing temperature. We shall show that the phase appearing next to the color-flavor-locked (CFL) phase down in density depends on the strength of the diquark coupling. In particular, the gapless CFL (gCFL) phase is realized only in a weak coupling regime. We give a qualitative argument on why the gCFL phase in the weak coupling region is replaced by some other phases in the strong coupling, once the competition between dynamical chiral symmetry breaking and the Cooper pair formation is taken into account.

hep-ph↗
arXiv2002

Color Superconductivity in Dense QCD and Structure of Cooper Pairs

The two-flavor color superconductivity is examined over a wide range of baryon density with a single model. To study the structural change of Cooper pairs, quark correlation in the color superconductor is calculated both in the momentum space and in the coordinate space. At extremely high baryon 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 due to strong color magnetic and electric interactions. The size of the Cooper pair is shown to become comparable to the averaged inter-quark distance at low densities, which indicates a crossover from BCS to BEC (Bose-Einstein condensation) of tightly bound Cooper pairs may take place at low density.

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