arXiv subjects

S. Aoki

Publications and source records attributed to S. Aoki.

At least 217 records · Page 12Linked to original sources

arXiv2008

Full QCD calculation of neutron electric dipole moment with the external electric field method

We have calculated the neutron electric dipole moment (NEDM) in the presence of the CP violating $θ$ term in lattice QCD with 2-flavor dynamical clover quarks, using the external electric field method. Accumulating a large number of statistics by the averages over 16 different source points and over forward and backward nucleon propagators, we have obtained non-zero signals of neutron and proton EDM beyond one standard deviation at each quark mass in full QCD. We have investigated the quark mass dependence of nucleon EDM in full QCD, and have found that nucleon EDM in full QCD does not decrease toward the chiral limit, as opposed to the theoretical expectation. We briefly discuss possible reasons for this behavior.

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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.

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arXiv2008

2+1 Flavor Lattice QCD toward the Physical Point

We present the first results of the PACS-CS project which aims to simulate 2+1 flavor lattice QCD on the physical point with the nonperturbatively $O(a)$-improved Wilson quark action and the Iwasaki gauge action. Numerical simulations are carried out at the lattice spacing of $a=0.0907(13)$fm on a $32^3\times 64$ lattice with the use of the DDHMC algorithm to reduce the up-down quark mass. Further algorithmic improvements make possible the simulation whose ud quark mass is as light as the physical value. The resulting PS meson masses range from 702MeV down to 156MeV, which clearly exhibit the presence of chiral logarithms. An analysis of the PS meson sector with SU(3) ChPT reveals that the NLO corrections are large at the physical strange quark mass. In order to estimate the physical ud quark mass, we employ the SU(2) chiral analysis expanding the strange quark contributions analytically around the physical strange quark mass. The SU(2) LECs ${\bar l}_3$ and ${\bar l}_4$ are comparable with the recent estimates by other lattice QCD calculations. We determine the physical point together with the lattice spacing employing $m_π$, $m_K$ and $m_Ω$ as input. The hadron spectrum extrapolated to the physical point shows an agreement with the experimental values at a few % level of statistical errors, albeit there remain possible cutoff effects. We also find that our results of $f_π=134.0(4.2)$MeV, $f_K=159.4(3.1)$MeV and $f_K/f_π=1.189(20)$ with the perturbative renormalization factors are compatible with the experimental values. For the physical quark masses we obtain $m_{\rm ud}^\msbar=2.527(47)$MeV and $m_{\rm s}^\msbar=72.72(78)$MeV extracted from the axial-vector Ward-Takahashi identity with the perturbative renormalization factors.

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arXiv2008

Topological susceptibility in 2-flavor lattice QCD with fixed topology

We determine the topological susceptibility $ χ_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 $ χ_t $ is extracted from the plateau (at large time separation) of the time-correlation function of the flavor-singlet pseudoscalar meson ($η'$), which arises from the finite size effect due to fixed topology. In the small $m_q$ regime, our result of $χ_t$ is proportional to $m_q$ as expected from chiral effective theory. Using the formula $χ_t=m_qΣ/N_f$ by Leutwyler-Smilga, we obtain the chiral condensate in $N_f=2$ QCD as $Σ^{\bar{\mathrm{MS}}}(\mathrm{2 GeV})=[252(5)(10) \mathrm{MeV}]^3 $, in good agreement with our previous result obtained in the $ε$-regime.

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arXiv2008

Topological susceptibility in two-flavor lattice QCD with exact chiral symmetry

We determine the topological susceptibility $χ_t$ 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 $16^3 \times 32$ lattice at lattice spacing $\sim$ 0.12 fm at six sea quark masses $m_q$ ranging in $m_s/6$--$m_s$ with $m_s$ the physical strange quark mass. The $χ_t$ 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 $m_q$ regime, our result of $χ_t$ is proportional to $m_q$ as expected from chiral effective theory. Using the formula $χ_t=m_qΣ/N_f$ by Leutwyler-Smilga, we obtain the chiral condensate in $N_f=2$ QCD as $Σ^{\bar{\mathrm{MS}}}(\mathrm{2 GeV}) = [252(5)(10) \mathrm{MeV}]^3 $, in good agreement with our previous result obtained in the $ε$-regime.

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arXiv2008

B_K with two flavors of dynamical overlap fermions

We present a two-flavor QCD calculation of $B_K$ on a $16^3 \times 32$ lattice at $a\sim 0.12$ fm (or equivalently $a^{-1}\sim$1.67 GeV). Both valence and sea quarks are described by the overlap fermion formulation. The matching factor is calculated non-perturbatively with the so-called RI/MOM scheme. We find that the lattice data are well described by the next-to-leading order (NLO) partially quenched chiral perturbation theory (PQChPT) up to around a half of the strange quark mass ($m_s^{\rm phys}/2$). The data at quark masses heavier than $m_s^{\rm phys}/2$ are fitted including a part of next-to-next-to-leading order terms. We obtain $B_K^{\bar{\rm MS}}(2 {\rm GeV})= 0.537(4)(40)$, 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.

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arXiv2008

Lattice study of meson correlators in the epsilon-regime of two-flavor QCD

We calculate mesonic two-point functions in the epsilon-regime of two-flavor QCD on the lattice with exact chiral symmetry. We use gauge configurations of size 16^3 32 at the lattice spacing a \sim 0.11 fm generated with dynamical overlap fermions. The sea quark mass is fixed at \sim 3 MeV and the valence quark mass is varied in the range 1-4 MeV, both of which are in the epsilon-regime. We find a good consistency with the expectations from the next-to-leading order calculation in the epsilon-expansion of (partially quenched) chiral perturbation theory. From a fit we obtain the pion decay constant F=87.3(5.6) MeV and the chiral condensate Sigma^{MS}=[239.8(4.0) MeV ]^3 up to next-to-next-to-leading order contributions.

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arXiv2007

Meson correlators in the epsilon-regime of two-flavor lattice QCD

We calculate the meson correlators in the $ε$-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 $ε$-expansion, we extract the leading-order low energy constants of ChPT, the pion decay constant $F$ and the chiral condensate $Σ$, as $F=87.3(5.5)$ MeV and $Σ^{\bar{\mathrm{MS}}}=[237.8(4.0){MeV}]^3$.

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arXiv2007

Two-flavor lattice QCD in the epsilon-regime and chiral Random Matrix Theory

The low-lying eigenvalue spectrum of the QCD Dirac operator in the epsilon-regime is expected to match with that of chiral Random Matrix Theory (ChRMT). We study this correspondence for the case including sea quarks by performing two-flavor QCD simulations on the lattice. Using the overlap fermion formulation, which preserves exact chiral symmetry at finite lattice spacings, we push the sea quark mass down to \sim 3 MeV on a 16^3\times 32 lattice at a lattice spacing a \simeq 0.11 fm. We compare the low-lying eigenvalue distributions and find a good agreement with the analytical predictions of ChRMT. By matching the lowest-lying eigenvalue we extract the chiral condensate, Σ(2 GeV)[MSbar] = [251(7)(11) MeV]^3, where errors represent statistical and higher order effects in the epsilon expansion. We also calculate the eigenvalue distributions on the lattices with heavier sea quarks at two lattice spacings. Although the epsilon expansion is not applied for those sea quarks, we find a reasonable agreement of the Dirac operator spectrum with ChRMT. The value of Sigma, after extrapolating to the chiral limit, is consistent with the estimate in the epsilon-regime.

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arXiv2007

Lattice QCD approach to nuclear force

We present our updated results of the nucleon-nucleon potential in quenched lattice QCD simulations with the plaquette gauge action and the Wilson quark action on the 32^4(\simeq (4.4 fm)^4) lattice. From the equal-time Bethe-Salpeter (BS) wave function, the NN potential is constructed through the Schroedinger-type equation. Resulting NN potential has all the qualitative features which phenomenological potentials commonly have: the repulsive core at short distance and the attractive well at medium and long distances. In the L \to \infty limit, our NN potential is guaranteed to reproduce the scattering length obtained from the Luescher's formula. The quark mass dependence of the NN potential is studied with m_{pi} \sim 380, 529, 731 MeV. The results suggest that both the repulsive core at short distance and the attractive well at medium distance are enhanced in the light quark mass region.

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arXiv2007

Hyperon-nucleon potentials from lattice QCD

We calculate $pΞ^0$ potentials from the equal-time Bethe-Salpeter amplitude measured in the quenched QCD simulation with the spatial lattice volume, (4.4 fm)$^3$. The standard Wilson gauge action with the gauge coupling $β=5.7$ on $32^4$ lattice together with the standard Wilson quark action are used. The hopping parameter $κ_{ud}=0.1678$ is chosen for $u$ and $d$ quarks, which corresponds to $m_π\simeq 0.37$ GeV. The physical strange quark mass is used by taking the parameter $κ_s=0.1643$ which is deduced from the physical $K$ meson mass. The lattice spacing $a=0.1420$ fm is determined by the physical $ρ$ meson mass. We find that the $pΞ^0$ potential has strong spin dependence. Strong repulsive core is found in $^1S_0$ channel while the effective central potential in the $^3S_1$ channel has relatively weak repulsive core. The potentials also have weak attractive parts in the medium to long distance region (0.6 fm $\lsim r \lsim 1.2$ fm) in both of the $^1S_0$ and $^3S_1$ channels.

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arXiv2007

Light hadron spectrum with 2+1 flavor dynamical $O(a)$-improved Wilson quarks

We present preliminary results for the light harden spectrum in $N_f=2+1$ lattice QCD obtained with the nonperturbatively $O(a)$-improved Wilson quark action and the Iwasaki gauge action. Simulations are carried out at $β=1.90$ on a $32^3 \times 64$ lattice using the PACS-CS computer. We employ Lüscher's domain-decomposed HMC algorithm to reduce the up-down quark masses toward the physical value. The pseudoscalar meson masses range from 730 MeV down to 210 MeV. We compare the light harden spectrum extrapolated to the physical point with the experimental values.

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arXiv2007

Application of chiral perturbation theory to 2+1 flavor lattice QCD with O(a)-improved Wilson quarks

We apply chiral perturbation theory to the pseudoscalar meson mass and decay constant data obtained in the PACS-CS Project toward 2+1 flavor lattice QCD simulations with the O(a)-improved Wilson quarks. We examine the existence of chiral logarithms in the quark mass range from m_{ud}=47 MeV down to 6 MeV on a (2.8 fm)^3 box with the lattice spacing a=0.09 fm. Several low energy constants are determined. We also discuss the magnitude of finite size effects based on chiral perturbation theory.

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arXiv2007

Lattice simulation of 2+1 flavors of overlap light quarks

We report on the status of the dynamical overlap QCD simulation project by the JLQCD collaboration. After completing two-flavor QCD simulation on a 16^3x32 lattice at lattice spacing a 0.12 fm, we started a series of runs with 2+1 flavors. In this report, we describe an outline of our algorithms, parameter choices, and some early physics results of this second phase of our project.

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arXiv2007

Light meson spectrum with $N_f=2$ dynamical overlap fermions

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.

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arXiv2007

Thermodynamics and heavy-quark free energies at finite temperature and density with two flavors of improved Wilson quarks

Thermodynamics of two-flavor QCD at finite temperature and density is studied on a $16^3 \times 4$ lattice, using a renormalization group improved gauge action and the clover improved Wilson quark action. In the simulations along lines of constant $m_{\rm PS}/m_{\rm V}$, we calculate the Taylor expansion coefficients of the heavy-quark free energy with respect to the quark chemical potential ($μ_q$) up to the second order. By comparing the expansion coefficients of the free energies between quark($Q$)and antiquark($\bar{Q}$), and between $Q$ and $Q$, we find a characteristic difference at finite $μ_q$ due to the first order coefficient of the Taylor expansion. We also calculate the quark number and isospin susceptibilities, and find that the second order coefficient of the quark number susceptibility shows enhancement around the pseudo-critical temperature.

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arXiv2007

B_K with dynamical overlap fermions

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.

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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.

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