S Hashimoto

High Energy Accelerator Research Organization, Tsukuba, Ibaraki, Japan

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Publications (227)254.47 Total impact

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    ABSTRACT: We calculate pion vector and scalar form factors in two-flavor lattice QCD and study the chiral behavior of the vector and scalar radii <r^2>_{V,S}. Numerical simulations are carried out on a 16^3 x 32 lattice at a lattice spacing of 0.12 fm with quark masses down to \sim m_s/6, where m_s is the physical strange quark mass. Chiral symmetry, which is essential for a direct comparison with chiral perturbation theory (ChPT), is exactly preserved in our calculation at finite lattice spacing by employing the overlap quark action. We utilize the so-called all-to-all quark propagator in order to calculate the scalar form factor including the contributions of disconnected diagrams and to improve statistical accuracy of the form factors. A detailed comparison with ChPT reveals that the next-to-next-to-leading-order contributions to the radii are essential to describe their chiral behavior in the region of quark mass from m_s/6 to m_s/2. Chiral extrapolation based on two-loop ChPT yields <r^2>_V=0.409(23)(37)fm and <r^2>_S=0.617(79)(66)fm, which are consistent with phenomenological analysis. We also present our estimates of relevant low-energy constants. Comment: 32 pages, 18 figures, typos corrected
    Physical review D: Particles and fields 05/2009;
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    ABSTRACT: We present a lattice calculation of L10, one of the low-energy constants in chiral perturbation theory, and the charged-neutral pion squared-mass splitting, using dynamical overlap fermion. The exact chiral symmetry of the overlap fermion allows us to reliably extract these quantities from the difference of the vacuum polarization functions for vector and axial-vector currents. In the context of the technicolor models, these two quantities are read as the S parameter and the pseudo Nambu-Goldstone boson mass, respectively, and play an important role in discriminating the models from others. This calculation can serve as a feasibility study of the lattice techniques for more general technicolor gauge theories.
    Physical Review Letters 01/2009; 101(24):242001. · 7.73 Impact Factor
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    ABSTRACT: We test the convergence property of the chiral perturbation theory using a lattice QCD calculation of pion mass and decay constant with two dynamical quark flavors. The lattice calculation is performed using the overlap fermion formulation, which realizes exact chiral symmetry at finite lattice spacing. By comparing various expansion prescriptions, we find that the chiral expansion is well saturated at the next-to-leading order for pions lighter than approximately 450 MeV. Better convergence behavior is found, in particular, for a resummed expansion parameter xi, with which the lattice data in the pion mass region 290-750 MeV can be fitted well with the next-to-next-to-leading order formulas. We obtain the results in two-flavor QCD for the low energy constants l[over ]_{3} and l[over ]_{4} as well as the pion decay constant, the chiral condensate, and the average up and down quark mass.
    Physical Review Letters 12/2008; 101(20):202004. · 7.73 Impact Factor
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    ABSTRACT: We present a calculation of the nucleon sigma term on two-flavor QCD configurations with dynamical overlap fermions. We analyse the lattice data for the nucleon mass using the baryon chiral perturbation theory. Using partially quenched data sets, we extract the connected and disconnected contributions to the nucleon sigma term separately. Chiral symmetry on the lattice simplifies the determination of the disconnected contribution. We find that the strange quark content, which determines the neutralino dark matter reaction rate with nucleon through the Higgs boson exchange, is much smaller than the previous lattice results. Comment: 7 pages, 6 figures, Talk given at 26th International Symposium on Lattice Field Theory (Lattice 2008), Williamsburg, Virginia, 14-20 Jul 2008
    10/2008;
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    ABSTRACT: We calculate the pion vector and scalar form factors in two-flavor QCD. Gauge configurations are generated with dynamical overlap quarks on a 16^3 x 32 lattice at a lattice spacing of 0.12 fm with sea quark masses down to a sixth of the physical strange quark mass. Contributions of disconnected diagrams to the scalar form factor is calculated employing the all-to-all quark propagators. We present a detailed comparison of the vector and scalar radii with chiral perturbation theory to two loops. Comment: 7 pages; Talk presented at the XXVI International Symposium on Lattice Field Theory, July 14 - 19 2008, Williamsburg, Virginia, USA; v2: a reference added
    10/2008;
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    ABSTRACT: We report on a numerical simulation with 2+1 dynamical flavors of overlap fermions. We calculate pseudo-scalar masses and decay constants on a $16^3\times 48 \times (0.11 {\rm fm})^4$ lattice at five different up and down quark masses and two strange quark masses. The lightest pion mass corresponds to $\approx 310$ MeV. We also study the validity of the chiral perturbation theory using the results of the numerical simulation with two dynamical flavors and conclude that the one-loop formulae cannot be directly applied in the strange quark mass region. We therefore extrapolate our 2+1-flavor results to the chiral limit by fitting the data to the two-loop formulae of the chiral perturbation theory. Comment: 7 pages, 7 figure files. Talk given at the XXVI International Symposium on Lattice Field Theory, July 14 - 19 2008, Williamsburg, Virginia, USA
    10/2008;
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    ABSTRACT: We determine the topological susceptibility \chi_t in the topologically-trivial sector generated by lattice simulations of N_f = 2+1 QCD with overlap Dirac fermion, on a 16^3 x 48 lattice with lattice spacing ~ 0.11 fm, for five sea quark masses m_q ranging from m_s/6 to m_s (where m_s is the physical strange quark mass). The \chi_t is extracted from the plateau (at large time separation) of the 2-point and 4-point time-correlation functions of the flavor-singlet pseudoscalar meson \eta', which arises from the finite size effect due to fixed topology. In the small m_q regime, our result of \chi_t agrees with the chiral effective theory. Using the formula \chi_t = \Sigma(m_u^{-1} + m_d^{-1} + m_s^{-1})^{-1} by Leutwyler-Smilga, we obtain the chiral condensate \Sigma^{MSbar}(2 GeV) = [249(4)(2) MeV]^3. Comment: 7 pages, 3 figures, talk presented at the XXVI International Symposium on Lattice Field Theory, July 14-19, 2008, Williamsburg, Virginia, USA
    10/2008;
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    ABSTRACT: We calculate the nucleon sigma term in two-flavor lattice QCD utilizing the Feynman-Hellman theorem. Both sea and valence quarks are described by the overlap fermion formulation, which preserves exact chiral and flavor symmetries on the lattice. We analyse the lattice data for the nucleon mass using the analytical formulae derived from the baryon chiral perturbation theory. From the data at valence quark mass set different from sea quark mass, we may extract the sea quark contribution to the sigma term, which corresponds to the strange quark content. We find that the strange quark content is much smaller than the previous lattice calculations and phenomenological estimates.
    Physical review D: Particles and fields 07/2008;
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    ABSTRACT: We calculate the vacuum polarization functions on the lattice using the overlap fermion formulation.By matching the lattice data at large momentum scales with the perturbative expansion supplemented by Operator Product Expansion (OPE), we extract the strong coupling constant $\alpha_s(\mu)$ in two-flavor QCD as $\Lambda^{(2)}_{\overline{MS}}$ = $0.234(9)(^{+16}_{- 0})$ GeV, where the errors are statistical and systematic, respectively. In addition, from the analysis of the difference between the vector and axial-vector channels, we obtain some of the four-quark condensates. Comment: 24 pages, 9 figures, enlarged version published in Phys. Rev. D
    Physical review D: Particles and fields 07/2008;
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    ABSTRACT: We determine the topological susceptibility $ \chi_t $ in the trivial topological sector generated by lattice simulations of two-flavor QCD with overlap Dirac fermion, on a $16^3 \times 32$ lattice with lattice spacing $\sim$ 0.12 fm, at six sea quark masses $m_q$ ranging from $m_s/6$ to $m_s$ (where $m_s$ is the physical strange quark mass). The $ \chi_t $ is extracted from the plateau (at large time separation) of the time-correlation function of the flavor-singlet pseudoscalar meson ($\eta'$), which arises from the finite size effect due to fixed topology. In the small $m_q$ regime, our result of $\chi_t$ is proportional to $m_q$ as expected from chiral effective theory. Using the formula $\chi_t=m_q\Sigma/N_f$ by Leutwyler-Smilga, we obtain the chiral condensate in $N_f=2$ QCD as $\Sigma^{\bar{\mathrm{MS}}}(\mathrm{2 GeV})=[252(5)(10) \mathrm{MeV}]^3 $, in good agreement with our previous result obtained in the $\epsilon$-regime.
    06/2008;
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    ABSTRACT: We present a two-flavor QCD calculation of BK on a 163×32 lattice at a~0.12 fm (or equivalently a-1=1.67 GeV). Both valence and sea quarks are described by the overlap fermion formulation. The matching factor is calculated nonperturbatively with the so-called RI/MOM scheme. We find that the lattice data are well described by the next-to-leading order (NLO) partially quenched chiral perturbation theory (PQChPT) up to around a half of the strange quark mass (msphys/2). The data at quark masses heavier than msphys/2 are fitted including a part of next-to-next-to-leading order terms. We obtain BK[overline MS](2 GeV)=0.537(4)(40), where the first error is statistical and the second is an estimate of systematic uncertainties from finite volume, fixing topology, the matching factor, and the scale setting.
    Physical review D: Particles and fields 05/2008;
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    ABSTRACT: 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.
    Physical review D: Particles and fields 04/2008;
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    ABSTRACT: We study the vacuum polarization functions on the lattice with exact chiral symmetry of over-lap fermion by matching the lattice data at large momentum scales with the Operator Product Expansion (OPE). We extract the strong coupling constant α s (µ) in two-flavor QCD as Λ (2) MS = 0.234(9)(+16 − 0) GeV. From the analysis of the difference between the vector and axial-vector chan-nels, we extract some of the four-quark (dimension-six) condensates.
    PoS. 01/2008;
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    ABSTRACT: We calculate the meson correlators in the $\epsilon$-regime of two-flavor QCD. On a $16^3\times 32$ lattice with $a\sim 0.11$ fm, the lattice simulations are performed with the dynamical overlap fermions. We reduce the sea quark mass down to $\sim$ 3 MeV and the valence quark masses are taken in the range 1-4 MeV. The meson correlators in various channels are compared with the predictions of (partially quenched) chiral perturbation theory (ChPT). Including the NLO order of the $\epsilon$-expansion, we extract the leading-order low energy constants of ChPT, the pion decay constant $F$ and the chiral condensate $\Sigma$, as $F=87.3(5.5)$ MeV and $\Sigma^{\bar{\mathrm{MS}}}=[237.8(4.0){MeV}]^3$.
    11/2007;
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    ABSTRACT: We report on our calculation of the pion electromagnetic form factor with two-flavors of dynamical overlap quarks. Gauge configurations are generated using the Iwasaki gauge action on a 16^3 \times 32 lattice at the lattice spacing of 0.12fm with sea quark masses down to m_s/6, where m_s is the physical strange quark mass. We describe our setup to measure the form factor through all-to-all quark propagators and present preliminary results.
    11/2007;
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    ABSTRACT: We present numerical simulation of QCD with two dynamical quark flavors described by the overlap fermion action on a $16^3\times 32\times (0.12 {\rm fm})^4$ lattice. We calculate pseudo-scalar masses and decay constants and investigate their chiral properties. We test the consistency of our data with the two-loop chiral perturbation theory predictions, which should also be valid at finite lattice spacings because of the exact chiral symmetry, including the finite size effects.
    11/2007;
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    ABSTRACT: We determine the topological susceptibility χt in two-flavor QCD using the lattice simulations at a fixed topological sector. The topological charge density is unambiguously defined on the lattice using the overlap-Dirac operator which possesses exact chiral symmetry. Simulations are performed on a 163×32 lattice at lattice spacing ∼ 0.12 fm at six sea quark masses mq ranging in ms/6−ms with ms the physical strange quark mass. The χt is extracted from the constant behavior of the time-correlation of flavor-singlet pseudo-scalar meson two-point function at large distances, which arises from the finite size effect due to the fixed topology. In the small mq regime, our result of χt is proportional to mq as expected from chiral effective theory. Using the formula χt=mqΣ/Nf by Leutwyler–Smilga, we obtain the chiral condensate in Nf=2 QCD as , in good agreement with our previous result obtained in the ϵ-regime.
    Physics Letters B 11/2007; · 4.57 Impact Factor
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    ABSTRACT: We calculate the electromagnetic contribution to the pion mass difference, $\Delta m^2_\pi=m^2_{\pi^+}-m^2_{\pi^0}$, in the chiral limit through the $VV-AA$ type vacuum polarization using Das-Guralnik-Mathur-Low-Young (DGMLY) sum rule. The calculation is made with two-flavors of dynamical overlap fermions on a $16^3\times 32$ lattice at $a\sim$0.12 fm. The exact chiral symmetry of the overlap fermion is essential to control the systematic error in the difference $VV-AA$. We obtain $\Delta m_\pi^2 = 1024(100) {\rm MeV^2}$ combining the lattice data with the perturbative contribution in the high momentum region evaluated by the operator product expansion. By analyzing the momentum dependence of the vacuum polarization, we also obtain pion decay constant $f_\pi$ and the low-energy constants $L_{10}^r$ in the chiral limit.
    11/2007;
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    ABSTRACT: We report on a calculation of $B_K$ with two-flavor dynamical overlap fermions on a $16^3 \times 32$ lattice at $a\sim 0.12$ fm. The results are compared with the PQChPT prediction of quark mass dependence. The systematic errors due to finite volume effects and fixing topology are discussed.
    11/2007;
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    ABSTRACT: 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.
    Physical review D: Particles and fields 05/2007;

Publication Stats

4k Citations
254.47 Total Impact Points

Institutions

  • 1997–2012
    • High Energy Accelerator Research Organization
      • • Institute of Particle and Nuclear Studies
      • • Computing Research Center
      Tsukuba, Ibaraki, Japan
  • 2010
    • Nagoya University
      • Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI)
      Nagoya, Aichi, Japan
  • 2008
    • The Graduate University for Advanced Studies
      Миура, Kanagawa, Japan
  • 1996–2003
    • University of Tsukuba
      • Centre for Computational Sciences
      Tsukuba, Ibaraki-ken, Japan
  • 1995–2000
    • Hiroshima University
      • Graduate School of Science
      Hiroshima-shi, Hiroshima-ken, Japan
  • 1999
    • Fermi National Accelerator Laboratory (Fermilab)
      Batavia, Illinois, United States