arXiv subjects

K. Kanaya

Publications and source records attributed to K. Kanaya.

At least 55 records · Page 3Linked to original sources

arXiv2008

Equation of state at finite density in two-flavor QCD with improved Wilson quarks

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 $μ_q$ up to the fourth order at $μ_q=0$. We adopt several improvement techniques in the evaluation. We study thermodynamic quantities and quark number susceptibilities at finite $μ_q$ using these derivatives. We find enhancement of the quark number susceptibility at finite $μ_q$, in accordance with previous observations using staggered-type quarks. This suggests the existence of a nearby critical point.

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arXiv2008

Charm quark system in 2+1 flavor lattice QCD using the PACS-CS configurations

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.

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arXiv2008

Thermodynamics of SU(3) gauge theory at fixed lattice spacing

We study thermodynamics of SU(3) gauge theory at fixed scales on the lattice, where we vary temperature by changing the temporal lattice size N_t=(Ta_t)^{-1}. In the fixed scale approach, finite temperature simulations are performed on common lattice spacings and spatial volumes. Consequently, we can isolate thermal effects in observables from other uncertainties, such as lattice artifact, renormalization factor, and spatial volume effect. Furthermore, in the EOS calculations, the fixed scale approach is able to reduce computational costs for zero temperature subtraction and parameter search to find lines of constant physics, which are demanding in full QCD simulations. As a test of the approach, we study the thermodynamics of the SU(3) gauge theory on isotropic and anisotropic lattices. In addition to the equation of state, we calculate the critical temperature and the static quark free energy at a fixed scale.

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arXiv2008

Charmonium dissociation temperatures in lattice QCD with a finite volume technique

Dissociation temperatures of J/ψ, ψ', and χ_c states play key roles in the sequential J/ψsuppression scenario for high energy heavy ion collisions. We report on a study of charmonium dissociation temperatures in quenched lattice QCD. On anisotropic lattices, we first subtract the effects of the constant mode in finite temperature meson correlators, which have lead to unphysical results in previous studies. We then extract ground and first exited state masses by diagonalizing correlation functions among different source and sink operators. To distinguish bound states from scattering states, we first compare the charmonium mass spectra under different spatial boundary conditions, and examine the shape and the volume-dependence of their Bethe-Salpeter wave functions. From these studies, we found so far no sign of scattering states up to about 2.3T_c.

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arXiv2008

2+1 flavor lattice QCD simulation with $O(a)$-improved Wilson quarks

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.

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arXiv2008

SU(2) and SU(3) chiral perturbation theory analyses on meson and baryon masses in 2+1 flavor lattice QCD

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.

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arXiv2008

Two-flavor QCD simulation with exact chiral symmetry

We perform numerical simulations of lattice QCD with two flavors of dynamical overlap quarks, which have exact chiral symmetry on the lattice. While this fermion discretization is computationally demanding, we demonstrate the feasibility to simulate reasonably large and fine lattices by a careful choice of the lattice action and algorithmic improvements. Our production runs are carried out on a 16^3 \times 32 lattice at a single lattice spacing around 0.12 fm. We explore the sea quark mass region down to m_s/6, where m_s is the physical strange quark mass, for a good control of the chiral extrapolation in future calculations of physical observables. We describe in detail our setup and algorithmic properties of the production simulations and present results for the static quark potential to fix the lattice scale and the locality of the overlap operator.

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arXiv2008

Light quark masses from unquenched lattice QCD

We calculate the light meson spectrum and the light quark masses by lattice QCD simulation, treating all light quarks dynamically and employing the Iwasaki gluon action and the nonperturbatively O(a)-improved Wilson quark action. The calculations are made at the squared lattice spacings at an equal distance a^2~0.005, 0.01 and 0.015 fm^2, and the continuum limit is taken assuming an O(a^2) discretization error. The light meson spectrum is consistent with experiment. The up, down and strange quark masses in the \bar{MS} scheme at 2 GeV are \bar{m}=(m_{u}+m_{d})/2=3.55^{+0.65}_{-0.28} MeV and m_s=90.1^{+17.2}_{-6.1} MeV where the error includes statistical and all systematic errors added in quadrature. These values contain the previous estimates obtained with the dynamical u and d quarks within the error.

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arXiv2008

2+1 Flavor Lattice QCD toward the Physical Point

We present the first results of the PACS-CS project which aims to simulate 2+1 flavor lattice QCD on the physical point with the nonperturbatively $O(a)$-improved Wilson quark action and the Iwasaki gauge action. Numerical simulations are carried out at the lattice spacing of $a=0.0907(13)$fm on a $32^3\times 64$ lattice with the use of the DDHMC algorithm to reduce the up-down quark mass. Further algorithmic improvements make possible the simulation whose ud quark mass is as light as the physical value. The resulting PS meson masses range from 702MeV down to 156MeV, which clearly exhibit the presence of chiral logarithms. An analysis of the PS meson sector with SU(3) ChPT reveals that the NLO corrections are large at the physical strange quark mass. In order to estimate the physical ud quark mass, we employ the SU(2) chiral analysis expanding the strange quark contributions analytically around the physical strange quark mass. The SU(2) LECs ${\bar l}_3$ and ${\bar l}_4$ are comparable with the recent estimates by other lattice QCD calculations. We determine the physical point together with the lattice spacing employing $m_π$, $m_K$ and $m_Ω$ as input. The hadron spectrum extrapolated to the physical point shows an agreement with the experimental values at a few % level of statistical errors, albeit there remain possible cutoff effects. We also find that our results of $f_π=134.0(4.2)$MeV, $f_K=159.4(3.1)$MeV and $f_K/f_π=1.189(20)$ with the perturbative renormalization factors are compatible with the experimental values. For the physical quark masses we obtain $m_{\rm ud}^\msbar=2.527(47)$MeV and $m_{\rm s}^\msbar=72.72(78)$MeV extracted from the axial-vector Ward-Takahashi identity with the perturbative renormalization factors.

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arXiv2007

Light hadron spectrum with 2+1 flavor dynamical $O(a)$-improved Wilson quarks

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 $β=1.90$ on a $32^3 \times 64$ lattice using the PACS-CS computer. We employ Lüscher'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.

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arXiv2007

Application of chiral perturbation theory to 2+1 flavor lattice QCD with O(a)-improved Wilson quarks

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.

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arXiv2007

Thermodynamics and heavy-quark free energies at finite temperature and density with two flavors of improved Wilson quarks

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 ($μ_q$) 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 $μ_q$ 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.

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arXiv2007

Lattice QCD Calculation of the $ρ$ Meson Decay Width

We present a lattice QCD calculation of the $ρ$ meson decay width via the $P$-wave scattering phase shift for the I=1 two-pion system. Our calculation uses full QCD gauge configurations for $N_f=2$ flavors generated using a renormalization group improved gauge action and an improved Wilson fermion action on a $12^3\times24$ lattice at $m_π/m_ρ=0.41$ and the lattice spacing $1/a=0.92 {\rm GeV}$. The phase shift calculated with the use of the finite size formula for the two-pion system in the moving frame shows a behavior consistent with the existence of a resonance at a mass close to the vector meson mass obtained in spectroscopy. The decay width estimated from the phase shift is consistent with the experiment, when the quark mass is scaled to the realistic value.

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arXiv2007

2+1 flavor light hadron spectrum and quark masses with the O(a) improved Wilson-clover quark formalism

We present a summary of results of the joint CP-PACS and JLQCD project toward a 2+1 flavor full QCD simulation with the O(a)-improved Wilson quark formalism and the Iwasaki gauge action. Configurations were generated during 2002-2005 at three lattice spacings, a~0.076, 0.100 and 0.122 fm, keeping the physical volume constant at (2.0fm)^3. Up and down quark masses are taken in the range m_{PS}/m_V~0.6-0.78. We have completed the analysis for the light meson spectrum and quark masses in the continuum limit using the full configuration set. The predicted meson masses reproduce experimental values in the continuum limit at a 1% level. The average up and down, and strange quark masses turn out to be m_{ud}^{\bar{MS}}(μ=2 GeV)=3.50(14)({}^{+26}_{-15}) MeV and m_s^{\bar{MS}}(μ=2 GeV)=91.8(3.9)({}^{+6.8}_{-4.1}) MeV. We discuss our future strategy toward definitive results on hadron spectroscopy with the Wilson-clover formalism.

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arXiv2007

Neutron electric dipole moment with external electric field method in lattice QCD

We discuss a possibility that the Neutron Electric Dipole Moment (NEDM) can be calculated in lattice QCD simulations in the presence of the CP violating $θ$ term. In this paper we measure the energy difference between spin-up and spin-down states of the neutron in the presence of an uniform and static external electric field. We first test this method in quenched QCD with the RG improved gauge action on a $16^3\times 32$ lattice at $a^{-1}\simeq$ 2 GeV, employing two different lattice fermion formulations, the domain-wall fermion and the clover fermion for quarks, at relatively heavy quark mass $(m_{PS}/m_V \simeq 0.85)$. We obtain non-zero values of NEDM from calculations with both fermion formulations. We next consider some systematic uncertainties of our method for NEDM, using $24^3\times 32$ lattice at the same lattice spacing only with the clover fermion. We finally investigate the quark mass dependence of NEDM and observe a non-vanishing behavior of NEDM toward the chiral limit. We interpret this behavior as a manifestation of the pathology in the quenched approximation.

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arXiv2007

Heavy-Quark Free Energy, Debye Mass, and Spatial String Tension at Finite Temperature in Two Flavor Lattice QCD with Wilson Quark Action

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.

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arXiv2007

First Nonperturbative Test of a Relativistic Heavy Quark Action in Quenched Lattice QCD

We perform a numerical test of a relativistic heavy quark(RHQ) action, recently proposed by Tsukuba group, in quenched lattice QCD at $a\simeq 0.1$ fm. With the use of the improvement parameters previously determined at one-loop level for the RHQ action, we investigate a restoration of rotational symmetry for heavy-heavy and heavy-light meson systems around the charm quark mass. We focused on two quantities, the meson dispersion relation and the pseudo-scalar meson decay constants. It is shown that the RHQ action significantly reduces the discretization errors due to the charm quark mass. We also calculate the S-state hyperfine splittings for the charmonium and charmed-strange mesons and the $D_s$ meson decay constant. The remaining discretization errors in the physical quantities are discussed.

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arXiv2007

Thermodynamics of two-flavor lattice QCD with an improved Wilson quark action at non-zero temperature and density

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.

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