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O. Kiriyama

Publications and source records attributed to O. Kiriyama.

15 recordsLinked to original sources
arXiv2009

Phase diagram of two-flavor quark matter: Gluonic phase at nonzero temperature

The phase structure of neutral two-flavor quark matter at nonzero temperature is studied. Our analysis is performed within the framework of a gauged Nambu--Jona-Lasinio model and the mean-field approximation. We compute the free energy of the gluonic phase (gluonic cylindrical phase II) in a self-consistent manner and investigate the phase transition from the gluonic phase to the 2SC/g2SC/NQ phases. We briefly consider the phase diagram in the plane of coupling strength versus temperature and discuss the mixed phase consisting of the normal quark and 2SC phases.

hep-ph↗
arXiv2006

Chromomagnetic instability in two-flavor quark matter at nonzero temperature

We calculate the effective potential of the 2SC/g2SC phases including vector condensates ($ $ and $ $) and study the gluonic phase and the single plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state at nonzero temperature. Our analysis is performed within the framework of the gauged Nambu--Jona-Lasinio model. We compute potential curvatures with respect to the vector condensates and investigate the temperature dependence of the Meissner masses squared of gluons of color 4--7 and 8 in the neutral 2SC/g2SC phases. The phase diagram is presented in the plane of temperature and coupling strength. The unstable regions for gluons 4--7 and 8 are mapped out on the phase diagram. We find that, apart from the case of strong coupling, the 2SC/g2SC phases at low temperatures are unstable against the vector condensation until the temperature reaches tens of MeV.

hep-ph↗
arXiv2006

Gluonic phase versus LOFF phase in two-flavor quark matter

We study the gluonic phase in a two-flavor color superconductor as a function of the ratio of the gap over the chemical potential mismatch,$Δ/δμ$. We find that the gluonic phase resolves the chromomagnetic instability encountered in a two-flavor color superconductor for $Δ/δμ< \sqrt{2}$. We also calculate approximately the free energies of the gluonic phase and the single plane-wave LOFF phase and show that the former is favored over the latter for a wide range of coupling strengths.

hep-ph↗
arXiv2006

Meissner screening mass in two-flavor quark matter at nonzero temperature

We calculate the Meissner screening mass of gluons 4--7 in two-flavor quark matter at nonzero temperature. To this end, we study the effective potential of the 2SC/g2SC phases including a vector condensate $ $ and calculate the Meissner mass from the potential curvature with respect to $ $. We find that the Meissner mass becomes real at the critical temperature which is about the half of the chemical potential mismatch. The phase diagram of the neutral two-flavor color superconductor is presented in the plane of temperature and coupling strength. We indicate the unstable region for gluons 4--7 on the phase diagram.

hep-ph↗
arXiv2006

Finite-size effects on the QCD phase diagram

We discuss the finite-size effects on the chiral phase transition in QCD. We employ the Nambu-Jona-Lasinio model and calculate the thermodynamic potential in the mean-field approximation. Finite-size effects on the thermodynamic potential are taken into account by employing the multiple reflection expansion. The critical temperature is lowered and the order of the phase transition is changed form first to second as the size of the system of interest is reduced.

hep-ph↗
arXiv2005

Stability of color-flavor locked strangelets

The stability of color-flavor locked (CFL) strangelets is studied in the three-flavor Nambu--Jona-Lasinio model. We consider all quark flavors to be massless, for simplicity. By making use of the multiple reflection expansion, we explicitly take into account finite size effects and formulate the thermodynamic potential for CFL strangelets. We find that the CFL gap could be large enough so that the energy per baryon number of CFL strangelets is greatly affected. In addition, if the quark-quark coupling constant is larger than a certain critical value, there is a possibility of finding absolutely stable CFL strangelets.

hep-ph↗
arXiv2005

Colour-superconducting strangelets in the Nambu--Jona-Lasinio model

The two-flavour colour-superconducting (2SC) phase in small strangelets is studied. In order to describe the 2SC phase we use the three-flavour Nambu--Jona-Lasinio model. We explicitly take into account finite size effects by making use of the approximation for the density of states in spherical cavities called multiple reflection expansion (MRE). The thermodynamic potential for the 2SC strangelets is derived in the mean-field approximation with the help of the MRE. We found that 2SC phase survives in small strangelets with a sizable gap. Consequences for the 2SC phases are also discussed.

hep-ph↗
arXiv2003

Chiral phase properties of finite size quark droplets in the Nambu--Jona-Lasinio model

Chiral phase properties of finite size hadronic systems are investigated within the Nambu--Jona-Lasinio model. Finite size effects are taken into account by making use of the multiple reflection expansion. We find that, for droplets with relatively small baryon numbers, chiral symmetry restoration is enhanced by the finite size effects. However the radius of the stable droplet does not change much, as compared to that without the multiple reflection expansion.

hep-ph↗
arXiv2001

Chiral meson masses at the critical point from QCD in the improved ladder approximation

Chiral meson masses at the critical point is investigated using QCD in the improved ladder approximation. We calculate the effective potential at finite temperature $T$ and quark chemical potential $μ$ and find the critical point at $T \simeq 95$ MeV, $μ\simeq 290$ MeV. The chiral meson masses are determined from the second derivative of the effective potential at its minimum. We find that the sigma goes to massless at the critical point while pion remains massive. Our results are consistent with that of the linear sigma model in contrast to the Nambu--Jona-Lasinio model.

hep-ph↗
arXiv2001

Color superconductivity at finite density and temperature with flavor asymmetry

Color superconductivity of QCD at finite density, temperature and flavor asymmetry is studied within an approximation which the interaction is modeled upon four--fermion interactions. We calculate the thermodynamic potential in the so-called NJL-type model and study the phase structure at finite density, temperature and flavor asymmetry. We find that a mixed phase appears at sufficiently large flavor asymmetry and low temperature. A tricritical point in the $T$-$μ_B$-$μ_I$ plane is also found.

hep-ph↗
arXiv2001

Current quark mass effects on chiral phase transition of QCD in the improved ladder approximation

Current quark mass effects on the chiral phase transition of QCD is studied in the improved ladder approximation. An infrared behavior of the gluon propagator is modified in terms of an effective running coupling. The analysis is based on a composite operator formalism and a variational approach. We use the Schwinger-Dyson equation to give a ``normalization condition'' for the Cornwall-Jackiw-Tomboulis effective potential and to isolate the ultraviolet divergence which appears in an expression for the quark-antiquark condensate. We study the current quark mass effects on the order parameter at zero temperature and density. We then calculate the effective potential at finite temperature and density and investigate the current quark mass effects on the chiral phase transition. We find a smooth crossover for $T>0$, $μ=0$ and a first-order phase transition for $μ>0$, T=0. Critical exponents are also studied and our model gives the classical mean-field values. We also study the temperature dependence of masses of scalar and pseudoscalar bosons. A critical end point in the $T$-$μ$ plane is found at $T \sim 100$ MeV, $μ\sim 300$ MeV.

hep-ph↗
arXiv2000

Chiral phase transition at high temperature and density in the QCD-like theory

The chiral phase transition at finite temperature T and/or chemical potential $μ$ is studied using the QCD-like theory with a variational approach. The ``QCD-like theory'' means the improved ladder approximation with an infrared cutoff in terms of a modified running coupling. The form of Cornwall-Jackiw-Tomboulis effective potential is modified by the use of the Schwinger-Dyson equation for generally nonzero current quark mass. We then calculate the effective potential at finite T and/or $μ$ and investigate the phase structure in the chiral limit. We have a second-order phase transition at $T_c=129$ MeV for $μ=0$ and a first-order one at $μ_c=422$ MeV for T=0. A tricritical point in the T-$μ$ plane is found at T=107 MeV, $μ=210$ MeV. The position is close to that of the random matrix model and some version of the Nambu-Jona-Lasinio model.

hep-ph↗
arXiv1998

Chiral phase transition at high temperature in the QCD-like gauge theory

The chiral phase transition at high temperature is investigated using the effect ive potential in the framework of the QCD-like gauge theory with a variational a pproach. We have a second order phase transition at $T_c=136$MeV. We also investigate numerically the temperature dependence of condensate, $f_π$ a nd $a_2(T)$(coefficient of the quadratic term in the effective potential) and es timate the critical exponents of these quantities.

hep-ph↗