Toshihiko Kaneko

Light hadron spectroscopy with two flavors of O(a)-improved dynamical quarks

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 \beta=5.2, where the lattice spacing is found to be a=0.0887(11)fm from \rho meson 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.

B0−B¯0Mixing in Unquenched Lattice QCD

We present an unquenched lattice calculation for the B(0)-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 O(a)-improved Wilson fermion action and heavy quarks described by nonrelativistic QCD. 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 constant, whereas it is small for 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), and xi=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.

Charmonium spectrum from quenched anisotropic lattice QCD.

We present a detailed study of the charmonium spectrum using anisotropic lattice QCD. We first derive a tree-level improved clover quark action on the anisotropic lattice for arbitrary quark mass by matching the Hamiltonian on the lattice and in the continuum. The heavy quark mass dependence of the improvement coefficients, i.e., the ratio of the hopping parameters z5Kt /Ks and the clover coefficients c s,t , is examined at the tree level, and effects of the choice of the spatial Wilson parameter r s are discussed. We then compute the charmonium spectrum in the quenched approximation employing j5as /at53 anisotropic lattices. Simulations are made with the standard anisotropic gauge action and the anisotropic clover quark action with r s51 at four lattice spacings in the range as50.07‐0.2 fm. The clover coefficientsc s,t are estimated from tree-level tadpole improvement. On the other hand, for the ratio of the hopping parameters z, we adopt both the tree-level tadpole-improved value and a non-perturbative one. The latter employs the condition that the speed of light calculated from the meson energy-momentum relation be unity. We calculate the spectrum of S and P states and their excitations using both the pole and kinetic masses. We find that the combination of the pole mass and the tadpole-improved value of z to yield the smoothest approach to the continuum limit, which we then adopt for the continuum extrapolation of the spectrum. The results largely depend on the scale input even in the continuum limit, showing a quenching effect. When the lattice spacing is determined from the 1 P-1S splitting, the deviation from the experimental value is estimated to be ;30% for the S-state hyperfine splitting and ;20% for the P-state fine structure. Our results are consistent with previous results at j52 obtained by Chen when the lattice spacing is determined from the Sommer scale r 0. We also address the problem with the hyperfine splitting that different choices of the clover coefficients lead to disagreeing results in the continuum limit. Making a leading order analysis based on potential models we show that a large hyperfine splitting ;95 MeV obtained by Klassen with a different choice of the clover coefficients is likely an overestimate.

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{sup 2}{approx_equal}0.005, 0.01, and 0.015 fm{sup 2}, and the continuum limit is taken assuming an O(a{sup 2}) discretization error. The light meson spectrum is consistent with experiment. The up, down, and strange quark masses in the MS scheme at 2 GeV are m=(m{sub u}+m{sub d})/2=3.55{sub -0.28}{sup +0.65} MeV and m{sub s}=90.1{sub -6.1}{sup +17.2} 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.

Dynamical quark effects on light quark masses.

Light quark masses are calculated in lattice QCD with two degenerate flavors of dynamical quarks. The calculations are made with improved actions with lattice spacing a = 0.22-0.11 fm. In the continuum limit we find m(M&Smacr;)(ud)(2 GeV) = 3.44(+0.14)(-0.22) MeV using the pi and rho meson masses as physical input, and m(M&Smacr;)(s)(2 GeV) = 88(+4)(-6) MeV or 90(+5)(-11) MeV with the K or straight phi meson mass as additional input. The quoted errors represent statistical and systematic combined, the latter including those from continuum and chiral extrapolations, and from renormalization factors. Compared to quenched results, two flavors of dynamical quarks reduce quark masses by about 25%.

S Parameter and Pseudo Nambu-Goldstone Boson Mass from Lattice QCD

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.

Nonperturbative O(a) improvement of the Wilson quark action with the renormalization-group-improved gauge action using the Schrödinger functional method

We perform a nonperturbative determination of the O(a)-improvement coefficient c{sub SW} and the critical hopping parameter {kappa}{sub c} for N{sub f}=3, 2, and 0 flavor QCD with the (RG) renormalization-group-improved gauge action using the Schroedinger functional method. In order to interpolate c{sub SW} and {kappa}{sub c} as a function of the bare coupling, a wide range of {beta} from the weak coupling region to the moderately strong coupling points used in large-scale simulations is studied. Corrections at finite lattice size of O(a/L) turned out to be large for the RG-improved gauge action, and hence we make the determination at a size fixed in physical units using a modified improvement condition. This enables us to avoid O(a) scaling violations which would remain in physical observables if c{sub SW} determined for a fixed lattice size L/a is used in numerical simulations.

Hybrid Quarkonia on Asymmetric Lattices

We report on a study of heavy quark bound states containing an additional excitation of the gluonic degrees of freedom. To this end we employ the nonrelativistic QCD approach on coarse and asymmetric lattices, where we discard vacuum polarization effects and neglect all spin-correction terms. We find a clear hybrid signal on all our lattices ( as › 0.15, . . . , 0.47 fm). We have studied in detail the lattice spacing artifacts, finite volume effects, and mass dependence. Within the above approximations we predict the lowest lying hybrid excitation in charmonium to be 1.323(13) GeV above the ground state, where we use the 1P-1S splitting to set the scale. The bottomonium hybrid was found to be 1.542(8) GeV above its ground state. [S0031-9007(99)09172-3]

B meson decay constant from two-flavor lattice QCD with non-relativistic heavy quarks

We present a study of leptonic

I=2 pion scattering length with the Wilson fermion.

B