S Hashimoto

The Graduate University for Advanced Studies, Миура, Kanagawa, Japan

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Publications (218)411.42 Total impact

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    ABSTRACT: We determine the strong coupling constant $\alpha_s$ from a lattice calculation of vacuum polarization functions (VPF) in three-flavor QCD with dynamical overlap fermions. Fitting lattice data of VPF to the continuum perturbative formula including the operator product expansion, we extract the QCD scale parameter $\Lambda_{\overline{MS}}^{(3)}$. At the $Z$ boson mass scale, we obtain $\alpha_s^{(5)}(M_Z)=0.1181(3)(^{+14}_{-12})$, where the first error is statistical and the second is our estimate of various systematic uncertainties. Comment: 15 pages, 7 figures, references updated
    Physical Review D 02/2010; 82(7). DOI:10.1103/PhysRevD.82.074505 · 4.86 Impact Factor
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    ABSTRACT: Using the nonperturbative renormalization technique, we calculate the renormalization factors for quark bilinear operators made of overlap fermions on the lattice. The background gauge field is generated by the JLQCD and TWQCD Collaborations, including dynamical effects of two or 2+1 flavors of light quarks on a 163×32 or 163×48 lattice at lattice spacing around 0.1 fm. By reducing the quark mass close to the chiral limit, where the finite volume system enters the so-called ϵ regime, the unwanted effect of spontaneous chiral symmetry breaking on the renormalization factors is suppressed. On the lattices in the conventional p regime, this effect is precisely subtracted by separately calculating the contributions from the chiral condensate.
    Physical review D: Particles and fields 01/2010; 81(3). DOI:10.1103/PhysRevD.81.034502 · 4.86 Impact Factor
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    ABSTRACT: We calculate the neutral pion ($\pi^0$) to off-shell two photon ($\gamma^*\gamma^*$) transition form factor in lattice QCD. The transition form factor can be extracted from the three-point function of the form (axial-vector)-(vector)-(vector) as a function of off-shell two-photon momentum. Since the axial-anomaly plays an important role in the $\pi^0\to\gamma\gamma$ decay process, we employ the overlap fermion, which preserves the exact chiral symmetery on the lattice. After extrapolating to the chiral and the vanishing photon momentum limit with a fit function based on vector meson dominance (VMD) model, we find that the Adler-Bell-Jackiw anomaly is correctly reproduced. Comment: 7 pages. Talk presented at The XXVII International Symposium on Lattice Field Theory, July 26-31 2009, Peking University, Beijing, China
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    ABSTRACT: We present an update of the light meson spectrum with $N_f$=2+1 overlap fermions on a $16^3\times 48$ lattice at five different up and down quark masses and two strange quark masses. Based on our experience with the previous simulation with $N_f=2$, we carry out the chiral extrapolation with the prediction of the chiral perturbation theory at the next-to-next-to leading order. We also check the consistency of our analysis by using alternative chiral extrapolation with a reduced theory in which the strange quark mass is integrated out. Comment: 7 pages, 6 figures, talk presented at The XXVII International Symposium on Lattice Field Theory - LAT2009, July 26-31 2009, Peking University, Beijing, China
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    ABSTRACT: We calculate the nucleon strange quark content directly from disconnected three-point functions. Numerical simulations are carried out in two-flavor QCD using the overlap quark action with up and down quark masses down to a fifth of the physical strange quark mass. To improve the statistical accuracy, we calculate the nucleon two-point functions with the low-mode averaging technique, whereas the all-to-all quark propagator is used for the disconnected quark loop. We obtain the y parameter, which is the ratio of the strange and light quark contents, y = 0.024(45) at the physical point. This is in a good agreement with our earlier calculation from the nucleon spectrum through the Feynman-Hellmann theorem. Comment: 7 pages, talk presented at The XXVII International Symposium on Lattice Field Theory (Lattice2009), July 26-31 2009, Peking University, Beijing, China
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    ABSTRACT: We report on our study of flavor-singlet mesons in three-flavor QCD with dynamical overlap quarks. Gauge ensembles are generated on a 16^3 x 48 lattice at a lattice spacing of 0.10 fm with the strange quark masses around its physical value m_s,phys and up and down quark masses down to m_s,phys/5. Connected and disconnected meson correlators are calculated using the all-to-all quark propagator. We present our preliminary results on the spectrum of flavor-singlet pseudoscalar and vector mesons. Comment: 7 pages, talk presented at The XXVII International Symposium on Lattice Field Theory - LAT2009, July 26-31 2009, Peking University, Beijing, China
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    ABSTRACT: We study the sigma term and the strange quark content of nucleon in 2+1-flavor QCD with dynamical overlap fermions. We analyze the lattice data of nucleon mass taken at two different strange quark masses with five values of up and down quark masses each. Using the reweighting technique, we study the strange quark mass dependence of the nucleon and extract the strange quark content. Comment: 7 pages, 4 figures, talk given at the XXVII International Symposium on Lattice Field Theory, July 26-31 2009, Peking University, Beijing, China
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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; 80(3). DOI:10.1103/PhysRevD.80.034508 · 4.86 Impact Factor
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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. DOI:10.1103/PhysRevLett.101.242001 · 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. DOI:10.1103/PHYSREVLETT.101.202004 · 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
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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
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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
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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
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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; 78(5). DOI:10.1103/PhysRevD.78.054502 · 4.86 Impact Factor
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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; 79(7). DOI:10.1103/PhysRevD.79.074510 · 4.86 Impact Factor
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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.
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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; 77(9). DOI:10.1103/PHYSREVD.77.094503 · 4.86 Impact Factor
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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; 78(1). DOI:10.1103/PHYSREVD.78.014508 · 4.86 Impact Factor

Publication Stats

4k Citations
411.42 Total Impact Points

Institutions

  • 2009–2014
    • The Graduate University for Advanced Studies
      • School of High Energy Accelerator Science
      Миура, Kanagawa, Japan
  • 2–2014
    • High Energy Accelerator Research Organization
      • • Institute of Particle and Nuclear Studies
      • • Computing Research Center
      Tsukuba, Ibaraki, Japan
  • 2000–2004
    • University of Tsukuba
      • Centre for Computational Sciences
      Tsukuba, Ibaraki-ken, Japan
  • 1999
    • Fermi National Accelerator Laboratory (Fermilab)
      • Theoretical Physics Department
      Batavia, Illinois, United States