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C. Sasaki

Publications and source records attributed to C. Sasaki.

At least 19 recordsLinked to original sources
arXiv2010

Hadron Production in Ultra-relativistic Nuclear Collisions: Quarkyonic Matter and a Triple Point in the Phase Diagram of QCD

We argue that features of hadron production in relativistic nuclear collisions, mainly at CERN-SPS energies, may be explained by the existence of three forms of matter: Hadronic Matter, Quarkyonic Matter, and a Quark-Gluon Plasma. We suggest that these meet at a triple point in the QCD phase diagram. Some of the features explained, both qualitatively and semi-quantitatively, include the curve for the decoupling of chemical equilibrium, along with the non-monotonic behavior of strange particle multiplicity ratios at center of mass energies near 10 GeV. If the transition(s) between the three phases are merely crossover(s), the triple point is only approximate.

hep-ph↗
arXiv2009

Enhancement of quark number susceptibility with an alternative pattern of chiral symmetry breaking in dense matter

We explore a possible phase where chiral $SU(2)_L \times SU(2)_R$ symmetry is spontaneously broken while its center $Z_2$ symmetry remains unbroken and its consequence on thermal quantities. In this phase, chiral symmetry breaking is driven by a quartic quark condensate although a bilinear quark condensate vanishes. A Ginzburg-Landau free energy leads to a new tricritical point (TCP) between the $Z_2$ broken and unbroken phases. Furthermore, a critical point can appear even in the chiral limit where explicit breaking is turned off, instead of a TCP at which restoration of chiral and its center symmetries takes place simultaneously. The net quark number density exhibits an abrupt change near the restoration of the center symmetry rather than that of the chiral symmetry. Hadron masses in possible phases are also studied in a linear sigma model. We show that, in the $Z_2$ symmetric phase, the $\bar{q}q$-type scalar meson with zero isospin I=0 splits from the $\bar{q}q$-type pseudoscalar meson with I=1.

hep-ph↗
arXiv2009

Bulk viscosity in quasi particle models

We discuss transport properties of dynamical fluid composed of quasi-particles whose masses depend on temperature and charge chemical potentials. Based on the relativistic kinetic theory formulated under the relaxation time approximation, we derive a general expression for the bulk viscosity in the quasi-particle medium. We show that dynamically generated particle masses imply an essential modification of the fluid compressibility. As an application of our results we consider a class of quasi-particle models with the chiral phase transition belonging to O(4) and Z(2) universality class. Based on the Ginzburg-Landau and the scaling theory we study the critical properties of the bulk viscosity zeta near the phase transition. We show that under the relaxation time approximation the zeta is not expected to show singular behavior near the O(4) and Z(2) critical point through static critical exponents.

hep-ph↗
arXiv2009

A novel spectral broadening from vector--axial-vector mixing in dense matter

The presence of baryonic matter leads to the mixing between transverse $ρ$ and $a_1$ mesons through a set of $ωρa_1$-type interactions, which results in the modification to the dispersion relation. We show that a clear enhancement of the vector spectral function appears below $\sqrt{s}=m_ρ$ for small three-momenta of the $ρ$ meson, and thus the vector spectrum exhibits broadening. We also discuss its relevance to dilepton measurements.

hep-ph↗
arXiv2009

Role of axial-vector mesons near the chiral phase transition

We present a systematic study of the vector--axial-vector mixing (V-A mixing) in the current correlation functions and its evolution with temperature within an effective field theory. The $a_1$-$ρ$-$π$ coupling vanishes at the critical temperature $T_c$ and thus the V-A mixing also vanishes. A remarkable observation is that even for finite $m_π$ the $ρ$ and $a_1$ meson masses are almost degenerate at $T_c$. The vanishing V-A mixing at $T_c$ stays approximately intact.

hep-ph↗
arXiv2008

Quarkyonic Matter and Chiral Symmetry Breaking

The appearance of a new phase of QCD, Quarkyonic Matter in the limit of large number of colors is studied within Nambu-Jona-Lassinio effective chiral model coupled to the Polyakov loop. The interplay of this novel QCD phase with chiral symmetry restoration and color deconfinement is discussed. We find that at vanishing temperature and at large $N_c$, the quarkyonic transition occurs at densities only slightly lower than that expected for the chiral transition. This property is also shown to be valid at finite temperature if the temperature is less than that of deconfinement. The position and $N_c$-dependence of chiral critical end point is also discussed.

hep-ph↗
arXiv2008

Transport coefficients near chiral phase transition

We analyze the transport properties of relativistic fluid composed of constituent quarks at finite temperature and density. We focus on the shear and bulk viscosities and study their behavior near chiral phase transition. We model the constituent quark interactions through the Nambu--Jona Lasinio Lagrangian. The transport coefficients are calculated within kinetic theory under relaxation time approximation including in--medium modification of quasi--particles dispersion relations. We quantify the influence of the order of chiral phase transition and the critical end point on dissipative phenomena in such a medium. Considering the changes of shear and bulk viscosities along the phase boundary we discuss their sensitivity to probe the existence of the critical end point.

hep-ph↗
arXiv2008

Vector-axialvector mixing from a chiral effective field theory at finite temperature

We study the vector-axialvector mixing in a hot medium and its evolution toward the chiral phase transition using different symmetry restoration scenarios based on the generalized hidden local symmetry framework. We show that the presence of the $a_1$ meson reduces the vector spectral function around $ρ$ meson mass and enhances it around $a_1$ meson mass. The coupling strength of $a_1$ to $ρ$ and $π$ vanishes at the critical temperature due to the degenerate $ρ$-$a_1$ masses. This feature holds rigorously in the chiral limit and still stays intact to good approximation for the physical pion mass.

hep-ph↗
arXiv2008

Density Fluctuations as Signature of a Non--Equilibrium First Order Phase Transition

We show that in the presence of spinodal instabilities which develop at a first order phase transition, the fluctuations of conserved charges can be as strong as those at the critical end point (CEP). In particular, the net baryon number susceptibility diverges as the system crosses the isothermal spinodal lines. This indicates that charge density fluctuations can be used not only to probe the CEP but also the non--equilibrium first order chiral phase transition in heavy ion collisions.

hep-ph↗
arXiv2008

Density fluctuations and a first-order chiral phase transition in non-equilibrium

The thermodynamics of a first-order chiral phase transition is considered in the presence of spinodal phase separation using the Nambu-Jona-Lasinio model in the mean field approximation. We focus on the behavior of conserved charge fluctuations. We show that in non-equilibrium the specific heat and charge susceptibilities diverge as the system crosses the isothermal spinodal lines.

hep-ph↗
arXiv2007

Density fluctuations and chiral phase transition

Based on an effective QCD Lagrangian we discuss the properties of charge density fluctuations in the vicinity of chiral phase transition. We explore thermodynamics in the presence of spinodal phase separation. We show that appearance of spinodal decomposition in a non-equilibrium first order phase transition results in divergence of the charge density fluctuations related with the electric charge and baryon number conservation. Consequently, divergent fluctuations at the chiral phase transition are not only attributed to the critical end point but are also there along the first order phase transition if the spinodal phase separation take place. Based on the mean field dynamics, the critical exponents for these singular behavior of charge susceptibilities are also discussed.

hep-ph↗
arXiv2007

Chiral phase transition in the presence of spinodal decomposition

The thermodynamics of a first order chiral phase transition is considered in the presence of spinodal phase separation within the Nambu-Jona-Lasinio (NJL) model. The properties of the basic thermodynamic observables in the coexistence phase are discussed for zero and non-zero quark masses. We focus on observables that probe the chiral phase transition. In particular, the behavior of the specific heat and entropy as well as charge fluctuations are calculated and analyzed. We show that the specific heat and charge susceptibilities diverge at the isothermal spinodal lines. We determine the scaling behavior and compute the critical exponent $γ$ of the net quark number susceptibility at the isothermal spinodal lines within the NJL model and the Ginsburg-Landau theory. We show that in the chiral limit the critical exponent $γ=1/2$ at the tricritical point as well as along the isothermal spinodal lines. On the other hand, for finite quark masses the critical exponent at the spinodal lines, $γ=1/2$, differs from that at the critical end point, $γ=2/3$, indicating a change in the universality class. These results are independent of the particular choice of the chiral Lagrangian and should be common for all mean field approaches.

hep-ph↗
arXiv2007

Density fluctuations in the presence of spinodal instabilities

Density fluctuations resulting from spinodal decomposition in a non-equilibrium first-order chiral phase transition are explored. We show that such instabilities generate divergent fluctuations of conserved charges along the isothermal spinodal lines appearing in the coexistence region. Thus, divergent density fluctuations could be a signal not only for the critical end point but also for the first order phase transition expected in strongly interacting matter. We also compute the mean-field critical exponent at the spinodal lines. Our analysis is performed in the mean-field approximation to the NJL model formulated at finite temperature and density. However, our main conclusions are expected to be generic and model independent.

hep-ph↗
arXiv2007

Exploring the QCD phase structure with density fluctuations

We briefly summarize the properties of conserved charge fluctuations as a sensitive probe for the QCD phase transitions. We discuss the density fluctuations which play a significant role to search for the critical end point. The importance of spinodal instabilities to distinguish the first-order phase transition is also indicated.

hep-ph↗
arXiv2007

Susceptibilities and the Phase Structure of a Chiral Model with Polyakov Loops

In an extension of the Nambu-Jona-Lasinio model where the quarks interact with the temporal gluon field, represented by the Polyakov loop, we explore the relation between the deconfinement and chiral phase transitions. The effect of Polyakov loop dynamics on thermodynamic quantities, on the phase structure at finite temperature and baryon density and on various susceptibilities is presented. Particular emphasis is put on the behavior and properties of the fluctuations of the (approximate) order parameters and their dependence on temperature and net--quark number density. We also discuss how the phase structure of the model is influenced by the coupling of the quarks to the Polyakov loop.

hep-ph↗
arXiv2007

Effective chiral model with Polyakov loops and its application to hot/dense medium

We study the thermodynamics of the Nambu--Jona-Lasinio model with Polyakov loops, where spontaneous chiral symmetry breaking and confinement are taken into account. We focus on the phase structure of the model and explore the susceptibilities associated with corresponding order parameters under the mean-field approximation.

hep-ph↗
arXiv2007

Quark Number Fluctuations in a Chiral Model at Finite Baryon Chemical Potential

We discuss the net quark and isovector fluctuations as well as off-diagonal quark flavor susceptibilities along the chiral phase transition line in the Nambu--Jona-Lasinio (NJL) model. The model is formulated at non-zero quark and isospin chemical potentials with non-vanishing vector couplings in the iso-scalar and iso-vector channels. We study the influence of the quark chemical potential on the quark flavour susceptibilities in detail and the dependence of the results on model parameters as well as on the quark mass. The NJL model findings are compared with recent lattice results obtained in two--flavor QCD at finite chemical potential. On a qualitative level, the NJL model provides a consistent description of the dependence of quark number fluctuations on temperature and baryon chemical potential. The phase diagram and the position of the tricritical point in the NJL model are also discussed for different parameter sets.

hep-ph↗