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K-I. Ishikawa

Publications and source records attributed to K-I. Ishikawa.

At least 19 recordsLinked to original sources
arXiv2015

Study of the conformal region of the SU(3) gauge theory with domain-wall fermions

We investigate the phase structure of the SU(3) gauge theory with $N_f=8$ by numerical simulations employing the massless Domain-Wall fermions.Our aim is to study directly the massless quark region, since it is the most important region to clarify the properties of conformal theories. When the number of flavor is within the conformal window, it is claimed recently with Wilson quarks that there is the conformal region at the small quark mass region in the parameter space in addition to the confining phase and the deconfining phase. We study the properties of the conformal region investing the spatial Polyakov loops and the temporal meson propagators. Our data imply that there is the conformal region, and a phase transition between the confining phase and the conformal region takes place. These results are consistent with the claim that the conformal window is between $7$ and $16$. Progress reports on other related studies are also presented.

hep-lat↗
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.

hep-lat↗
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.

hep-lat↗
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.

hep-lat↗
arXiv2006

Mobility edge and locality of the overlap-Dirac operator with and without dynamical overlap fermions

We perform a systematic study of low-lying eigenmodes of $H_w$ with various gauge actions to find the optimal choice for dynamical overlap fermion simulations, with which one may achieve lower numerical cost for HMC and better locality property of the overlap kernel. For this purpose, our study is made with emphasis on the distribution of low-lying eigenvalues and the mobility edge with and without dynamical overlap fermions.

hep-lat↗
arXiv2006

An application of the UV-filtering preconditioner to the Polynomial Hybrid Monte Carlo algorithm

We apply the UV-filtering preconditioner, previously used to improve the Multi-Boson algorithm, to the Polynomial Hybrid Monte Carlo (UV-PHMC) algorithm. The performance test for the algorithm is given for the plaquette gauge action and the $O(a)$-improved Wilson action at $β=5.2, c_{\mathrm{sw}}=2.02, M_π/M_ρ\sim 0.8$ and 0.7 on a $16^3\times 48$ lattice. We find that the UV-filtering reduces the magnitude of the molecular dynamics force from the pseudo fermion by a factor 3 by tuning the UV-filter parameter. Combining with the multi-time scale molecular dynamics integrator we achieve a factor 2 improvement.

hep-lat↗
arXiv2006

JLQCD's dynamical overlap project

We present an overview of our project of large-scale simulations with dynamical overlap fermions. The first production run in two-flavor QCD is on-going using the Iwasaki gauge action on a 16^3 \times 32 lattice at the lattice spacing of 0.12fm with six sea quark masses down to m_{s,phys}/6, where m_{s,phys} is the physical strange quark mass. We briefly introduce our choice of the lattice action and simulation algorithm, and describe the present status of the production run. Preliminary results on the light meson masses and the static quark potential are also reported.

hep-lat↗
arXiv2006

Nonperturbative $O(a)$ improvement of the Wilson quark action with the RG-improved gauge action using the Schrödinger functional method

We perform a nonperturbative determination of the $O(a)$-improvement coefficient $c_{\rm SW}$ and the critical hopping parameter $κ_c$ for $N_f$=3, 2, 0 flavor QCD with the RG-improved gauge action using the Schrödinger functional method. In order to interpolate $c_{\rm SW}$ and $κ_c$ as a function of the bare coupling, a wide range of $β$ from the weak coupling region to the moderately strong coupling points used in large-scale simulations is studied. Corrections at finite lattice size of $O(a/L)$ turned out to be large for the RG-improved gauge action, and hence we make the determination at a size fixed in physical units using a modified improvement condition. This enables us to avoid $O(a)$ scaling violations which would remain in physical observables if $c_{\rm SW}$ determined for a fixed lattice size $L/a$ is used in numerical simulations.

hep-lat↗
arXiv2005

Kaon semileptonic decay form factors in two-flavor QCD

We present a calculation of the kaon form factors in two-flavor QCD with the non-perturbatively $O(a)$-improved Wilson quark action. In order to achieve a few percent accuracy in the study of SU(3) breaking effects, we use a set of double ratios of the matrix elements, with which the bulk of the statistical fluctuation and the multiplicative renormalization factors cancel.

hep-lat↗
arXiv2005

Non-perturbative O(a)-improvement of Wilson quark action in three-flavor QCD with plaquette gauge action

We perform a non-perturbative determination of the O(a)-improvement coefficient c_SW for the Wilson quark action in three-flavor QCD with the plaquette gauge action. Numerical simulations are carried out in a range of β=12.0-5.2 on a single lattice size of 8^3x16 employing the Schrödinger functional setup of lattice QCD. As our main result, we obtain an interpolation formula for c_SW and the critical hopping parameter K_c as a function of the bare coupling. This enables us to remove O(a) scaling violation from physical observables in future numerical simulation in the wide range of β. Our analysis with a perturbatively modified improvement condition for c_SW suggests that finite volume effects in c_SW are not large on the 8^3x16 lattice. We investigate N_f dependence of c_SW by additional simulations for N_f=4, 2 and 0 at β=9.6. As a preparatory step for this study, we also determine c_SW in two-flavor QCD at β=5.2. At this β, several groups carried out large-scale calculations of the hadron spectrum, while no systematic determination of c_SW has been performed.

hep-lat↗
arXiv2005

I=2 Pion Scattering Length from Two-Pion Wave Functions

We calculate the two-pion wave function in the ground state of the I=2 $S$-wave system and find the interaction range between two pions, which allows us to examine the validity of the necessary condition for the finite-volume method for the scattering length proposed by Lüscher. We work in the quenched approximation employing a renormalization group improved gauge action for gluons and an improved Wilson action for quarks at $1/a=1.207(12) {\rm GeV}$ on $16^3 \times 80$, $20^3 \times 80$ and $24^3 \times 80$ lattices. We conclude that the necessary condition is satisfied within the statistical errors for the lattice sizes $L\ge 24$ ($3.92 {\rm fm}$) when the quark mass is in the range that corresponds to $m_π^2 = 0.273-0.736 {\rm GeV}^2$. We obtain the scattering length with a smaller statistical error from the wave function than from the two-pion time correlator.

hep-lat↗
arXiv2004

Light hadron spectrum in 2+1 flavor full QCD by CP-PACS and JLQCD Collaborations

CP-PACS and JLQCD Collaborations are carrying out a joint project of the 2+1 flavor full QCD with the RG-improved gauge action and the non-perturbatively ${\cal O}(a)$-improved Wilson quark action. This simulation removes quenching effects of all three light quarks, which is the last major uncertainty in lattice QCD. In this report we present our results for the light meson spectrum and quark masses on a $20^3\times 40$ lattice at the lattice spacing $a\simeq 0.10$ fm.

hep-lat↗
arXiv2004

A scaling study of the step scaling function of quenched QCD with improved gauge actions

We study the scaling behavior of the step scaling function for SU(3) gauge theory, employing the Iwasaki gauge action and the Luescher-Weisz gauge action. In particular, we test the choice of boundary counter terms and apply a perturbative procedure for removal of lattice artifacts for the simulation results in the extrapolation procedure. We confirm the universality of the step scaling functions at both weak and strong coupling regions. We also measure the low energy scale ratio with the Iwasaki action, and confirm its universality.

hep-lat↗
arXiv2004

Charmed meson spectra and decay constants with one-loop $O(a)$ improved relativistic heavy quark action

We calculate charmed meson spectra and decay constants in lattice QCD employing one-loop $O(a)$ improved heavy quark action and axial-vector currents. In quenched simulations at $a \sim 0.1$ fm with the plaquette gauge action as well as a renormalization-group improved one, it is shown that the deviation from the continuum dispersion relation and the violation of space-time symmetry for the pseudoscalar meson decay constants are substantially reduced, once the $O(a)$ improvement is applied. Preliminary results with two flavors of dynamical quarks are also presented.

hep-lat↗
arXiv2004

I=2 Pion Scattering Length from Two-Pion Wave Function

We present a report on a calculation of scattering length for I=2 $S$-wave two-pion system from two-pion wave function. Calculations are made with an RG-improved action for gluons and improved Wilson action for quarks at $a^{-1}=1.207(12) {\rm GeV}$ on $16^3 \times 80$, $20^3 \times 80$ and $24^3 \times 80$ lattices. We investigate the validity of necessary condition for application of Lüscher's formula through the wave function. We find that the condition is satisfied for lattice volumes $L\ge 3.92 {\rm fm}$ for the quark mass range $m_π^2 = 0.273-0.736 {\rm GeV}^2$. We also find that the scattering length can be extracted with a smaller statistical error from the wave function than with a time correlation function used in previous studies.

hep-lat↗
arXiv2004

Light hadron spectroscopy in two-flavor QCD with small sea quark masses

We extend the study of the light hadron spectrum and the quark mass in two-flavor QCD to smaller sea quark mass, corresponding to $m_{PS}/m_{V}=0.60$--0.35. Numerical simulations are carried out using the RG-improved gauge action and the meanfield-improved clover quark action at $β=1.8$ ($a = 0.2$ fm from $ρ$ meson mass). We observe that the light hadron spectrum for small sea quark mass does not follow the expectation from chiral extrapolations with quadratic functions made from the region of $m_{PS}/m_{V}=0.80$--0.55. Whereas fits with either polynomial or continuum chiral perturbation theory (ChPT) fails, the Wilson ChPT (WChPT) that includes $a^2$ effects associated with explicit chiral symmetry breaking successfully fits the whole data: In particular, WChPT correctly predicts the light quark mass spectrum from simulations for medium heavy quark mass, such as $m_{PS}/m_V \simgt 0.5$. Reanalyzing the previous data %at $m_{PS}/m_{V}=0.80$--0.55 with the use of WChPT, we find the mean up and down quark mass being smaller than the previous result from quadratic chiral extrapolation by approximately 10%, $m_{ud}^{\bar{\rm MS}}(μ=2 {GeV}) = 3.11(17)$ [MeV] in the continuum limit.

hep-lat↗
arXiv2004

Chiral extrapolations with small sea quark mass data in two-flavor lattice QCD

We present results on the light hadron spectrum and quark mass in two-flavor QCD calculated with small sea quark masses down to $m_{PS}/m_{V}=0.35$. The configurations are generated using the RG improved gauge and tadpole-improved clover quark action at $β=1.8$, where $a^{-1} \simeq 1$ GeV. Chiral extrapolations are made using not only polynomials and ChPT in the continuum but also formulae of Wilson chiral perturbation theory (WChPT) including $O(a^2)$ chiral breaking terms. We examine the viability of WChPT and its influence on quark masses.

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