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Ken-Ichi Ishikawa

Publications and source records attributed to Ken-Ichi Ishikawa.

At least 55 records · Page 3Linked to original sources
arXiv2018

Nucleon form factors on a large volume lattice near the physical point in 2+1 flavor QCD

We present results for the isovector nucleon form factors measured on a $96^4$ lattice at almost the physical pion mass with a lattice spacing of 0.085 fm in 2+1 flavor QCD. The configurations are generated with the stout-smeared $O(a)$-improved Wilson quark action and the Iwasaki gauge action at $β$=1.82. The pion mass at the simulation point is about 146 MeV. A large spatial volume of $(8.1~{\rm fm})^3$ allows us to investigate the form factors in the small momentum transfer region. We determine the isovector electric radius and magnetic moment from nucleon electric ($G_E$) and magnetic ($G_M$) form factors as well as the axial-vector coupling $g_A$. We also report on the results of the axial-vector ($F_A$), induced pseudoscalar ($F_P$) and pseudoscalar ($G_P$) form factors in order to verify the axial Ward-Takahashi identity in terms of the nucleon matrix elements, which may be called the generalized Goldberger-Treiman relation.

hep-lat↗
arXiv2018

Utility of geometry in lattice QCD simulations

We propose a way to improve the resolution of the spatial momentum and the time interval for hadron propagators utilizing the lattice geometry. We demonstrate the validity of the method presenting results for pseudoscalar meson energies with and without finite momenta in a large-scale quenched QCD simulation. The method should be useful especially for master-field simulations.

hep-lat↗
arXiv2018

Construction of lattice Möbius domain wall fermions in the Schrödinger functional scheme

In this paper we construct the Möbius domain wall fermions (MDWF) in the Schrödinger functional (SF) scheme for the SU(3) gauge theory by adding a boundary operator at the temporal boundary of the SF scheme setup and investigate the property using the perturbation theory. The MDWFs we investigated include the optimal type domain wall, the overlap, the truncated domain wall, and the truncated overlap fermions. We observe the universality of the spectrum of the effective four-dimensional operator at the tree-level. The fermionic contribution to the universal one-loop beta function is reproduced for the MDWFs with a sufficiently large fifth dimensional extent.

hep-lat↗
arXiv2017

Nucleon structure from 2+1 flavor lattice QCD near the physical point

We present an update on our results of nucleon form factors measured on a large-volume lattice $(8.1\rm{fm})^4$ at almost the physical point in 2+1 flavor QCD. The configurations are generated with the stout-smeared $\mathcal{O}(a)$ improved Wilson quark action and Iwasaki gauge action at $β= 1.82$, which corresponds to the lattice spacing of 0.085 fm. The pion mass at the simulation point is about 145 MeV. We determine the iso- vector electric radius and magnetic moment from nucleon electric ($G_E$) and magnetic ($G_M$) form factors. We also report on preliminary results of the axial-vector ($F_A$), induced pseudo-scalar ($F_P$) and pseudo-scalar ($G_P$) form factors in order to verify the axial Ward- Takahashi identity in terms of the nucleon matrix elements, which may be called as the generalized Goldberger-Treiman relation.

hep-lat↗
arXiv2017

Mixed Precision Solver Scalable to 16000 MPI Processes for Lattice Quantum Chromodynamics Simulations on the Oakforest-PACS System

Lattice Quantum Chromodynamics (Lattice QCD) is a quantum field theory on a finite discretized space-time box so as to numerically compute the dynamics of quarks and gluons to explore the nature of subatomic world. Solving the equation of motion of quarks (quark solver) is the most compute-intensive part of the lattice QCD simulations and is one of the legacy HPC applications. We have developed a mixed-precision quark solver for a large Intel Xeon Phi (KNL) system named "Oakforest-PACS", employing the $O(a)$-improved Wilson quarks as the discretized equation of motion. The nested-BiCGSTab algorithm for the solver was implemented and optimized using mixed-precision, communication-computation overlapping with MPI-offloading, SIMD vectorization, and thread stealing techniques. The solver achieved 2.6 PFLOPS in the single-precision part on a $400^3\times 800$ lattice using 16000 MPI processes on 8000 nodes on the system.

physics.comp-ph↗
arXiv2017

Non-perturbative determination of the $Λ$-parameter in the pure SU(3) gauge theory from the twisted gradient flow coupling

We evaluate the $Λ$-parameter in the $\overline{\mathrm{MS}}$ scheme for the pure SU(3) gauge theory with the twisted gradient flow (TGF) method. A running coupling constant $g_{\mathrm{TGF}}^2(1/L)$ is defined in a finite volume box with size of $L^4$ with the twisted boundary condition. This defines the TGF scheme. Using the step scaling method for the TGF coupling with lattice simulations, we can evaluate the $Λ$-parameter non-perturbatively in the TGF scheme. In this paper we determine the dimensionless ratios, $Λ_{\mathrm{TGF}}/\sqrtσ$ and $r_{0}Λ_{\mathrm{TGF}}$ together with the $Λ$-parameter ratio $Λ_{\mathrm{SF}}/Λ_{\mathrm{TGF}}$ on the lattices numerically. Combined with the known ratio $Λ_{\overline{\mathrm{MS}}}/Λ_{\mathrm{SF}}$, we obtain $Λ_{\overline{\mathrm{MS}}}/\sqrtσ = 0.517(10)(^{+8}_{-7})$ and $r_{0}Λ_{\overline{\mathrm{MS}}}=0.593(12)(^{+12}_{-9})$, where the first error is statistical one and the second is our estimate of systematic uncertainty.

hep-lat↗
arXiv2017

Systematic study of operator dependence in nucleus calculation at large quark mass

Recently it is claimed that there is a significant systematic error from excited state contributions in the nucleus correlation functions by comparing with calculations using the exponential and wall source operators. However, the wall source result is obtained in much earlier time than the plateau region. In order to investigate the systematic error in the plateau region, we calculate the correlation functions with both the operators in quenched QCD at 0.8 GeV pion mass and in $N_f=2+1$ QCD at 0.7 GeV pion mass in high accuracy. In this report we present preliminary results of those calculations, and show that the energy shift obtained from the two sources agree with each other, if those are determined from a region, where both the nucleon and two-nucleon correlation functions have plateaus.

hep-lat↗
arXiv2017

The one-loop analysis of the beta-function in the Schroedinger Functional for Moebius Domain Wall Fermions

We proposed a construction of the Schroedinger functional scheme for the Moebius domain wall fermions (MDWF) by introducing a proper boundary operator to the original MDWF in the last conference. The spectrum of the effective four-dimensional operator was investigated. This year we investigate the fermionic contribution to the beta-function with the Moebius domain wall fermion with the SF boundary term up to the one-loop level and find that our construction properly reproduce the one-loop beta-function.

hep-lat↗
arXiv2016

A performance evaluation of CCS QCD Benchmark on the COMA (Intel(R) Xeon Phi$^{TM}$, KNC) system

The most computationally demanding part of Lattice QCD simulations is solving quark propagators. Quark propagators are typically obtained with a linear equation solver utilizing HPC machines. The CCS QCD Benchmark is a benchmark program solving the Wilson-Clover quark propagator, and is developed at the Center for Computational Sciences (CCS), University of Tsukuba. We optimized the benchmark program for a \Intel \XeonPhi (Knights Corner, KNC) system named "COMA (PACS-IX)" at CCS Tsukuba under the Intel Parallel Computing Center program. A single precision BiCGStab solver with the overlapped Restricted Additive Schwarz (RAS) preconditioner was implemented using SIMD intrinsics, OpenMP and MPI in the offload mode. With the reverse-offloading technique, we could reduce the communication and offloading overheads. We observed a performance of $\sim 200$ GFlops sustained for the Wilson-Clover hopping matrix multiplication on the lattice sizes larger than $24^3\times 32$ on a sinlge card of the COMA system. A good weak scaling perofmace was observed on the local lattice sizes larger than $24^3\times 32$.

hep-lat↗
arXiv2014

A construction of the Schrödinger Functional for Möbius Domain Wall Fermions

We construct the Schrödinger Functional (SF) setup for the Möbius domain wall fermions (MDWF). The method is an extension of the method proposed by Takeda for the standard domain wall fermion. In order to fulfill the requirement that the lattice Dirac operator with the SF boundary obeys the Lüscher's universality argument: the lattice chiral fermion with the SF boundary condition breaks the chiral symmetry at the temporal boundary, we impose the parity symmetry with respect to the fifth-direction on the MDWF operator. This additional symmetry restricts the choice of the parameter of the MDWF so that the optimal parameter from the Zolotarev optimal approximation cannot be applied. We introduce a modified parameter set having the fifth-dimensional parity symmetry. We investigate the MDWF with the SF boundary by observing eigenvalues of the Hermitian operator and the Ginsparg-Wilson relation violation at the tree-level. We compare the computational cost with that of the standard DWF with the SF scheme.

hep-lat↗
arXiv2013

Lattice study on chiral dynamics of two-color six-flavors QCD

The electroweak symmetry breaking and origin of masses may be attributed to the breakdown of chiral symmetry due to a strong gauge dynamics. Among several candidates of such gauge systems, we focus on two-color QCD with $N_f=6$ massless Dirac fermions in the fundamental representation, and study on whether the dynamics of this gauge system trigger chiral symmetry breaking or not by simulating with Wilson fermions on lattices up to $L/a=32$. We show the result for the quark mass dependence and the volume dependence of some quantities such as the mass of the lightest pseudoscalar meson, decay constant, and give the three evidences supporting the absence of chiral symmetry breaking in the six-flavor theory.

hep-lat↗
arXiv2013

Testing reweighting method for truncated Overlap fermions

The lattice QCD simulation with the lattice chiral symmetry is very attractive, however, it is difficult to maintain the symmetry at a modest numerical computation cost. A candidate to reduce the computational cost during the configuration generation with the HMC algorithm is to relax the requirement of the chiral symmetry and to use the reweighing method recovering the symmetry at the measurement phase. In this talk, we presented the reweighing method to restore the chiral symmetry of the truncated overlap fermion operator. In order to avoid the large discrepancy between the truncated overlap operator and the exact overlap operator, we split the reweighting factor into several steps gradually increasing the order of truncation. We investigated the truncation dependence of the reweighting factor on a set of quenched $8^3\times 32$ lattice configurations generated with the DBW2 gauge action. We found that a large fluctuation on the reweighting factor between a high-order truncated overlap operator and the exact overlap operator on a couple of configurations. The origin of the large fluctuation seems to be due to a small eigenvalue of the overlap kernel on these configurations.

hep-lat↗
arXiv2013

Multi-nucleon bound states in $N_f=2+1$ lattice QCD

We report on our on-going effort to calculate the properties of light nuclei directly from quarks and gluons based on lattice QCD. After briefly introducing our motivations and aims, we describe our strategy of fixing the strange quark mass at its physical value and approaching the physical point for the up and down quark masses step by step from the region of heavy quark masses. A successful calculation for the pion mass of 0.51GeV is reviewed, and the status for a lighter pion mass of 0.30GeV is reported.

hep-lat↗
arXiv2013

A numerical evaluation of vacuum polarization tensor in constant external magnetic fields

Hattori-Itakura have recently derived the full Landau-level summation form for the photon vacuum polarization tensor in constant external magnetic fields at the one-loop level. The Landau-level summation form is essential when the photon momentum exceeds the threshold of the pair creation of charged particles in a magnetic field stronger than the squared mass of the charged particle. The tensor has three different form factors depending on the tensor direction with respect to the external magnetic field. The renormalization is nontrivial because these form factors are expressed in terms of double or triple summation forms. We give a numerical UV subtraction method which can be applied to numerically evaluate the form factors in constant external magnetic fields. We numerically investigate the photon vacuum polarization tensor in the form of the Landau-level summation and estimate the systematic errors coming from truncation of the Landau-level summation in a parameter region realized in heavy ion collision experiments. We find that the error is practically controllable at an $O(10^{-2})$ level for electrons and muons in strong magnetic fields expected in heavy ion collisions in the experimentally feasible kinematic parameter regions.

hep-ph↗
arXiv2011

Electromagnetic form factor of pion from N_f=2+1 dynamical flavor QCD

We present a calculation of the electromagnetic form factor of the pion in $N_f=2+1$ flavor lattice QCD. Calculations are made on the PACS-CS gauge field configurations generated using Iwasaki gauge action and Wilson-clover quark action on a $32^3\times64$ lattice volume with the lattice spacing estimated as $a=0.0907(13)$ fm at the physical point. Measurements of the form factor are made using the technique of partially twisted boundary condition to reach small momentum transfer as well as periodic boundary condition with integer momenta. Additional improvements including random wall source techniques and a judicious choice of momenta carried by the incoming and outgoing quarks are employed for error reduction. Analyzing the form factor data for the pion mass at $M_π\approx 411$ MeV and 296 MeV, we find that the NNLO SU(2) chiral perturbation theory fit yields $ =0.441 \pm 0.046 {\rm fm}^2$ for the pion charge radius at the physical pion mass. Albeit the error is quite large, this is consistent with the experimental value of $0.452\pm 0.011 {\rm fm}^2$. Below $M_π\approx 300$ MeV, we find that statistical fluctuations in the pion two- and three-point functions become too large to extract statistically meaningful averages on a $32^3$ spatial volume. We carry out a sample calculation on a $64^4$ lattice with the quark masses close to the physical point, which suggests that form factor calculations at the physical point become feasible by enlarging lattice sizes to $M_πL\approx 4$.

hep-lat↗
arXiv2010

Domain Decomposition method on GPU cluster

Pallalel GPGPU computing for lattice QCD simulations has a bottleneck on the GPU to GPU data communication due to the lack of the direct data exchanging facility. In this work we investigate the performance of quark solver using the restricted additive Schwarz (RAS) preconditioner on a low cost GPU cluster. We expect that the RAS preconditioner with appropriate domaindecomposition and task distribution reduces the communication bottleneck. The GPU cluster we constructed is composed of four PC boxes, two GPU cards are attached to each box, and we have eight GPU cards in total. The compute nodes are connected with rather slow but low cost Gigabit-Ethernet. We include the RAS preconditioner in the single-precision part of the mixedprecision nested-BiCGStab algorithm and the single-precision task is distributed to the multiple GPUs. The benchmarking is done with the O(a)-improved Wilson quark on a randomly generated gauge configuration with the size of $32^4$. We observe a factor two improvment on the solver performance with the RAS precoditioner compared to that without the preconditioner and find that the improvment mainly comes from the reduction of the communication bottleneck as we expected.

hep-lat↗
arXiv2008

Recent algorithm and machine developments for lattice QCD

I review recent machine trends and algorithmic developments for dynamical lattice QCD simulations with the HMC algorithm for Wilson-type fermions. The topics include the trend toward multi-core processors and general purpose GPU (GPGPU) computing, and improvements on the quark determinant preconditioning, molecular dynamics integrator, and quark solvers. I also discuss the prospect on the use of these techniques on the forthcoming petaflops machines.

hep-lat↗