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Layers of twisted bilayer graphene exhibit varieties of exotic quantum phenomena1-5. Today, the twist angle {\Theta} has become an important degree of freedom for exploring novel states of matters, i.e. two-dimensional superconductivity ( {\Theta} = 1.1{\deg})6, 7 and a two-dimensional quasicrystal ({\Theta} = 30{\deg})8, 9. We report herein experimental observation on the photo-induced ultrafast dynamics of Dirac fermions in the quasicrystalline 30{\deg} twisted bilayer graphene (QCTBG). We discover that hot carriers are asymmetrically distributed between the two graphene layers, followed by the opposing femtosecond relaxations, by using time- and angle-resolved photoemission spectroscopy. The key mechanism involves the differing carrier transport between layers and the transient doping from the substrate interface. The ultrafast dynamics scheme continues after the Umklapp scattering, which is induced by the incommensurate interlayer stacking of the quasi-crystallinity. The dynamics in the atomic layer opens the possibility of new applications and creates interdisciplinary links in the optoelectronics of van der Waals crystals.

Transient single-particle spectral function of BaFe$_{2}$As$_{2}$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin-density-wave (SDW) ordered state within $\sim$ 1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_{F}$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From the comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim$ 0.60 ps both around the zone center and the zone corner.

We present a lattice QCD study of the phase shift of $I{=}2$ $\pi\pi$ scattering on the basis of two different approaches: the standard finite volume approach by Luscher and the recently introduced HAL QCD potential method. Quenched QCD simulations are performed on lattices with extents $N_s{=}16,24,32,48$ and $N_t{=}128$ as well as lattice spacing $a{\sim}0.115\,\mathrm{fm}$ and a pion mass of $m_\pi{\sim}940\,\mathrm{MeV}$. The phase shift and the scattering length are calculated in these two methods. In the potential method, the error is dominated by the systematic uncertainty associated with the violation of rotational symmetry due to finite lattice spacing. In Luscher's approach, such systematic uncertainty is difficult to be evaluated and thus is not included in this work. A systematic uncertainty attributed to the quenched approximation, however, is not evaluated in both methods. In case of the potential method, the phase shift can be calculated for arbitrary energies below the inelastic threshold. The energy dependence of the phase shift is also obtained from Luscher's method using different volumes and/or nonrest-frame extension of it. The results are found to agree well with the potential method., Comment: Version 2, consistent with published version

We present a first attempt to determine nucleon-nucleon potentials in the parity-odd sector, which appear in 1P1, 3P0, 3P1, 3P2-3F2 channels, in Nf=2 lattice QCD simulations. These potentials are constructed from the Nambu-Bethe-Salpeter wave functions for J^P=0^-, 1^- and 2^-, which correspond to A1^-, T1^- and T2^- + E^- representation of the cubic group, respectively. We have found a large and attractive spin-orbit potential VLS(r) in the isospin-triplet channel, which is qualitatively consistent with the phenomenological determination from the experimental scattering phase shifts. The potentials obtained from lattice QCD are used to calculate the scattering phase shifts in 1P1, 3P0, 3P1 and 3P2-3F2 channels. The strong attractive spin-orbit force and a weak repulsive central force in spin-triplet P-wave channels lead to an attraction in the 3P2 channel, which is related to the P-wave neutron paring in neutron stars., Comment: 14 pages, 5 figures, Physics Letters B published version

The WHOT-QCD Collaboration is pushing forward lattice studies of QCD at finite temperatures and densities using improved Wilson quarks. We first present results on QCD at zero and finite densities with two flavors of degenerate quarks (N_F=2 QCD) adopting the conventional fixed-Nt approach. We then report on the status of a study of N_F=2+1 QCD adopting a fixed-scale approach armed with the T-integration method which we have developed., Comment: 6 pages, 12 figures, invited talk presented at the YITP workshop on "New Frontiers in QCD 2010", March 1-19, 2010, Kyoto, Japan

Screenings of the quark-gluon plasma in electric and magnetic sectors are studied on the basis of generalized Polyakov-line correlation functions in lattice QCD simulations with two flavors of improved Wilson quarks. Using the Euclidean-time reflection ($\R$) and the charge conjugation ($\Ca$), electric and magnetic screening masses are extracted in a gauge invariant manner. Long distance behavior of the standard Polyakov-line correlation in the quark-gluon plasma is found to be dictated by the magnetic screening. Also, ratio of the two screening masses agrees with that obtained from the dimensionally-reduced effective field theory and the ${\cal N}=4$ supersymmetric Yang-Mills theory.

Recently a new approach to calculate the nuclear potential from lattice QCD has been proposed. In the approach the nuclear potential is constructed from Bethe-Salpeter (BS) wave functons through the Schroedinger equation. The procedure leads to non-local but energy independent potential, which can be expanded in terms of local functions. In several recent applications of this method, local potentials, which correspond to the leading order (LO) terms of the expansion, are calculated from the BS wave function at E~0 MeV, where E is the center of mass energy. It is therefore important to check the validity of the LO approximation obtained at E~0. In this report, in order to check how well the LO approximation for the NN potentials works, we compare the LO potentials determined from the BS wave function at E~45 MeV with those at E~0 MeV in quenched QCD. We find that the difference of the LO potentials between two energies are not found wihin the statistical errors. This shows that the LO approximation for the potential is valid at low energies to describe the NN interactions., Comment: 7 pages, talk presented at The XXVII International Symposium on Lattice Field Theory (Lattice2009), July 26-31 2009, Peking University, Beijing, China

We study the KN interactions in the I(J^{\pi})=0(1/2^-) and 1(1/2^-) channels and associated exotic state \Theta^+ from 2+1 flavor full lattice QCD simulation for relatively heavy quark mass corresponding to m_{\pi}=871 MeV. The s-wave KN potentials are obtained from the Bethe-Salpeter wave function by using the method recently developed by HAL QCD (Hadrons to Atomic nuclei from Lattice QCD) Collaboration. Potentials in both channels reveal short range repulsions: Strength of the repulsion is stronger in the I=1 potential, which is consistent with the prediction of the Tomozawa-Weinberg term. The I=0 potential is found to have attractive well at mid range. From these potentials, the $KN$ scattering phase shifts are calculated and compared with the experimental data., Comment: Talk given at 19th International IUPAP Conference on Few-Body Problems in Physics (fb19), Bonn, Germany, 30 Aug - 5 Sep 2009

We report on the status of our study towards the equation of state in 2+1 flavor QCD with improved Wilson quarks. To reduce the computational cost which is quite demanding for Wilson-type quarks, we adopt the fixed scale approach, i.e. the temperature T is varied by N_t at fixed lattice spacing. Since the conventional integral method to obtain the pressure is inapplicable at a fixed scale, we adopt the "T-integral method", to calculate the pressure non-perturbatively. Reduction of the computational cost of T=0 simulations thus achieved is indispensable to study EOS in QCD with dynamical quarks., Comment: 8 pages, poster presented at The XXVII International Symposium on Lattice Field Theory (Lattice 2009), July 26-31 2009, Peking University, Beijing, China

We study the equation of state at finite temperature and density in two-flavor QCD with the RG-improved gluon action and the clover-improved Wilson quark action on a $ 16^3 \times 4$ lattice. Along the lines of constant physics at $m_{\rm PS}/m_{\rm V} = 0.65$ and 0.80, we compute the second and forth derivatives of the grand canonical partition function with respect to the quark chemical potential $\mu_q = (\mu_u+\mu_d)/2$ and the isospin chemical potential $\mu_I = (\mu_u-\mu_d)/2$ at vanishing chemical potentials, and study the behaviors of thermodynamic quantities at finite $\mu_q$ using these derivatives for the case $\mu_I=0$. In particular, we study density fluctuations at none-zero temperature and density by calculating the quark number and isospin susceptibilities and their derivatives with respect to $\mu_q$. To suppress statistical fluctuations, we also examine new techniques applicable at low densities. We find a large enhancement in the fluctuation of quark number when the density increased near the pseudo-critical temperature, suggesting a critical point at finite $\mu_q$ terminating the first order transition line between hadronic and quark gluon plasma phases. This result agrees with the previous results using staggered-type quark actions qualitatively. Furthermore, we study heavy-quark free energies and Debye screening masses at finite density by measuring the first and second derivatives of these quantities for various color channels of heavy quark-quark and quark-anti-quark pairs. The results suggest that, to the leading order of $\mu_q$, the interaction between two quarks becomes stronger at finite densities, while that between quark and anti-quark becomes weaker., Comment: 38 pages, 63 figures

The free energy between a static quark and an antiquark is studied by using the color-singlet Polyakov-line correlation at finite temperature in lattice QCD with 2+1 flavors of improved Wilson quarks. From the simulations on $32^3 \times 12$, 10, 8, 6, 4 lattices in the high temperature phase, based on the fixed scale approach, we find that, the heavy-quark free energies at short distance converge to the heavy-quark potential evaluated from the Wilson loop at zero temperature, in accordance with the expected insensitivity of short distance physics to the temperature. At long distance, the heavy-quark free energies approach to twice the single-quark free energies, implying that the interaction between heavy quarks is screened. The Debye screening mass obtained from the long range behavior of the free energy is compared with the results of thermal perturbation theory., Comment: 4 pages, 3 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee

We propose a fixed scale approach to calculate the equation of state (EOS) in lattice QCD. In this approach, the temperature T is varied by Nt at fixed lattice spacings. This enables us to reduce T=0 simulations which are required to provide basic data in finite temperature studies but are quite expensive in the conventional fixed-Nt approach. Since the conventional integral method to obtain the pressure is inapplicable at fixed scale, we introduce a new method, "T-integration method", to calculate pressure non-perturbatively. We test the fixed scale approach armed with the T-integral method in quenched QCD on isotropic and anisotropic lattices. Our method is found to be powerful to obtain reliable results for the equation of state, especially at intermediate and low temperatures. Reduction of the computational cost of T=0 simulations is indispensable to study EOS in QCD with dynamical quarks. The status of our study in Nf=2+1 QCD with improved Wilson quarks is also reported., Comment: 4 pages, 4 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee. Fonts in the figures magnified

Two of recent progress in lattice QCD approach to nuclear force are reported. (i) Tensor force from quenched lattice QCD: By truncating the derivative expansion of inter-nucleon potential to the strictly local terms, we obtain central force V_C(r) and tensor force V_T(r) separately from s-wave and d-wave components of Bethe-Salpeter wave function for two nucleon state with J^P=1^+. Numerical calculation is performed with quenched QCD on 32^4 lattice using the standard plaquette action at beta=5.7 with the standard Wilson quark action with kappa=0.1640, 0.1665, 0.1678. Preliminary results show that the depths of the resulting tensor force amount to 20 to 40 MeV, which is enhanced in the light quark mass region. (ii) Nuclear force from 2+1 flavor QCD with PACS-CS gauge configuration: Preliminary full QCD results are obtained by using 2+1 flavor gauge configurations generated by PACS-CS collaboration. The resulting potential has the midium range attraction of about 30 MeV similar to the preceding quenched calculations. However, the repulsive core at short distance is significantly stronger than the corresponding quenched QCD result., Comment: A talk given at the XXVI International Symposium on Lattice Field Thoery(LATTICE2008), July 14-19, 2008, Williamsburg, Virginia, USA

We study the $\Lambda$-nucleon ($\Lambda N$) force by using lattice QCD. The Bethe-Salpeter amplitude is calculated for the lowest scattering state of the $\Lambda N$ so as to obtain the $\Lambda N$ potential. The numerical calculation is twofold: (i) Full lattice QCD by using 2+1 flavor PACS-CS gauge configurations with, $\beta=1.9$, corresponding to the lattice spacing of $a=0.0907(13)$ fm, on a $32^3\times 64$ lattice. A set of parameter $(\kappa_{ud},\kappa_s)=(0.13770,0.13640)$ is used, which corresponds to $m_\pi\approx 300$ MeV and $m_K\approx 594$ MeV. The spatial lattice volume corresponds to (2.86 fm)$^3$. (ii) Quenched lattice QCD with $\beta=5.7$, the lattice spacing of $a=0.1416(9)$ fm, on the $32^3\times48$ lattice. Two sets of hopping parameters $(\kappa_{ud},\kappa_s)=(0.1665,0.1643),(0.1670,0.1643)$ are used. The spatial lattice volume is (4.5 fm)$^3$. For the full QCD, we find that the $\Lambda p$ has a relatively strong (weak) repulsive core in the $^1S_0$ ($^3S_1$) channel at short distance, while the potential has slight attractive region at medium distance. The lowest scattering energy in the finite lattice volume is calculated; Slightly negative values obtained in both spin channels. For the quenched QCD, we find that the results are qualitatively in agreement with those in the full QCD calculation., Comment: 7 pages, 4 figures, talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

Screening properties of the quark gluon plasma are studied from Polyakov-loop correlation in lattice QCD simulations with two flavors of improved Wilson quarks at temperatures $T/\Tpc \simeq 1$--4 where $\Tpc$ is the pseudocritical temperature. Using the Euclidean-time reflection symmetry and the charge conjugation symmetry, we introduce various types of Polyakov-loop correlation functions and extract screening masses in magnetic and electric sectors. We find that the temperature dependence of the screening masses are well described by the weak coupling expansion. We also find that a ratio of the screening masses in the electric sector to the magnetic sector shows qualitative agreement with a prediction from the dimensionally-reduced effective field theory and the N=4 supersymmetric Yang-Mills theory at $1.3 < T/\Tpc < 3$., Comment: 7 pages, 5 figures, Talk given at the XXVI International Symposium on Lattice Field Theory (LATTICE 2008), July 14-19, 2008, Williamsburg, Virginia, USA

We study the equation of state in two-flavor QCD at finite temperature and density. Simulations are made with the RG-improved gluon action and the clover-improved Wilson quark action. Along the lines of constant physics for $m_{\rm PS}/m_{\rm V} = 0.65$ and 0.80, we compute the derivatives of the quark determinant with respect to the quark chemical potential $\mu_q$ up to the fourth order at $\mu_q=0$. We adopt several improvement techniques in the evaluation. We study thermodynamic quantities and quark number susceptibilities at finite $\mu_q$ using these derivatives. We find enhancement of the quark number susceptibility at finite $\mu_q$, in accordance with previous observations using staggered-type quarks. This suggests the existence of a nearby critical point., Comment: 7 pages, 16 figures, presented at the XXVI International Symposium on Lattice Field Theory (LATTICE 2008), July 14-19, 2008, Williamsburg, Virginia, USA

We study heavy-heavy and heavy-light quark systems for charm with a relativistic heavy quark action in 2+1 flavor lattice QCD. Configurations are generated by the PACS-CS Collaboration at the lattice spacing is $a=0.09$ fm with the lattice size of $32^3\times 64$ employing the $O(a)$-improved Wilson quark action and the Iwasaki gauge action. We present preliminary results for the charmonium spectrum and the $D$ and $D_s$ meson decay constants evaluated at 3.5 MeV$< m_{\rm ud}<$ 12 MeV with $m_{\rm s}$ around the physical value. We investigate the dynamical quark mass dependences of the hyperfine and the orbital splittings. The decay constants are compared with the recent experimental values., Comment: 7 Pages, 8 figures, talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We present simulation details and results for the light hadron spectrum in N f = 2 + 1 lattice QCD with the nonperturbatively O(a)-improved Wilson quark action and the Iwasaki gauge action. Simulations are carried out at a lattice spacing of 0.09 fm on a (2.9fm)^3 box using the PACS-CS computer. We employ the Luscher's domain-decomposed HMC algorithm with several improvements to reduce the degenerate up-down quark mass toward the physical value. So far the resulting pseudoscalar meson mass is ranging from 702MeV down to 156MeV. We discuss on the stability and the efficiency of the algorithm. The light harden spectrum extrapolated at the physical point is compared with the experimental values. We also present the values of the quark masses and the pseudoscalar meson decay constants., Comment: 7 pages, 15 figures, 1 table talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We investigate the quark mass dependence of meson and baryon masses obtained from 2+1 flavor dynamical quark simulations performed by the PACS-CS Collaboration. With the use of SU(2) and SU(3) chiral perturbation theories up to NLO, we examine the chiral behavior of the pseudoscalar meson masses and the decay constants in terms of the degenerate up-down quark mass ranging form 3 MeV to 24 MeV and two choices of the strange quark mass around the physical value. We discuss the convergence of the SU(2) and SU(3) chiral expansions and present the results for the low energy constants. We find that the SU(3) expansion is not convergent at NLO for the physical strange quark mass. The chiral behavior of the nucleon mass is also discussed based on the SU(2) heavy baryon chiral perturbation theory up to NNLO. Our results show that the expansion is well behaved only up to m_pi^2 ~ 0.2 GeV^2., Comment: 7 Pages, 8 figures, talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA

We calculate potentials between a proton and a $\Xi^0$ (hyperon with strangeness -2) through the equal-time Bethe-Salpeter wave function, employing quenched lattice QCD simulations with the plaquette gauge action and the Wilson quark action on (4.5 fm)^4 lattice at the lattice spacing $a \simeq 0.14$ fm. The ud quark mass in our study corresponds to $m_{\pi}\simeq 0.37$ and 0.51 GeV, while the s quark mass corresponds to the physical value of $m_K$. The central $p \Xi^0$ potential has a strong (weak) repulsive core in the $^1S_0$ ($^3S_1$) channel for $r \lsim 0.6$ fm, while the potential has attractive well at the medium and long distances (0.6 fm $\lsim r \lsim 1.2$ fm) in both channels. The sign of the $p \Xi^0$ scattering length and its quark mass dependence indicate a net attraction in both channels at low energies., Comment: 10 gages, 4 figures, version accepted for publication in Physics Letters B

We find a strong evidence for the survival of $J/\Psi$ and $\eta_c$ as spatially-localized $c\bar c$ (quasi-)bound states above the QCD critical temperature $T_c$, by investigating the boundary-condition dependence of their energies and spectral functions. In a finite-volume box, there arises a boundary-condition dependence for spatially spread states, while no such dependence appears for spatially compact states. In lattice QCD, we find almost {\it no} spatial boundary-condition dependence for the energy of the $c\bar c$ system in $J/\Psi$ and $\eta_c$ channels for $T\simeq(1.11-2.07)T_c$. We also investigate the spectral function of charmonia above $T_c$ in lattice QCD using the maximum entropy method (MEM) in terms of the boundary-condition dependence. There is {\it no} spatial boundary-condition dependence for the low-lying peaks corresponding to $J/\Psi$ and $\eta_c$ around 3GeV at $1.62T_c$. These facts indicate the survival of $J/\Psi$ and $\eta_c$ as compact $c\bar c$ (quasi-)bound states for $T_c < T < 2T_c$., Comment: 4 pages, 1 figure

The nuclear force acting between protons and neutrons is studied in the Monte Carlo simulations of the fundamental theory of the strong interaction, the quantum chromodynamics defined on the hypercubic space-time lattice. After a brief summary of the empirical nucleon-nucleon (NN) potentials which can fit the NN scattering experiments in high precision, we outline the basic formulation to derive the potential between the extended objects such as the nucleons composed of quarks. The equal-time Bethe-Salpeter amplitude is a key ingredient for defining the NN potential on the lattice. We show the results of the numerical simulations on a $32^4$ lattice with the lattice spacing $a \simeq 0.137 $fm (lattice volume (4.4 fm)$^4$) in the quenched approximation. The calculation was carried out using the massively parallel computer Blue Gene/L at KEK. We found that the calculated NN potential at low energy has basic features expected from the empirical NN potentials; attraction at long and medium distances and the repulsive core at short distance. Various future directions along this line of research are also summarized., Comment: 13 pages, 4 figures, version accepted for publication in "Computational Science & Discovery" (IOP)

A new analysis is performed in QCD sum rule for the lightest negative parity baryon Lambda (1405). Mixings of three-quark and five-quark Fock components are taken into account. Terms containing up to dimension 12 condensates are computed in the operator product expansion. It is found that the sum rule gives much stronger coupling of Lambda* to the five-quark operator so that the five-quark components occupy about 90% of Lambda (1405)., Comment: 14pages

The International Lattice Datagrid (ILDG) is a federation of several regional grids. Since most of these grids have reached production level, an increasing number of lattice scientists start to benefit from this new research infrastructure. The ILDG Middleware Working Group has the task of specifying the ILDG middleware such that interoperability among the different grids is achieved. In this paper we will present the architecture of the ILDG middleware and describe what has actually been achieved in recent years. Particular focus is given to interoperability and security issues. We will conclude with a short overview on issues which we plan to address in the near future., Comment: 7 pages, poster presented at Lattice2007, Regensburg, Germany (Algorithms and machines)

We present our updated results of the nucleon-nucleon potential in quenched lattice QCD simulations with the plaquette gauge action and the Wilson quark action on the 32^4(\simeq (4.4 fm)^4) lattice. From the equal-time Bethe-Salpeter (BS) wave function, the NN potential is constructed through the Schroedinger-type equation. Resulting NN potential has all the qualitative features which phenomenological potentials commonly have: the repulsive core at short distance and the attractive well at medium and long distances. In the L \to \infty limit, our NN potential is guaranteed to reproduce the scattering length obtained from the Luescher's formula. The quark mass dependence of the NN potential is studied with m_{pi} \sim 380, 529, 731 MeV. The results suggest that both the repulsive core at short distance and the attractive well at medium distance are enhanced in the light quark mass region., Comment: Talk given at the XXV International Symposium on Lattice Field Theory (LATTICE 2007), Regensburg, Germany, July 30-August 4, 2007

We calculate $p\Xi^0$ potentials from the equal-time Bethe-Salpeter amplitude measured in the quenched QCD simulation with the spatial lattice volume, (4.4 fm)$^3$. The standard Wilson gauge action with the gauge coupling $\beta=5.7$ on $32^4$ lattice together with the standard Wilson quark action are used. The hopping parameter $\kappa_{ud}=0.1678$ is chosen for $u$ and $d$ quarks, which corresponds to $m_{\pi}\simeq 0.37$ GeV. The physical strange quark mass is used by taking the parameter $\kappa_s=0.1643$ which is deduced from the physical $K$ meson mass. The lattice spacing $a=0.1420$ fm is determined by the physical $\rho$ meson mass. We find that the $p\Xi^0$ potential has strong spin dependence. Strong repulsive core is found in $^1S_0$ channel while the effective central potential in the $^3S_1$ channel has relatively weak repulsive core. The potentials also have weak attractive parts in the medium to long distance region (0.6 fm $\lsim r \lsim 1.2$ fm) in both of the $^1S_0$ and $^3S_1$ channels., Comment: UTCCS-P-38, UT-554, TKYNT-07-16, 7pages, talk presented at the XXV International Symposium on Lattice Field Theory (Lattice 2007), 30 Jul - 4 Aug 2007, Regensburg, Germany

We apply chiral perturbation theory to the pseudoscalar meson mass and decay constant data obtained in the PACS-CS Project toward 2+1 flavor lattice QCD simulations with the O(a)-improved Wilson quarks. We examine the existence of chiral logarithms in the quark mass range from m_{ud}=47 MeV down to 6 MeV on a (2.8 fm)^3 box with the lattice spacing a=0.09 fm. Several low energy constants are determined. We also discuss the magnitude of finite size effects based on chiral perturbation theory., Comment: 7 pages, 6 figures, talk presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, Germany

We present preliminary results for the light harden spectrum in $N_f=2+1$ lattice QCD obtained with the nonperturbatively $O(a)$-improved Wilson quark action and the Iwasaki gauge action. Simulations are carried out at $\beta=1.90$ on a $32^3 \times 64$ lattice using the PACS-CS computer. We employ L\"uscher's domain-decomposed HMC algorithm to reduce the up-down quark masses toward the physical value. The pseudoscalar meson masses range from 730 MeV down to 210 MeV. We compare the light harden spectrum extrapolated to the physical point with the experimental values., Comment: 7 pages, 11 figures, talk presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4, 2007, Regensburg, Germany

Thermodynamics of two-flavor QCD at finite temperature and density is studied on a $16^3 \times 4$ lattice, using a renormalization group improved gauge action and the clover improved Wilson quark action. In the simulations along lines of constant $m_{\rm PS}/m_{\rm V}$, we calculate the Taylor expansion coefficients of the heavy-quark free energy with respect to the quark chemical potential ($\mu_q$) up to the second order. By comparing the expansion coefficients of the free energies between quark($Q$)and antiquark($\bar{Q}$), and between $Q$ and $Q$, we find a characteristic difference at finite $\mu_q$ due to the first order coefficient of the Taylor expansion. We also calculate the quark number and isospin susceptibilities, and find that the second order coefficient of the quark number susceptibility shows enhancement around the pseudo-critical temperature., Comment: Talk given at the XXV International Symposium on Lattice Field Theory (Lattice 2007), July 30 - August 4, 2007, Regensburg, Germany

Mixings of 4-quark components in the non-singlet scalar mesons are studied in the QCD sum rules. We propose a formulation to evaluate the cross correlators of q\bar q and qq\bar q \bar q operators and to define the mixings of different Fock states in the sum rule. It is applied to the non-singlet scalar mesons, a_0 and K_0^\ast. It is found that the 4-quark operators predict lower masses than the q\bar q operators and that the 4-quark states occupy about 70-90% of the lowest mass states., Comment: 8 pages, 9 figures

We report the current status of our systematic studies of the QCD thermodynamics by lattice QCD simulations with two flavors of improved Wilson quarks. We evaluate the critical temperature of two flavor QCD in the chiral limit at zero chemical potential and show the preliminary result. Also we discuss fluctuations at none-zero temperature and density by calculating the quark number and isospin susceptibilities and their derivatives with respect to chemical potential., Comment: Talk given at The 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2006), Shanghai, China, 14-20 Nov 2006

We study Polyakov loop correlations and spatial Wilson loop at finite Temperature in two-flavor QCD simulations with the RG-improved gluon action and the clover-improved Wilson quark action on a $ 16^3 \times 4$ lattice. From the line of constant physics at $m_{\rm PS}/m_{\rm V}=0.65$ and 0.80, we extract the heavy-quark free energies, the effective running coupling $g_{\rm eff}(T)$ and the Debye screening mass $m_D(T)$ for various color channels of heavy quark--quark and quark--anti-quark pairs above the critical temperature. The free energies are well approximated by the screened Coulomb form with the appropriate Casimir factors at high temperature. The magnitude and the temperature dependence of the Debye mass are compared to those of the next-to-leading order thermal perturbation theory and to a phenomenological formula in terms of $g_{\rm eff}(T)$. We make a comparison between our results with the Wilson quark action and the previous results with the staggered quark action. The spatial string tension is also studied in the high temperature phase and is compared to the next-to-next-leading order prediction in an effective theory with dimensional reduction., Comment: 25 pages, 37 EPS figures

Nucleon-nucleon (NN) potential is studied by lattice QCD simulations in the quenched approximation, using the plaquette gauge action and the Wilson quark action on a 32^4 (\simeq (4.4 fm)^4) lattice. A NN potential V_{NN}(r) is defined from the equal-time Bethe-Salpeter amplitude with a local interpolating operator for the nucleon. By studying the NN interaction in the ^1S_0 and ^3S_1 channels, we show that the central part of V_{NN}(r) has a strong repulsive core of a few hundred MeV at short distances (r \alt 0.5 fm) surrounded by an attractive well at medium and long distances. These features are consistent with the known phenomenological features of the nuclear force., Comment: Physical Review Letters accepted version. 4 pages, 4 figures

We study the phase structure of QCD at finite temperatures with two flavors of dynamical quarks on a lattice with the size $N_s^3 \times N_t=16^3 \times 4$, using a renormalization group improved gauge action and a clover improved Wilson quark action. The simulations are made along the lines of constant physics determined in terms of $m_{\rm PS}/m_{\rm V}$ at zero-temperature. We show preliminary results for the spatial string tension in the high temperature phase., Comment: 7 pages, 7 figures, talk presented at Lattice 2006 (high temperature and density)

Polyakov loop correlations at finite temperature in two-flavor QCD are studied in lattice simulations with the RG-improved gluon action and the clover-improved Wilson quark action. From the simulations on a $16^3 \times 4$ lattice, we extract the free energies, the effective running coupling $g_{\rm eff}(T)$ and the Debye screening mass $m_D(T)$ for various color channels of heavy quark--quark and quark--anti-quark pairs above the critical temperature. The free energies are well approximated by the screened Coulomb form with the appropriate Casimir factors. The magnitude and the temperature dependence of the Debye mass are compared to those of the next-to-leading order thermal perturbation theory and to a phenomenological formula given in terms of $g_{\rm eff}(T)$. Also we made a comparison between our results with the Wilson quark and those with the staggered quark previously reported., Comment: 7 pages, 9 figures, talk given at Lattice 2006 (high temperature and density)

The QCD thermodynamics on the lattice provides fundamental theoretical grounds to analyze the various experimental data in relativistic heavy ion collisions. So far, most of the numerical simulations on the lattice have been performed by using the staggered-type fermion actions. Therefore it is important to carry out studies using different fermion formulations to test the uncertainties of the lattice QCD results. For this purpose, we perform systematic simulations of two-flavor QCD with an improved Wilson quark action to investigate the equation of state. We report the current status of our project and show the preliminary results of the Taylor expansion coefficients of the thermodynamic grand partition function in terms of chemical potential., Comment: 7 pages, 6 figures, poster at Lattice 2006 (high temperature and density)

J/Psi and eta_c above the QCD critical temperature T_c are studied in anisotropic quenched lattice QCD, considering whether the c\bar c systems above T_c are spatially compact (quasi-)bound states or scattering states. We adopt the standard Wilson gauge action and O(a)-improved Wilson quark action with renormalized anisotropy a_s/a_t =4.0 at \beta=6.10 on 16^3\times (14-26) lattices, which correspond to the spatial lattice volume V\equiv L^3\simeq(1.55{\rm fm})^3 and temperatures T\simeq(1.11-2.07)T_c. We investigate the c\bar c system above T_c from the temporal correlators with spatially-extended operators, where the overlap with the ground state is enhanced. To clarify whether compact charmonia survive in the deconfinement phase, we investigate spatial boundary-condition dependence of the energy of c\bar c systems above T_c. In fact, for low-lying S-wave c \bar c scattering states, it is expected that there appears a significant energy difference \Delta E \equiv E{\rm (APBC)}-E{\rm (PBC)}\simeq2\sqrt{m_c^2+3\pi^2/L^2}-2m_c (m_c: charm quark mass) between periodic and anti-periodic boundary conditions on the finite-volume lattice. In contrast, for compact charmonia, there is no significant energy difference between periodic and anti-periodic boundary conditions. As a lattice QCD result, almost no spatial boundary-condition dependence is observed for the energy of the c\bar c system in J/\Psi and \eta_c channels for T\simeq(1.11-2.07)T_c. This fact indicates that J/\Psi and \eta_c would survive as spatially compact c\bar c (quasi-)bound states below 2T_c. We also investigate a $P$-wave channel at high temperature with maximally entropy method (MEM) and find no low-lying peak structure corresponding to \chi_{c1} at 1.62T_c., Comment: 13 pages, 11 figures

Anisotropic lattice QCD studies of penta-quarks(5Q) with J^P=1/2^\pm and 3/2^{\pm} are presented at the quenched level together with tetra-quarks(4Q). The standard gauge action at \beta=5.75 and O(a) improved quark (clover) action with \kappa=0.1410(0.010)0.1440 are employed on the anisotropic lattice with the renormalized anisotropy a_s/a_t = 4. The ``hybrid boundary condition(HBC)'' is adopted to discriminate a compact resonance state from scattering states. Only massive 5Q states are found for J^P=1/2^+ and 3/2^{\pm}, which cannot be identified as \Theta^+(1540). A low-lying 5Q state is found for J^P=1/2^- at m_{5Q}\simeq 1.75 GeV, which however turns out to be an NK scattering state through the HBC analysis. A preliminary result for 4Q states is presented suggesting an existence of a compact 4Q resonance at m_{4Q} \simeq 1.1 GeV in the idealized SU(4)_f chiral limit., Comment: A talk given at Particles and Nuclei International Conference(PANIC05), Santa Fe, NM, Oct. 24-28, 2005. 3 pages, 2 figures

Motivated by the recent experimental discoveries of multi-quark candidates, e.g., the $\Theta^+(1540)$, we study multi-quark systems in lattice QCD. First, we perform accurate mass measurements of low-lying 5Q states with $J=1/2$ and I=0 in both positive- and negative-parity channels in anisotropic lattice QCD. The lowest positive-parity 5Q state is found to have a large mass of about 2.24GeV after the chiral extrapolation. To single out the compact 5Q state from $NK$ scattering states, we develop a new method with the hybrid-boundary condition (HBC), and find no evidence of the compact 5Q state below 1.75GeV in the negative-parity channel. Second, we perform the first study of the multi-quark potential in lattice QCD to clarify the inter-quark interaction in multi-quark systems. The 5Q potential $V_{\rm 5Q}$ for the QQ-${\rm \bar{Q}}$-QQ system is found to be well described by the ``OGE Coulomb plus multi-Y Ansatz": the sum of the one-gluon-exchange (OGE) Coulomb term and the multi-Y-type linear term based on the flux-tube picture. The 4Q potential $V_{\rm 4Q}$ for the QQ-${\rm \bar{Q}\bar{Q}}$ system is also described by the OGE Coulomb plus multi-Y Ansatz, when QQ and $\rm \bar Q \bar Q$ are well separated. The 4Q system is described as a "two-meson" state with disconnected flux tubes, when the nearest quark and antiquark pair is spatially close. We observe a lattice-QCD evidence for the ``flip-flop'', i.e., the flux-tube recombination between the connected 4Q state and the ``two-meson'' state. On the confinement mechanism, the lattice QCD results indicate the flux-tube-type linear confinement in multi-quark hadrons., Comment: 22 pages, 3 tables, 16 figures. Talk given at International Workshop on Quark Nuclear Physics 2005 (QNP05), Phoenix Park, Korea, 22-24, Feb., 2005

A high-precision mass measurement for the pentaquark (5Q) Theta^+ in J^P=3/2^{\pm} channel is performed in anisotropic quenched lattice QCD using a large number of gauge configurations as N_{conf}=1000. We employ the standard Wilson gauge action at beta=5.75 and the O(a) improved Wilson (clover) quark action with kappa=0.1210(0.0010)0.1240 on a 12^3 \times 96 lattice with the renormalized anisotropy as a_s/a_t = 4. The Rarita-Schwinger formalism is adopted for the interpolating fields. Several types of the interpolating fields with isospin I=0 are examined such as (a) the NK^*-type, (b) the (color-)twisted NK^*-type, (c) a diquark-type. The chiral extrapolation leads to only massive states, i.e., m_{5Q} \simeq 2.1-2.2 GeV in J^P=3/2^- channel, and m_{5Q} = 2.4-2.6 GeV in J^P=3/2^+ channel. The analysis with the hybrid boundary condition(HBC) is performed to investigate whether these states are compact 5Q resonances or not. No low-lying compact 5Q resonance states are found below 2.1GeV., Comment: 15 pages, 6 figures, 4 tables

Penta-quark (5Q) baryons are studied using anisotropic lattice QCD for high-precision measurement of temporal correlators. A non-NK-type interpolating field is employed to study the 5Q states with J^P=1/2^{\pm} and I=0. In J^P=1/2^+ channel, the lowest-lying state is found at m_{5Q} \simeq 2.25 GeV, which is too massive to be identified as the Theta^+(1540). In J^P=1/2^- channel, the lowest-lying state is found at m_{5Q} \simeq 1.75 GeV. To distinguish a compact 5Q resonance state from an NK scattering state, a new method with ``hybrid boundary condition (HBC)'' is proposed. As a result of the HBC analysis, the observed state in the negative-parity channel turns out to be an $NK$ scattering state., Comment: A talk presented at 10th International Conference BARYONS04, Ecole Polytechnique, Palaiseau, France, 25-29, Oct 2004; A single sentence is added in Sect.1