arXiv subjects
Publications and source records attributed to H. Matsufuru.
We report on the two-flavor QCD simulation in the epsilon-regime using the overlap fermion formulation. Sea quark mass is reduced to ~ 2 MeV on a 16^3x32 lattice with the lattice spacing ~ 0.11fm. Topological charge is fixed at Q=0. We compare the eigenvalue distribution of the overlap-Dirac operator with the prediction of the chiral random matrix theory. Preliminary results on meson correlators are also reported.
We launched a project to perform dymanical fermion simulations using the overlap fermion formulation for sea quarks. In order to avoid the appearace of near-zero modes of the hermitian Wilson-Dirac operator $H_W$, we introduce a pair of extra Wilson fermions with a large negative mass term. Crossing of the topological boundary is then strictly prohibited, and the topological charge is conserved during simulations. It makes the simulations substantially faster compared to the algorithms which allow the topology change. We discuss on the finite volume effects due to the fixed global topology.
We investigate the distribution of low-lying eigenmodes of hermitian Wilson-Dirac operator, $H_W$, with the gauge action whose form is designed to avoid topology change, as well as with the standard plaquette action. On the quenched lattices, the former gauge action exhibits less density of low-lying eigenmodes of $H_W$ compared to the latter at the same lattice spacing. We also show preliminary results for the dynamical simulation with two flavors of overlap fermions.
The properties of hadron screening masses around the deconfinement phase transition at finite baryonic density can be studied by evaluating the Taylor coefficients with respect to the iso-scalar and iso-vector chemical potentials. We simulate 2-flavour lattice QCD with staggered fermions on a 12*12*24*6 lattice with ma = 0.05 and 0.10 and report investigations of nucleon, pseudo-scalar and vector mesons. We present new, strong evidence that in the confining phase, the screening masses at vanishing chemical potential have significant temperature dependence, but the effect of the iso-scalar chemical potential is very small. Above the critical temperature, the second derivative terms of mesons rapidly increase as contrasted to the case of baryon. We also study the responses of the screening masses to an iso-vector chemical potential and discuss some of the issues related to the properties of hadron masses at finite density.
We study quark confinement physics using lattice QCD. In the maximally abelian (MA) gauge, the off-diagonal gluon amplitude is strongly suppressed, and then the off-diagonal gluon phase shows strong randomness, which leads to a large effective off-diagonal gluon mass, M_off=1.2GeV. Due to the large off-diagonal gluon mass in the MA gauge, low-energy QCD is abelianized like nonabelian Higgs theories. In the MA gauge, there appears a macroscopic network of the monopole world-line covering the whole system. We extract and analyze the dual gluon field B_mu from the monopole-current system in the MA gauge, and evaluate the dual gluon mass as m_B = 0.4-0.5GeV in the infrared region, which is a lattice-QCD evidence of the dual Higgs mechanism by monopole condensation. Even without explicit use of gauge fixing, we can define the maximal abelian projection by introducing a ``gluonic Higgs field'' phi(x), whose hedgehog singularities lead to monopoles. From infrared abelian dominance and infrared monopole condensation, infrared QCD is describable with the dual Ginzburg-Landau theory. In relation to the color-flux-tube picture for baryons, we study the three-quark (3Q) ground-state potential V_3Q in SU(3) lattice QCD at the quenched level, with the smearing technique for enhancement of the ground-state component. With accuracy better than a few %, V_3Q is well described by a sum of the two-body Coulomb part and the three-body linear confinement part sigma_3Q L_min, where L_min denotes the minimal value of the total length of the color flux tube linking the three quarks. Comparing with the Q-barQ potential, we find a universal feature of the string tension and the OGE result for the Coulomb coefficient.
We are aiming to construct Quark Hadron Physics and Confinement Physics based on QCD. Using SU(3)$_c$ lattice QCD, we are investigating the three-quark potential at T=0 and $T \ne 0$, mass spectra of positive and negative-parity baryons in the octet and the decuplet representations of the SU(3) flavor, glueball properties at T=0 and $T \ne 0$. We study also Confinement Physics using lattice QCD. In the maximally abelian (MA) gauge, the off-diagonal gluon amplitude is strongly suppressed, and then the off-diagonal gluon phase shows strong randomness, which leads to a large effective off-diagonal gluon mass, $M_{\rm off} \simeq 1.2 {\rm GeV}$. Due to the large off-diagonal gluon mass in the MA gauge, infrared QCD is abelianized like nonabelian Higgs theories. In the MA gauge, there appears a macroscopic network of the monopole world-line covering the whole system. From the monopole current, we extract the dual gluon field $B_μ$, and examine the longitudinal magnetic screening. We obtain $m_B \simeq$ 0.5 GeV in the infrared region, which indicates the dual Higgs mechanism by monopole condensation. From infrared abelian dominance and infrared monopole condensation, low-energy QCD in the MA gauge is described with the dual Ginzburg-Landau (DGL) theory.
We study abelianization of QCD in the maximally abelian (MA) gauge. In the MA gauge, the off-diagonal gluon amplitude is strongly suppressed, and then the off-diagonal gluon phase shows strong randomness, which leads to a large off-diagonal gluon mass. Using lattice QCD, we find a large effective off-diagonal gluon mass in the MA gauge: $M_{\rm off} \simeq 1.2 {\rm GeV}$ in SU(2) QCD, $M_{\rm off} \simeq 1.1 {\rm GeV}$ in SU(3) QCD. Due to the large off-diagonal gluon mass in the MA gauge, infrared QCD is well abelianized like nonabelian Higgs theories. We investigate the inter-monopole potential and the dual gluon field $B_μ$ in the MA gauge, and find longitudinal magnetic screening with $m_B \simeq$ 0.5 GeV in the infrared region, which indicates the dual Higgs mechanism by monopole condensation. We propose a gauge invariant description of the MA projection by introducing the ``gluonic Higgs scalar field''.
We aim to construct quark hadron physics based on QCD. First, using lattice QCD, we study mass spectra of positive-parity and negative-parity baryons in the octet, the decuplet and the singlet representations of the SU(3) flavor. In particular, we consider the lightest negative-parity baryon, the $Λ$(1405), which can be an exotic hadron as the $N \bar K$ molecular state or the flavor-singlet three-quark state. We investigate the negative-parity flavor-singlet three-quark state in lattice QCD using the quenched approximation, where the dynamical quark-anitiquark pair creation is absent and no mixing occurs between the three-quark and the five-quark states. Our lattice QCD analysis suggests that the flavor-singlet three-quark state is so heavy that the $Λ$(1405) cannot be identified as the three-quark state, which supports the possibility of the molecular-state picture of the $Λ$(1405). Second, we study thermal properties of the scalar glueball in an anisotropic lattice QCD, and find about 300 MeV mass reduction near the QCD critical temperature from the pole-mass analysis. Finally, we study the three-quark potential, which is responsible to the baryon properties. The detailed lattice QCD analysis for the 3Q potential indicates the Y-type flux-tube formation linking the three quarks.
We review briefly recent studies of the Lambda(1405) spectrum in Lattice QCD. Ordinary three-quark pictures of the Lambda(1405) in quenched Lattice QCD fail to reproduce the mass of the experimental value, which seems to support the penta-quark picture for the Lambda(1405) such as a Kbar-N molecule-like state. It is also noted that the present results suffer from relatively large systematic uncertainties coming from the finite volume effect, the chiral extrapolation and the quenching effect.
We study negative-parity baryon spectra in quenched anisotropic lattice QCD. The negative-parity baryons are measured as the parity partner of the ground-state baryons. In addition to the flavor octet and decuplet baryons, we pay much attention to the flavor-singlet negative-parity baryon as a three-quark state and compare it with the Lambda(1405) baryon. Numerical results of the flavor octet and decuplet negative-parity baryon masses are close to experimental values of lowest-lying negative-parity baryons, while the flavor-singlet baryon is much heavier than Lambda(1405). This indicates that the Lambda(1405) would be a multi-quark state such as the N-Kbar molecule rather than the flavor-singlet 3 quark state.
We investigate the negative-parity baryon spectra in quenched lattice QCD. We employ the anisotropic lattice with standard Wilson gauge and O(a) improved Wilson quark actions at three values of lattice spacings with renormalized anisotropy xi=a_sigma/a_tau=4, where a_sigma and a_tau are spatial and temporal lattice spacings, respectively. The negative-parity baryons are measured with the parity projection. In particular, we pay much attention to the lowest SU(3) flavor-singlet negative-parity baryon, which is assigned as the Lambda(1405) in the quark model. For the flavor octet and decuplet negative-parity baryons, the calculated masses are close to the experimental values of corresponding lowest-lying negative-parity baryons. In contrast, the flavor-singlet baryon is found to be about 1.7 GeV, which is much heavier than the Lambda(1405). Therefore, it is difficult to identify the Lambda(1405) to be the flavor-singlet three-quark state, which seems to support an interesting picture of the penta-quark (uds qbar q) state or the N-Kbar molecule for the Lambda(1405).
The static three-quark(3Q) potential is investigated in the SU(3) lattice QCD at the quenched level, using the standard Wilson action with $β=5.7 (a\simeq 0.2{\rm fm})$ and $12^3\times 24$. With the 3Q Wilson loop, the ground-state 3Q potential $V_{\rm 3Q}$ is calculated for 16 patterns of the 3Q configuration, using the smearing technique, which enhances the ground-state overlap of the 3Q operator. With the accuracy better than a few %, the lattice QCD data for $V_{\rm 3Q}$ are well described by a sum of the Coulomb term, a constant and the linear term which is proportional to the minimal length $L_{\rm min}$ of the flux-tube linking the 3 quarks. Our results seem to support the Y-type flux picture rather than $Δ$-type one. From the comparison of $V_{\rm 3Q}$ with the ${\rm Q}$-${\rm \bar{Q}}$ potential, we find the universality of the string tension, $σ_{\rm 3Q}\simeq σ_{\rm Q\bar{Q}}$ as well as the OGE relation on the Coulomb coefficient, $A_{\rm 3Q}\simeq {1/2}A_{\rm Q\bar{Q}}$.
The ground-state three-quark (3Q) potential $V_{\rm 3Q}^{\rm g.s.}$ and the excited-state 3Q potential $V_{\rm 3Q}^{\rm e.s.}$ are studied using SU(3) lattice QCD at the quenched level. For more than 300 patterns of the 3Q systems, the ground-state potential $V_{\rm 3Q}^{\rm g.s.}$ is investigated in detail in lattice QCD with $12^3\times 24$ at $β=5.7$ and with $16^3\times 32$ at $β=5.8, 6.0$. As a result, the ground-state potential $V_{\rm 3Q}^{\rm g.s.}$ is found to be well described with Y-ansatz within the 1%-level deviation. From the comparison with the Q-$\rm\bar Q$ potential, we find the universality of the string tension as $σ_{\rm 3Q}\simeqσ_{\rm Q\bar Q}$ and the one-gluon-exchange result as $A_{\rm 3Q}\simeq\frac12 A_{\rm Q\bar Q}$. The excited-state potential $V_{\rm 3Q}^{\rm e.s.}$ is also studied in lattice QCD with $16^3\times 32$ at $β=5.8$ for 24 patterns of the 3Q systems.The energy gap between $V_{\rm 3Q}^{\rm g.s.}$ and $V_{\rm 3Q}^{\rm e.s.}$, which physically means the gluonic excitation energy, is found to be about 1GeV in the typical hadronic scale, which is relatively large compared with the excitation energy of the quark origin. This large gluonic excitation energy justifies the great success of the simple quark model.
We study charmonium correlators at finite temperature in quenched anisotropic lattice QCD. The smearing technique is applied to enhance the low energy part of the correlator. We use two analysis procedures: the maximum entropy method for extraction of the spectral function without assuming specific form, as an estimate of the shape of spectral function, and the $χ^2$ fit assuming typical forms as quantitative evaluation of the parameters associated to the forms. We find that at $T\simeq 0.9T_c$ the ground state peak has almost the same mass as at T=0 and almost vanishing width. At $T\simeq 1.1T_c$, our result suggests that the correlator still has nontrivial peak structure at almost the same position as below $T_c$ with finite width.
We examine whether the $O(a)$ improved quark action on anisotropic lattices can be used as a framework for the heavy quark, which enables precision computation of matrix elements of heavy-light mesons. To this end, it is crucial to verify that a mass independent and nonperturbative tuning of the parameters is possible. As a first step, we observe the dispersion relation of heavy-light mesons on a quenched lattice using the action which is nonperturbatively tuned only for the leading terms. On a lattice with the spatial cutoff $a_σ^{-1} \simeq$ 1.6 GeV and the anisotropy $ξ=4$, the relativity relation holds within 2% accuracy in the quark mass region $a_σm_Q \leq 1.2$ with the bare anisotropy parameter tuned for the massless quark. We also apply the action to a calculation of heavy-light decay constants in the charm quark mass region.
We investigate the renormalization group (RG) flow of SU(3) lattice gauge theory in a two coupling space with couplings $β_{11}$ and $β_{12}$ corresponding to $1\times 1$ and $1\times 2$ loops respectively. Extensive numerical calculations of the RG flow are made in the fourth quadrant of this coupling space, i.e., $β_{11}>0$ and $β_{12}<0$. Swendsen's factor two blocking and the Schwinger-Dyson method are used to find an effective action for the blocked gauge field. The resulting renormalization group flow runs quickly towards an attractive stream which has an approximate line shape. This is numerical evidence of a renormalized trajectory which locates close to the two coupling space. A model flow equation which incorporates a marginal coupling (asymptotic scaling term), an irrelevant coupling and a non-perturbative attraction towards the strong coupling limit reproduces qualitatively the observed features. We further examine the scaling properties of an action which is closer to the attractive stream than the currently used improved actions. It is found that this action shows excellent restoration of rotational symmetry even for coarse lattices with $a \sim 0.3$ fm.
With the smearing technique, the three-quark (3Q) ground-state potential $V_{\rm 3Q}$ is numerically extracted in the SU(3)$_c$ lattice QCD Monte Carlo simulation with $12^3 \times 24$ and $β=5.7$ at the quenched level. With accuracy better than a few %, $V_{\rm 3Q}$ is well described by a sum of a constant $C_{\rm 3Q}$, the two-body Coulomb part $-A_{\rm 3Q}\sum_{i<j} \frac1{|{\bf r}_i-{\bf r}_j|}$ and the three-body linear confinement part $σ_{\rm 3Q} L_{\rm min}$, where $L_{\rm min}$ denotes the minimal length of the color flux tube linking the three quarks. By comparing with the Q-$\bar {\rm Q}$ potential, we find a universal feature of the string tension as $σ_{\rm 3Q} \simeq σ_{\rm Q \bar Q}$ and the one-gluon-exchange result for the Coulomb coefficient as $A_{\rm 3Q} \simeq \frac12 A_{\rm Q \bar Q}$. All our results including the constant term are consistent with the requirement on the diquark limit in the lattice regularization.
The spectra of octet and decuplet baryons are studied using SU(3) lattice QCD at the quenched level. As an implementation to reduce the statistical fluctuation, we employ the anisotropic lattice with $O(a)$ improved quark action. In relation to $Λ(1405)$, we measure also the mass of the SU(3) flavor-singlet negative-parity baryon, which is described as a three quark state in the quenched lattice QCD, and its lowest mass is measured about 1.6 GeV. Since the experimentally observed negative-parity baryon $Λ(1405)$ is much lighter than 1.6 GeV, $Λ(1405)$ may include a large component of a $N \bar K$ bound state rather than the three quark state. The mass splitting between the octet and the decuplet baryons are also discussed in terms of the current quark mass.