arXiv subjects

Shinji Ejiri

Publications and source records attributed to Shinji Ejiri.

At least 55 records · Page 3Linked to original sources

arXiv2014

Complex singularities around the QCD critical point at finite densities

Partition function zeros provide alternative approach to study phase structure of finite density QCD. The structure of the Lee-Yang edge singularities associated with the zeros in the complex chemical potential plane has a strong influence on the real axis of the chemical potential. In order to investigate what the singularities are like in a concrete form, we resort to an effective theory based on a mean field approach in the vicinity of the critical point. The crossover is identified as a real part of the singular point. We consider the complex effective potential and explicitly study the behavior of its extrema in the complex order parameter plane in order to see how the Stokes lines are associated with the singularity. Susceptibilities in the complex plane are also discussed.

hep-lat↗

arXiv2013

Nature of finite temperature and density phase transitions in many-flavor QCD

We investigate the phase structure of (2+Nf)-flavor QCD, where two light flavors and Nf massive flavors exist, to discuss the feasibility of the electroweak baryogenesis in realistic technicolor scenario and to understand properties of finite density QCD. Because an appearance of a first order phase transition at finite temperature is a necessary condition for the baryogenesis, it is important to study the nature of finite temperature phase transition. Applying the reweighting method, the probability distribution function of the plaquette is calculated in the many-flavor QCD. Through the shape of the distribution function, we determine the critical mass of heavy flavors terminating the first order region, and find it to become larger with Nf. We moreover study the critical line at finite density and the first order region is found to become wider as increasing the chemical potential. We discuss how the properties of real (2+1)-flavor QCD at finite temperature and density can be extracted from simulations of many-flavor QCD.

hep-lat↗

arXiv2013

Singularities around the QCD critical point in the complex chemical potential plane

We consider thermodynamic singularities appearing in the complex chemical potential plane in the vicinity of QCD critical point. In order to investigate what the singularities are like in a concrete form, we resort to an effective theory based on a mean field approach. We study the behavior of extrema of the real part of the complex effective potential in the complex order parameter plane.

hep-lat↗

arXiv2013

Phase structure of hot dense QCD by a histogram method

We study the phase structure of QCD at high temperature and density by lattice QCD simulations adopting a histogram method. The quark mass dependence and the chemical potential dependence of the nature of phase transition are investigated focusing on the probability distribution function (histogram). The shape of the distribution function changes with the quark mass and chemical potential. Through the shape of the distribution, the critical surface which separates the first order transition and crossover regions in the heavy quark region is determined for the (2+1)-flavor case. Moreover, we determined the critical point at finite density for two-flavor QCD with an intermediate quark mass, using a Gaussian approximation of the complex phase distribution of the quark determinant. The chemical potential dependence of the critical quark mass is also evaluated in the situation where two light quarks and many massive quarks exist. We find that the first order transition region becomes wider with the chemical potential in the many-flavor QCD.

hep-lat↗

arXiv2013

End Point of a First-Order Phase Transition in Many-Flavor Lattice QCD at Finite Temperature and Density

Towards the feasibility study of the electroweak baryogenesis in realistic technicolor scenario, we investigate the phase structure of (2+Nf)-flavor QCD, where the mass of two flavors is fixed to a small value and the others are heavy. For the baryogenesis, an appearance of a first order phase transition at finite temperature is a necessary condition. Using a set of configurations of two-flavor lattice QCD and applying the reweighting method, the effective potential defined by the probability distribution function of the plaquette is calculated in the presence of additional many heavy flavors. Through the shape of the effective potential, we determine the critical mass of heavy flavors separating the first order and crossover regions and find it to become larger with Nf. We moreover study the critical line at finite density and the first order region is found to become wider as increasing the chemical potential. Possible applications to real (2+1)-flavor QCD are discussed.

hep-lat↗

arXiv2013

Phase structure of many flavor lattice QCD at finite temperature

In realistic technicolor models containing many fermions, the electroweak baryogenesis offers a natural scenario for generating baryon number asymmetry. One of the key ingredients is the occurrence of the first order phase transition at finite temperature. As a first step toward the exploration of this possibility on the lattice, we develop an agile method to identify the critical mass for a given Nf, separating the first order and the crossover transition. We explain the outline of our method and demonstrate it by determining the critical mass of Nf-flavors in the presence of light two-flavors. It is found that the critical mass becomes larger with Nf.

hep-lat↗

arXiv2012

Phase structure of finite density QCD with a histogram method

We study the phase structure of QCD in the $T-μ$ plane using a histogram method and the reweighting technique by performing phase quenched simulations of two-flavor QCD with RG-improved gauge action and O($a$) improved Wilson quark action. Taking the effects of the complex phase of the quark determinant using the cumulant expansion method, we calculate the probability distribution function of plaquette and phase-quenched determinant as a function of $T$ and $μ$. We discuss the order of the QCD phase transition consulting the shape of the probability distribution function.

hep-lat↗

arXiv2012

Ab initio study of QCD thermodynamics on the lattice at zero and finite densities

The WHOT-QCD Collaboration is pushing forward a series of lattice studies of QCD at finite temperatures and densities using improved Wilson quarks. Because Wilson-type quarks require more computational resources than the more widely adopted staggered-type quarks, various theoretical and computational techniques have to be developed and applied. In this paper, we introduce the fixed-scale approach armed with the T-integration method, the Gaussian method based on the cumulant expansion, and the histogram method combined with the reweighting technique. Adopting these methods, we have carried out the first study of finite-density QCD with Wilson-type quarks and the first calculation of the equation of state with 2+1 flavors of Wilson-type quarks. We present results of these studies and discuss perspectives towards a clarification of the properties of 2+1 flavor QCD at the physical point, at finite temperatures and densities.

hep-lat↗

arXiv2012

Numerical study of QCD phase diagram at high temperature and density by a histogram method

We study the QCD phase structure at high temperature and density adopting a histogram method. Because the quark determinant is complex at finite density, the Monte-Carlo method cannot be applied directly. We use a reweighting method and try to solve the problems which arise in the reweighting method, i.e. the sign problem and the overlap problem. We discuss the chemical potential dependence of the probability distribution function in the heavy quark mass region and examine the applicability of the approach in the light quark region.

hep-lat↗

arXiv2010

Lattice QCD at finite temperature and density

We study the phase structure of QCD at finite temperature and density by numerical simulations on a lattice. The most important point for the numerical study at finite density is treatment of the sign problem. We propose a method to avoid the sign problem, which is based on a cumulant expansion of the complex phase in the density of state method combined with the reweighting method. Using the method, we study the critical point terminating a first order phase transition line in lattice QCD at high temperature and density.

hep-lat↗

arXiv2009

Third moments of conserved charges as probes of QCD phase structure

The third moments of conserved charges, the baryon and electric charge numbers, and energy, as well as their mixed moments, carry more information on the state around the QCD phase boundary than previously proposed fluctuation observables and higher order moments. In particular, their signs give plenty of information on the location of the state created in relativistic heavy ion collisions in the temperature and baryon chemical potential plane. We demonstrate this with an effective model.

nucl-th↗

arXiv2009

Singularities of QCD in the complex chemical potential plane

We study the thermodynamic singularities of QCD in the complex chemical potential plane by a numerical simulation of lattice QCD, and discuss a method to understand the nature of the QCD phase transition at finite density from the information of the singularities. The existence of singular points at which the partition function (Z) vanishes is expected in the complex plane. These are called Lee-Yang zeros or Fisher zeros. We investigate the distribution of these singular points using the data obtained by a simulation of two-flavor QCD with p4-improved staggered quarks. The convergence radius of a Taylor expansion of ln Z in terms of the chemical potential is also discussed.

hep-lat↗

arXiv2009

Third moments of conserved charges in QCD phase diagram

We point out that the third moments of conserved charges, the baryon and electric charge numbers, and energy, as well as their mixed moments, change their signs around the QCD phase boundary in the temperature and baryon chemical potential plane. These signs can be measured in relativistic heavy ion collisions, and will give clear information on the phase structure of QCD and the state of the system in the early stage of relativistic heavy ion collisions. The behaviors of these moments on the temperature axis and at small quark chemical potential can be analyzed in lattice QCD simulations. We emphasize that the third moments obtained on the lattice, together with the experimental results, will provide a deep understanding about the QCD phase diagram and the location of the state created in heavy ion collisions.

hep-lat↗

arXiv2009

Critical point in finite density lattice QCD by canonical approach

We propose a method to find the QCD critical point at finite density calculating the canonical partition function ${\cal Z}_{\rm C} (T,N)$ by Monte-Carlo simulations of lattice QCD, and analyze data obtained by a simulation with two-flavor p4-improved staggered quarks with pion mass $m_π \approx 770 {\rm MeV}$. It is found that the shape of an effective potential changes gradually as the temperature decreases and a first order phase transition appears in the low temperature and high density region. This result strongly suggests the existence of the critical point in the $(T, μ_q)$ phase diagram.

hep-lat↗

arXiv2008

Recent progress in lattice QCD at finite density

We review recent progress in lattice QCD at finite density. The phase diagram of QCD and the equation of state at finite temperature and density are discussed. In particular, we focus on the critical point terminating a first order phase transition line in the high density region. The critical point is one of the most interesting features that may be discovered in heavy-ion collision experiments. We summarize the current discussion on the existence of a critical point in the QCD phase diagram and discuss some attempts to find the critical point by numerical simulations.

hep-lat↗

arXiv2008

Canonical partition function and finite density phase transition in lattice QCD

We discuss the nature of the phase transition for lattice QCD at finite temperature and density. We propose a method to calculate the canonical partition function by fixing the total quark number introducing approximations allowed in the low density region. An effective potential as a function of the quark number density is discussed from the canonical partition function. We analyze data obtained in a simulation of two-flavor QCD using p4-improved staggered quarks with bare quark mass $m/T = 0.4$ on a $16^3 \times 4$ lattice. The results suggest that the finite density phase transition at low temperature is of first order.

hep-lat↗

arXiv2007

On the existence of the critical point in finite density lattice QCD

We propose a method to probe the nature of phase transitions in lattice QCD at finite temperature and density, which is based on the investigation of an effective potential as a function of the average plaquette. We analyze data obtained in a simulation of two-flavor QCD using p4-improved staggered quarks with bare quark mass $m/T = 0.4$, and find that a first order phase transition line appears in the high density regime for $μ_q/T \simge 2.5$. We also discuss the difference between the phase structures of QCD with non-zero quark chemical potential and non-zero isospin chemical potential.

hep-lat↗

arXiv2007

Study of the critical point in lattice QCD at high temperature and density

We propose a method to probe the nature of phase transitions in lattice QCD at finite temperature and density, which is based on the investigation of an effective potential as a function of the average plaquette. We analyze data obtained in a simulation of two-flavor QCD using p4-improved staggered quarks with bare quark mass $m/T = 0.4$, and find that a first order phase transition line appears in the high density regime for $μ_q/T \sim 2.5$. The effective potential as a function of the quark number density is also studied. We calculate the chemical potential as a function of the density from the canonical partition function and discuss the existence of the first order phase transition line.

hep-lat↗