arXiv subjects

Masahiro Imachi

Publications and source records attributed to Masahiro Imachi.

11 recordsLinked to original sources

arXiv2008

String Junction Model, Cluster Hypothesis, Penta-Quark Baryon and Tetra-Quark Meson

Thirty years ago we proposed string junction model of hadrons and examined structure and reaction of hadrons including exotic ones. Mass $m$ of exotic hadrons of light quarks is roughly given by $m \sim N_J\cdot m_B$, where $N_J$ is the total number of junctions and $m_B \sim 1$ GeV is the ordinary light baryon mass. In this paper we introduce "cluster hypothesis" into the model by which mass of a complex hadron is given by the sum of masses of clusters composing it. The hypothesis guarantees the established picture that mass differences of hadrons of the same string junction structure are due to those of the constituent quarks. A candidate for penta-quark baryon $Θ$(1530 MeV, $S=+1)$ including a strange anti-quark ${\sb}$ and that for tetra-quark meson $Z^+$(4430 MeV) recently reported by the Bell collaboration are examined in parallel. $Θ$ is considered to have non-strange partners, which are lighter by the mass difference $Δ_s$ between strange and non-strange quarks. Mass of such light penta-quark baryons with $N_J=3$ is expected to be about 3 GeV. Several parameters of the model are estimated such as mass of junction of $m_J \sim O(10)$ MeV. While mass of light tetra-quark meson with $N_J=2$ is expected to be about 2 GeV, $Z^+$(4430 MeV) containing $(u,c,{\db},{\cb})$ gives a clue to determine some parameters of the model, e.g., inter-junction string energy $m_{IJ}$.

hep-ph↗

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.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗

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 ${\cal Z}(θ)$.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗

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.

hep-lat↗