arXiv subjects
Publications and source records attributed to T. Onogi.
We report on the two-flavor QCD simulation in the epsilon-regime using the overlap fermion formulation. Sea quark mass is reduced to ~ 2 MeV on a 16^3x32 lattice with the lattice spacing ~ 0.11fm. Topological charge is fixed at Q=0. We compare the eigenvalue distribution of the overlap-Dirac operator with the prediction of the chiral random matrix theory. Preliminary results on meson correlators are also reported.
We launched a project to perform dymanical fermion simulations using the overlap fermion formulation for sea quarks. In order to avoid the appearace of near-zero modes of the hermitian Wilson-Dirac operator $H_W$, we introduce a pair of extra Wilson fermions with a large negative mass term. Crossing of the topological boundary is then strictly prohibited, and the topological charge is conserved during simulations. It makes the simulations substantially faster compared to the algorithms which allow the topology change. We discuss on the finite volume effects due to the fixed global topology.
We apply the topology conserving gauge action proposed by Luescher to the four-dimensional lattice QCD simulation in the quenched approximation. With this gauge action the topological charge is stabilized along the hybrid Monte Carlo updates compared to the standard Wilson gauge action. The quark potential and renormalized coupling constant are in good agreement with the results obtained with the Wilson gauge action. We also investigate the low-lying eigenvalue distribution of the hermitian Wilson-Dirac operator, which is relevant for the construction of the overlap-Dirac operator.
The topology conserving gauge action proposed by Luescher is expected to reduce the number of non-smooth gauge configurations as well as the topology change compared to the conventional actions for the same lattice spacings. We report our quenched QCD study of the topological stability and the scaling violation of the static quark potential. We find that the the topology change is indeed suppressed when the parameter epsilon is of order one. We also find that the scaling violation in the static quark potential remain reasonably small in the parameter range of our study. Our study is done at the inverse lattice spacing a^{-1}=1.4-2.5 GeV with the lattice size L=1.0-1.6fm.
We investigate the distribution of low-lying eigenmodes of hermitian Wilson-Dirac operator, $H_W$, with the gauge action whose form is designed to avoid topology change, as well as with the standard plaquette action. On the quenched lattices, the former gauge action exhibits less density of low-lying eigenmodes of $H_W$ compared to the latter at the same lattice spacing. We also show preliminary results for the dynamical simulation with two flavors of overlap fermions.
We study the spin $\frac12$ hadronic state in quenched lattice QCD to search for a possible $S=+1$ pentaquark resonance. Simulations are carried out on $8^3\times 24$, $10^3\times 24$, $12^3\times 24$ and $16^3\times 24$ lattices at $β$=5.7 at the quenched level with the standard plaquette gauge action and Wilson quark action. We adopt two independent operators with I=0 and $J^P=\frac12$ to construct a $2\times 2$ correlation matrix. After the diagonalization of the correlation matrix, we successfully obtain the energies of the ground-state and the 1st excited-state in this channel. The volume dependence of the energies suggests the existence of a possible resonance state slightly above the NK threshold in I=0 and $J^P=\frac12^-$ channel.
CP-PACS and JLQCD Collaborations are carrying out a joint project of the 2+1 flavor full QCD with the RG-improved gauge action and the non-perturbatively ${\cal O}(a)$-improved Wilson quark action. This simulation removes quenching effects of all three light quarks, which is the last major uncertainty in lattice QCD. In this report we present our results for the light meson spectrum and quark masses on a $20^3\times 40$ lattice at the lattice spacing $a\simeq 0.10$ fm.
This report presents the results of studies that investigate the physics reach at a Super B factory, an asymmetric-energy e^+e^- collider with a design luminosity of 5 x 10^35 cm^-2s^-1, which is around 40 times as large as the peak luminosity achieved by the KEKB collider. The studies focus on flavor physics and CP violation measurements that could be carried out in the LHC era. The physics motivation, key observables, measurement methods and expected precisions are presented. The sensitivity studies are a part of the activities associated with the preparation of a Letter of Intent for SuperKEKB, which has been submitted recently.
We present a quenched lattice QCD calculation of the αand βparameters of the proton decay matrix element. The simulation is carried out using the Wilson quark action at three values of the lattice spacing in the range a\approx 0.1-0.064 fm to study the scaling violation effect. We find only mild scaling violation when the lattice scale is determined by the nucleon mass. We obtain in the continuum limit, |α(NDR,2GeV)|=0.0090(09)(^{+5}_{-19})GeV^3 and |β(NDR,2GeV)|=0.0096(09)(^{+6}_{-20})GeV^3 with αand βin a relatively opposite sign, where the first error is statistical and the second is due to the uncertainty in the determination of the physical scale.
We present an unquenched lattice calculation for the $B^0-\bar{B}^0$ transition amplitude. The calculation, carried out at an inverse lattice spacing $1/a$ = 2.22(4) GeV, incorporates two flavors of dynamical quarks described by the fully $O(a)$-improved Wilson fermion action and heavy quarks described by NRQCD. A particular attention is paid to the uncertainty that arises from the chiral extrapolation, especially the effect of pion loops, for light quarks, which we find could be sizable for the leptonic decay of the $B_d$ meson, whereas it is small for the $B_s$ meson and the $B$ parameters. We obtain $f_{B_d}=191(10)(^{+12}_{-22})$ MeV, $f_{B_s}/f_{B_d}=1.13(3)(^{+13}_{-\ 2})$, $B_{B_d}(m_b)=0.836(27)(^{+56}_{-62})$, $B_{B_s}/B_{B_d}=1.017(16)(^{+56}_{-17})$, $ξ=1.14(3)(^{+13}_{- 2})$, where the first error is statistical, and the second is systematic, including uncertainties due to chiral extrapolation, finite lattice spacing, heavy quark expansion and perturbative operator matching.
We present a lattice QCD calculation of the parameters αand βwhich are necessary in the theoretical estimation of the proton lifetime in grand unified theories (GUTs) using chiral lagrangian approach. The simulation is carried out using the Wilson quark action at three gauge coupling constants in the quenched approximation. We obtain |α(2GeV)|=0.0091(08)(^{+10}_{-19})GeV^3 and |β(2GeV)|=0.0098(08)(^{+10}_{-20})GeV^3 in the continuum limit where the first error is statistical and the second one is due to scale setting.
We present a high statistics study of the light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Numerical simulations are carried out using the plaquette gauge action and the O(a)-improved Wilson quark action at β=5.2, where the lattice spacing is found to be a=0.0887(11)fm from ρmeson mass, on a 20^3\times 48 lattice. At each of five sea quark masses corresponding to m_{PS}/m_{V} \simeq 0.8-0.6, we generate 12000 trajectories using the symmetrically preconditioned Hybrid Monte Carlo algorithm. Finite spatial volume effects are investigated employing 12^3 \times 48, 16^3 \times 48 lattices. We also perform a set of simulations in quenched QCD with the same lattice actions at a similar lattice spacing to those for the full QCD runs. In the meson sector we find clear evidence of sea quark effects. The J parameter increases for lighter sea quark masses, and the full QCD meson masses are systematically closer to experiment than in quenched QCD. Careful finite-size studies are made to ascertain that these are not due to finite-size effects. Evidence of sea quark effects is less clear in the baryon sector due to larger finite-size effects. We also calculate light quark masses and find m_{ud}^{MS}(2GeV) =3.223(+0.046/-0.069)MeV and m_s^{MS}(2GeV)=84.5(+12.0/-1.7)MeV which are about 20% smaller than in quenched QCD.
We present our results of lattice calculations of $B$ parameters, which parameterize $ΔB$=2 transition amplitudes together with the leptonic decay constant. Calculations are made in the quenched approximation at $β$=5.7, 5.9, 6.0 and 6.1, using NRQCD action for heavy quark and the $O(a)$-improved Wilson action for light quark. The operators are perturbatively renormalized including the correction of $O(α_s/(aM)^m)$ ($m\ge$0). We examine the scaling behavior of $B$ parameters, and discuss the systematic uncertainties based on the results with several different truncations of higher order terms in 1/M and $α_s$ expansions. We find $B_{B_d}(m_b)=0.84(3)(5)$, $B_{B_s}/B_{B_d}=1.020(21)(^{+15}_{-16})(^{+5}_{-0})$ and $B_{S_s}(m_b)=0.85(1)(5)(^{+1}_{-0})$ in the quenched approximation. The errors represent statistical and systematic as well as the uncertainty in the determination of strange quark mass.
The calculation of the I=2 pion scattering length in quenched lattice QCD is revisited. The calculation is carried out with the Wilson fermion action employing Lüscher's finite size scaling method at $β=5.9$, 6.1, and 6.3 corresponding to the range of lattice spacing $a\simeq 0.12 - 0.07$ fm. We obtain in the continuum limit $a_0m_π= -0.0410(69)$, which is consistent with the prediction of chiral perturbation theory $a_0m_π=-0.0444(10)$.
We examine whether the $O(a)$ improved quark action on anisotropic lattices can be used as a framework for the heavy quark, which enables precision computation of matrix elements of heavy-light mesons. To this end, it is crucial to verify that a mass independent and nonperturbative tuning of the parameters is possible. As a first step, we observe the dispersion relation of heavy-light mesons on a quenched lattice using the action which is nonperturbatively tuned only for the leading terms. On a lattice with the spatial cutoff $a_σ^{-1} \simeq$ 1.6 GeV and the anisotropy $ξ=4$, the relativity relation holds within 2% accuracy in the quark mass region $a_σm_Q \leq 1.2$ with the bare anisotropy parameter tuned for the massless quark. We also apply the action to a calculation of heavy-light decay constants in the charm quark mass region.
We present a polynomial hybrid Monte Carlo (PHMC) algorithm for lattice QCD with odd numbers of flavors of O(a)-improved Wilson quark action. The algorithm makes use of the non-Hermitian Chebyshev polynomial to approximate the inverse square root of the fermion matrix required for an odd number of flavors. The systematic error from the polynomial approximation is removed by a noisy Metropolis test for which a new method is developed. Investigating the property of our PHMC algorithm in the N_f=2 QCD case, we find that it is as efficient as the conventional HMC algorithm for a moderately large lattice size (16^3 times 48) with intermediate quark masses (m_{PS}/m_V ~ 0.7-0.8). We test our odd-flavor algorithm through extensive simulations of two-flavor QCD treated as an N_f = 1+1 system, and comparing the results with those of the established algorithms for N_f=2 QCD. These tests establish that our PHMC algorithm works on a moderately large lattice size with intermediate quark masses (16^3 times 48, m_{PS}/m_V ~ 0.7-0.8). Finally we experiment with the (2+1)-flavor QCD simulation on small lattices (4^3 times 8 and 8^3 times 16), and confirm the agreement of our results with those obtained with the R algorithm and extrapolated to a zero molecular dynamics step size.
We investigate the anisotropic lattice with $O(a)$ improved quark action as a candidate of framework in which we can treat both the heavy and light quark region in the same manner and systematically reduce the systematic uncertainties. To examine applicability of anisotropic lattice, we calculate the charmed meson spectrum and decay constants in quenched approximation. We find consistent result with most advanced results on isotropic lattices.
Using the lattice NRQCD action for heavy quark, we calculate the heavy quark expansion parameters $μ_π^2$ and $μ_G^2$ for heavy-light mesons and heavy-light-light baryons. The results are compared with the mass differences among heavy hadrons to test the validity of HQET relations on the lattice.