arXiv subjects

Hiroshi Yoneyama

Publications and source records attributed to Hiroshi Yoneyama.

16 recordsLinked to original sources

arXiv2015

Analytic continuation of finite density QCD with heavy quarks in the strong coupling region

Complex nature of finite density QCD with heavy quarks in the strong coupling region is studied. For this purpose, we consider the effective potential as a function of Polyakov line, and study thermodynamic singularities and associated Stokes boundaries in the complex chemical potential plane. We also perform an explicit analytic continuation of the first order transition and crossover lines in the complex chemical potential plane.

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arXiv2015

First order transition regions in the quark masses and chemical potential parameter space of QCD

We investigate the phase transitions of (2+Nf)-flavor QCD, where two light flavors and Nf massive flavors exist, aiming to understand the phase structure of (2+1)-flavor QCD. Performing simulations of 2-flavor QCD with improved staggered and Wilson fermions and using the reweighting method, we calculate probability distribution functions in the many-flavor QCD. Through the shape of distribution functions, we determine the critical surface terminating first order phase transitions in the parameter space of the light quark mass, heavy quark mass and the chemical potential, and find that the first order region becomes larger with Nf. We then study the critical surface at finite density for large Nf and the first order region is found to become wider with the increasing chemical potential. On the other hand, the light quark mass dependence of the critical mass of heavy quarks seems weak in the region we investigated. The result of this weak dependence suggests that the critical mass of heavy quark remains finite in the chiral limit of 2-flavors and there exists a second order transition region on the line of the 2-flavor massless limit above the tri-critical point. Moreover, we extend the study of 2-flavor QCD at finite density to the case of a complex chemical potential and investigate the singularities where the partition function vanishes, so-called Lee-Yang zeros. The plaquette effective potential is computed in the complex plane. We find that the shape of the effective potential changes from single-well on the real axis to double-well at large imaginary chemical potential and the double-well potential causes the singularities.

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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.

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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.

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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.

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arXiv2009

Analytic Continuation in Two-color Finite Density QCD and Chiral Random Matrix Model

Two-color finite density QCD is free from the sign problem, and it is thus regarded as a good model to check the validity of the analytic continuation method. We study the method in terms of the corresponding chiral random matrix model. It is found that at temperatures slightly higher than the pseudo critical temperature, the ratio type of extrapolated function works well in accordance with the results of the Monte Carlo simulations.

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arXiv2007

Lattice Field Theory with the Sign Problem and the Maximum Entropy Method

Although numerical simulation in lattice field theory is one of the most effective tools to study non-perturbative properties of field theories, it faces serious obstacles coming from the sign problem in some theories such as finite density QCD and lattice field theory with the $θ$ term. We reconsider this problem from the point of view of the maximum entropy method.

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arXiv2006

The $θ$-term, CP$^{N-1}$ Model and the Inversion Approach in the Imaginary $θ$ Method

The weak coupling region of CP$^{N-1}$ lattice field theory with the $θ$-term is investigated. Both the usual real theta method and the imaginary theta method are studied. The latter was first proposed by Bhanot and David. Azcoiti et al. proposed an inversion approach based on the imaginary theta method. The role of the inversion approach is investigated in this paper. A wide range of values of $h=-{\rm Im} θ$ is studied, where $θ$ denotes the magnitude of the topological term. Step-like behavior in the $x$-$h$ relation (where $x=Q/V$, $Q$ is the topological charge, and $V$ is the two dimensional volume) is found in the weak coupling region. The physical meaning of the position of the step-like behavior is discussed. The inversion approach is applied to weak coupling regions.

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arXiv2006

Sign problem and MEM in lattice field theory with the $θ$ term

Lattice field theory with the $θ$ term suffers from the sign problem. The sign problem appears as flattening of the free energy. As an alternative to the conventional method, the Fourier transform method (FTM), we apply the maximum entropy method (MEM) to Monte Carlo data obtained using the CP$^3$ model with the $θ$ term. For data without flattening, we obtain the most probable images of the partition function ${\hat{\cal Z}}(θ)$ with rather small errors. The results are quantitatively close to the result obtained with the FTM. Motivated by this fact, we systematically investigate flattening in terms of the MEM. Obtained images ${\hat{\cal Z}}(θ)$ are consistent with the FTM for small values of $θ$, while the behavior of ${\hat{\cal Z}}(θ)$ depends strongly on the default model for large values of $θ$. This behavior of ${\hat{\cal Z}}(θ)$ reflects the flattening phenomenon.

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arXiv2006

Sign problem and MEM

The sign problem is notorious in Monte Carlo simulations of lattice QCD with the finite density, lattice field theory (LFT) with a $θ$ term and quantum spin models. In this report, to deal with the sign problem, we apply the maximum entropy method (MEM) to LFT with the $θ$ term and investigate to what extent the MEM is applicable to this issue. Based on this study, we also make a brief comment about lattice QCD with the finite density in terms of the MEM.

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arXiv2005

True or Fictitious Flattening? -MEM and the $θ$ Term-

We study the sign problem in lattice field theory with a $θ$ term. We apply the maximum entropy method (MEM) to flattening phenomenon of the free energy density $f(θ)$, which originates from the sign problem. In our previous paper, we applied the MEM by employing the Gaussian topological charge distribution $P(Q)$ as mock data. In the present paper, we consider models in which `true' flattening of $f(θ)$ occurs. These may be regarded as good examples for studying whether the MEM could correctly detect non trivial phase structure.

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arXiv2004

MEM study of true flattening of free energy and the $θ$ term

We study the sign problem in lattice field theory with a $θ$ term, which reveals as flattening phenomenon of the free energy density $f(θ)$. We report the result of the MEM analysis, where such mock data are used that `true' flattening of $f(θ)$ occurs. This is regarded as a simple model for studying whether the MEM could correctly detect non trivial phase structure in $θ$ space. We discuss how the MEM distinguishes fictitious and true flattening.

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arXiv2004

CP$^{N-1}$ model with the theta term and maximum entropy method

A $θ$ term in lattice field theory causes the sign problem in Monte Carlo simulations. This problem can be circumvented by Fourier-transforming the topological charge distribution $P(Q)$. This strategy, however, has a limitation, because errors of $P(Q)$ prevent one from calculating the partition function ${\cal Z}(θ)$ properly for large volumes. This is called flattening. As an alternative approach to the Fourier method, we utilize the maximum entropy method (MEM) to calculate ${\cal Z}(θ)$. We apply the MEM to Monte Carlo data of the CP$^3$ model. It is found that in the non-flattening case, the result of the MEM agrees with that of the Fourier transform, while in the flattening case, the MEM gives smooth ${\cal Z}(θ)$.

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arXiv2004

Maximum Entropy Method Approach to $θ$ Term

In Monte Carlo simulations of lattice field theory with a $θ$ term, one confronts the complex weight problem, or the sign problem. This is circumvented by performing the Fourier transform of the topological charge distribution $P(Q)$. This procedure, however, causes flattening phenomenon of the free energy $f(θ)$, which makes study of the phase structure unfeasible. In order to treat this problem, we apply the maximum entropy method (MEM) to a Gaussian form of $P(Q)$, which serves as a good example to test whether the MEM can be applied effectively to the $θ$ term. We study the case with flattening as well as that without flattening. In the latter case, the results of the MEM agree with those obtained from the direct application of the Fourier transform. For the former, the MEM gives a smoother $f(θ)$ than that of the Fourier transform. Among various default models investigated, the images which yield the least error do not show flattening, although some others cannot be excluded given the uncertainty related to statistical error.

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arXiv1999

Two dimensional CP^2 Model with θ-term and Topological Charge Distributions

Topological charge distributions in 2 dimensional CP^2 model with theta-term is calculated. In strong coupling regions, topological charge distribution is approximately given by Gaussian form as a function of topological charge and this behavior leads to the first order phase transition at θ=π. In weak coupling regions it shows non-Gaussian distribution and the first order phase transition disappears. Free energy as a function of θshows "flattening" behavior at theta=theta_f<pi, when we calculate the free energy directly from topological charge distribution. Possible origin of this flattening phenomena is prensented.

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arXiv1997

Phase Structures of U(2) Gauge Theory with $θ$-Term in 2 dimensions

U(2) lattice gauge theory with $θ$-term in 2 space-time dimensions is investigated. It has non-Abelian real action and Abelian( U(1) type) imaginary action. The imaginary action is defined as the standard $θ$-term. As the effect of renormalization group (RG) transformation, non-Abelian imaginary action is induced. After many steps of RG transformation, non-Abelian part will die away. After several steps of RG transformations, renormalized action approaches so called heat kernel action. Phase transition is found at $θ=π$ only.

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