Publications (305)613.17 Total impact

Article: Topology density correlator on dynamical domainwall ensembles with nearly frozen topological charge
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ABSTRACT: Global topological charge decorrelates very slowly or even freezes in fine lattice simulations. On the other hand, its local fluctuations are expected to survive and lead to the correct physical results as long as the volume is large enough. We investigate this issue on recently generated configurations including dynamical domainwall fermions at lattice spacings a = 0.08 fm and finer. We utilize the YangMills gradient flow to define the topological charge density operator and calculate its longdistance correlation, through which we propose a new method for extracting the topological susceptibility in a subvolume. This method takes care of the finite volume correction, which reduces the bias caused by the global topological charge. Our lattice data clearly show a shorter autocorrelation time than that of the naive definition using the whole lattice, and are less sensitive to the global topological history. Numerical results show a clear seaquark mass dependence, which agrees well with the prediction of chiral perturbation theory. 
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ABSTRACT: In the Monte Carlo study of QCD at finite baryon density based upon the phase reweighting method, the pion condensation in the phasequenched theory and associated zeromode prevent us to go to the lowtemperature highdensity region. We propose a method to circumvent them by a simple modification of the density of state method. We first argue that the standard version of the density of state method, which is invented to solve the overlapping problem, is effective only for a certain `good' class of observables. We then modify it so as to solve the overlap problem for `bad' observables as well. While, in the standard version of the density of state method, we usually constrain an observable we are interested in, we fix a different observable in our new method which has a sharp peak at some particular value characterizing the correct vacuum of the target theory. In the finitedensity QCD, such an observable is the pion condensate. The average phase becomes vanishingly small as the value of the pion condensate becomes large, hence it is enough to consider configurations with small values of pion condensate, where the zero mode does not appear. We demonstrate an effectiveness of our method by using a toy model (the chiral random matrix theory) which captures the properties of finitedensity QCD qualitatively. We also argue how to apply our method to other theories including finitedensity QCD. Although the example we study numerically is based on the phase reweighting method, the same idea can be applied to more general reweighting methods and we show how this idea can be applied to find a possible QCD critical point. 
Journal of Physics Conference Series 06/2014; 523:012046. DOI:10.1088/17426596/523/1/012046

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ABSTRACT: We calculate the electromagnetic pion form factor in lattice QCD with 2+1 flavors of the dynamical overlap quarks. Up and down quark masses are set below their physical values so that the system is in the socalled epsilon regime with the small size of our lattice ~ 1.8 fm. The finite volume corrections are generally expected to be ~ 100% in the epsilon regime. We, however, find a way to automatically cancel the dominant part of them. Inserting nonzero momenta and taking appropriate ratios of the two and three point functions, we can eliminate the contribution from the zeromomentum pion mode. Then the remaining finite volume effect is a small perturbation from the nonzero modes. Our lattice data agree with this theoretical prediction and the extracted pion charge radius is consistent with the experiment.Physical Review D 05/2014; 90(3). DOI:10.1103/PhysRevD.90.034506 · 4.86 Impact Factor 
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ABSTRACT: We are developing a new code set "Bridge++" for lattice QCD (Quantum Chromodynamics) simulations. It aims at an extensible, readable, and portable workbench, while achieving high performance. Bridge++ covers popular lattice actions and numerical algorithms. The code set is constructed in C++ with an object oriented programming. In this paper, we describe our code design focusing on the use of accelerators such as GPGPUs. For portability our implementation employs OpenCL to control the devices while encapsulates the details of manipulation by providing generalized interfaces. The code is successfully applied to several recent accelerators.Procedia Computer Science 01/2014; 29:17011710. DOI:10.1016/j.procs.2014.05.155 
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ABSTRACT: We carry out a comparative study among fivedimensional formulations of chirally symmetric fermions about the algorithmic performance, chiral symmetry violation and topological tunneling to find a computationally inexpensive formulation with good chiral symmetry. With our choice of the lattice action, we have launched largescale simulations on fine lattices aiming at a precision study of light and heavy quark physics. We report on the comparative study, current status of the largescale simulations, and preliminary results on the residual quark mass and autocorrelation. 
Article: Overlap/Domainwall reweighting
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ABSTRACT: We investigate the eigenvalues of nearly chiral lattice Dirac operators constructed with fivedimensional implementations. Allowing small violation of the GinspargWilson relation, the HMC simulation is made much faster while the eigenvalues are not significantly affected. We discuss the possibility of reweighting the gauge configurations generated with domainwall fermions to those of exactly chiral lattice fermions. 
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ABSTRACT: We study the phase structure of lattice QCD with heavy quarks at finite temperature and density by a histogram method. We determine the location of the critical point at which the firstorder deconfining transition in the heavyquark limit turns into a crossover at intermediate quark masses through a change of the shape of the histogram under variation of coupling parameters. We estimate the effect of the complex phase factor which causes the sign problem at finite density, and show that, in heavyquark QCD, the effect is small around the critical point. We determine the critical surface in 2+1 flavor QCD in the heavyquark region at all values of the chemical potential mu including mu=infty.Physical Review D 09/2013; 89(3). DOI:10.1103/PhysRevD.89.034507 · 4.86 Impact Factor 
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ABSTRACT: We present a lattice QCD study of the phase shift of I = 2 ππ scattering on the basis of two different approaches: the standard finite volume approach by Lüscher 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 ~ 0.115 fm and a pion mass of m π ~ 940 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 Lüscher’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 Lüscher’s method using different volumes and/or nonrestframe extension of it. The results are found to agree well with the potential method.Journal of High Energy Physics 05/2013; 2013(12). DOI:10.1007/JHEP12(2013)015 · 6.22 Impact Factor 
Article: Spinorbit force from lattice QCD
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ABSTRACT: We present a first attempt to determine nucleonnucleon potentials in the parityodd sector, which appear in 1P1, 3P0, 3P1, 3P23F2 channels, in Nf=2 lattice QCD simulations. These potentials are constructed from the NambuBetheSalpeter 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 spinorbit potential VLS(r) in the isospintriplet 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 3P23F2 channels. The strong attractive spinorbit force and a weak repulsive central force in spintriplet Pwave channels lead to an attraction in the 3P2 channel, which is related to the Pwave neutron paring in neutron stars.Physics Letters B 05/2013; 735. DOI:10.1016/j.physletb.2014.05.061 · 6.02 Impact Factor 
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ABSTRACT: We investigate the charmed baryon mass spectrum using the relativistic heavy quark action on 2+1 flavor PACSCS configurations previously generated on $32^3 \times 64$ lattice. The dynamical updown and strange quark masses are tuned to their physical values, reweighted from those employed in the configuration generation. At the physical point, the inverse lattice spacing determined from the $\Omega$ baryon mass gives $a^{1}=2.194(10)$ GeV, and thus the spatial extent becomes $L = 32 a = 2.88(1)$ fm. Our results for the charmed baryon masses are consistent with experimental values, except for the mass of $\Xi_{cc}$, which has been measured by only one experimental group so far and has not been confirmed yet by others. In addition, we report values of other doubly and triply charmed baryon masses, which have never been measured experimentally.Physical review D: Particles and fields 01/2013; 87(9). DOI:10.1103/PhysRevD.87.094512 · 4.86 Impact Factor 
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ABSTRACT: We study thermodynamic properties of 2+1 flavor QCD with improved Wilson quarks coupled with the RG improved Iwasaki glue, using the fixed scale approach. We present the results for the equation of state, renormalized Polyakov loop, and chiral condensate. 
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ABSTRACT: We study the phase structure of QCD at high temperature and density by lattice QCD simulations adopting a histogram method. We try to solve the problems which arise in the numerical study of the finite density QCD, focusing on the probability distribution function (histogram). As a first step, we investigate the quark mass dependence and the chemical potential dependence of the probability distribution function as a function of the Polyakov loop when all quark masses are sufficiently large, and study the properties of the distribution function. The effect from the complex phase of the quark determinant is estimated explicitly. The shape of the distribution function changes with the quark mass and the chemical potential. Through the shape of the distribution, the critical surface which separates the first order transition and crossover regions in the heavy quark region is determined for the 2+1flavor case. 
Article: Chiral behavior of kaon semileptonic form factors in lattice QCD with exact chiral symmetry
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ABSTRACT: We calculate the kaon semileptonic form factors in lattice QCD with three flavors of dynamical overlap quarks. Gauge ensembles are generated at pion masses as low as 290 MeV and at a strange quark mass near its physical value. We precisely calculate relevant meson correlators using the alltoall quark propagator. Twisted boundary conditions and the reweighting technique are employed to vary the momentum transfer and the strange quark mass. We discuss the chiral behavior of the form factors by comparing with chiral perturbation theory and experiments. 
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ABSTRACT: We consider how to extract the pion form factors in the epsilon regime. Using the correlators with nonzero momenta and taking appropriate ratios of them, we eliminate the dominant finite volume effect from the zeromomentum pion mode. Our preliminary lattice result for the pion charge radius is consistent with the experiment. 
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ABSTRACT: We review the work of the PACSCS Collaboration, which aimed to realize lattice quantum chromodynamics (QCD) calculations at the physical point, i.e., those with quark masses set at physical values. This has been a longterm goal of lattice QCD simulation since its inception in 1979. After reviewing the algorithmic progress, which played a key role in this development, we summarize the simulations that explored the quark mass dependence of hadron masses down to values close to the physical point. In addition to allowing a reliable determination of the light hadron mass spectrum, this work provided clues on the validity range of chiral perturbation theory, which is widely used in phenomenology. We then describe the application of the technique of quark determinant reweighting, which enables lattice QCD calculations exactly on the physical point. The physical quark masses and the strong coupling constants are fundamental constants of the strong interaction. We describe a nonperturbative Schrodinger functional approach to figure out the nonperturbative renormalization needed to calculate them. There are a number of physical applications that can benefit from lattice QCD calculations carried out either near or at the physical point. We take up three illustrative examples: calculation of the physical properties of the rho meson as a resonance, the electromagnetic form factor and charge radius of the pion, and charmed meson spectroscopy. Bringing single hadron properties under control opens up a number of new areas for serious lattice QCD research. One such area is electromagnetic effects in hadronic properties. We discuss the combined QCD plus QED simulation strategy and present results on electromagnetic mass difference. Another area is multihadron states, or nuclei. We discuss the motivations and difficulties in this area, and describe our work for deuteron and helium as our initial playground. We conclude with a brief discussion on the future perspective of lattice QCD.08/2012; DOI:10.1093/ptep/pts002 
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ABSTRACT: We calculate the strange quark content of the nucleon in 2+1flavor lattice QCD. Chirally symmetric overlap fermion formulation is used to avoid the contamination from up and down quark contents due to an operator mixing between strange and light scalar operators, \bar{s}s and \bar{u}u+\bar{d}d. At a lattice spacing a=0.112(1) fm, we perform calculations at four values of degenerate up and down quark masses, which cover a range of the pion mass M_pi \simeq 300540 MeV. We employ two different methods: one is a direct method where we calculate the strange quark content by directly inserting the strange scalar operator. The other is an indirect method where the quark content is extracted from a derivative of the nucleon mass in terms of the strange quark mass. With these two methods we obtain consistent results with each other. Our best estimate f_{T_s}=0.009(15)(16) is in good agreement with our previous studies in twoflavor QCD.Physical review D: Particles and fields 08/2012; 87(3). DOI:10.1103/PhysRevD.87.034509 · 4.86 Impact Factor 
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ABSTRACT: We present the status of our study on the equation of state in 2+1 flavor QCD with nonperturbatively improved Wilson quarks coupled with the RG improved glue. We apply the Tintegration method to nonperturbatively calculate the equation of state by the fixedscale approach.Physical review D: Particles and fields 05/2012; 85(9). DOI:10.1103/PhysRevD.85.094508 · 4.86 Impact Factor 
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ABSTRACT: We present the results of 1+1+1 flavor QCD+QED simulation at the physical point, in which the dynamical quark effects in QED and the updown quark mass difference are incorporated by the reweighting technique. The physical quark masses together with the lattice spacing are determined with $m_{\pi^+}$, $m_{K^+}$, $m_{K^0}$ and $m_{\Omega^}$ as physical inputs. Calculations are carried out using a set of 2+1 flavor QCD configurations near the physical point generated by the nonperturbatively $O(a)$improved Wilson quark action and the Iwasaki gauge action at $\beta=1.9$ on a $32^3\times 64$ lattice. We evaluate the values of the up, down and strange quark masses individually with nonperturbative QCD renormalization.Physical review D: Particles and fields 05/2012; 86(3). DOI:10.1103/PhysRevD.86.034507 · 4.86 Impact Factor 
Physical review D: Particles and fields 04/2012; 85(7). DOI:10.1103/PhysRevD.85.079902 · 4.86 Impact Factor
Publication Stats
5k  Citations  
613.17  Total Impact Points  
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Institutions

2013–2014

Kyoto University
 Yukawa Institute for Theoretical Physics
Kioto, Kyōto, Japan


1994–2013

University of Tsukuba
 • Centre for Computational Sciences
 • Graduate School of Pure and Applied Sciences
Tsukuba, Ibaraki, Japan
