Nonperturbative structure of the quark-gluon vertex

The complete tensor structure of the quark--gluon vertex in Landau gauge is determined at two kinematical points (`asymmetric' and `symmetric') from lattice QCD in the quenched approximation. The simulations are carried out at beta=6.0, using a mean-field improved Sheikholeslami-Wohlert fermion action, with two quark masses ~ 60 and 115 MeV. We find substantial deviations from the abelian form at the asymmetric point. The mass dependence is found to be negligible. At the symmetric point, the form factor related to the chromomagnetic moment is determined and found to contribute significantly to the infrared interaction strength.Comment: 16 pages, 11 figures, JHEP3.cl

Improving Algorithms to Compute All Elements of the Lattice Quark Propagator

We present a new exact algorithm for estimating all elements of the quark propagator. The advantage of the method is that the exact all-to-all propagator is reproduced in a large but finite number of inversions. The efficacy of the algorithm is tested in Monte Carlo simulations of Wilson quarks in quenched QCD. Applications that are difficult to probe with point propagators are discussed.Comment: Talks presented by AOC and KJJ at Lattice2004(machines), Fermilab, June 21-26, 2004. 6 pages, 6 figure

Practical all-to-all propagators for lattice QCD

A new method for computing all elements of the lattice quark propagator is proposed. The method combines the spectral decomposition of the propagator, computing the lowest eigenmodes exactly, with noisy estimators which are 'diluted', i.e. taken to have support only on a subset of time, space, spin or colour. We find that the errors are dramatically reduced compared to traditional noisy estimator techniques.Comment: 24 pages, 18 figure

Charmonium at high temperature in two-flavor QCD

We compute charmonium spectral functions in 2-flavor QCD on anisotropic lattices using the maximum entropy method. Our results suggest that the S-waves (J/psi and eta_c) survive up to temperatures close to 2Tc, while the P-waves (chi_c0 and chi_c1) melt away below 1.2Tc.Comment: 11 pages, 19 figures. v2: expanded discussion and modified conclusions. One figure changed. To appear in PR

Static-light matrix elements on a dynamical anisotropic lattice

The static-light matrix element needed to determine $f_B$ is studied on an anisotropic lattice with $N_f=2$. The improvement in precision due to stout links and all-to-all propagators is investigated.Comment: Lattice2004(heavy), Fermilab, June 21-26, 2004. 3 page

The spectrum of radial, orbital and gluonic excitations of charmonium

We present results for the charmonium spectrum from $N_f=2$ dynamical QCD simulations on $12^3\times 80$ anisotropic lattices. Using all-to-all propagators we determine the ground and excited states of S, P and D waves and hybrids. We also evaluate the disconnected (OZI suppressed) contribution to the $\eta_c$ and $J/\Psi$Comment: 6 pages, 3 figures, Presented at 24th International Symposium on Lattice Field Theory (Lattice 2006), Tucson, Arizona, 23-28 Jul 200

Nonperturbative improvement and tree-level correction of the quark propagator

We extend an earlier study of the Landau gauge quark propagator in quenched QCD where we used two forms of the O(a)-improved propagator with the Sheikholeslami-Wohlert quark action. In the present study we use the nonperturbative value for the clover coefficient c_sw and mean-field improvement coefficients in our improved quark propagators. We compare this to our earlier results which used the mean-field c_sw and tree-level improvement coefficients for the propagator. We also compare three different implementations of tree-level correction: additive, multiplicative, and hybrid. We show that the hybrid approach is the most robust and reliable and can successfully deal even with strong ultraviolet behavior and zero-crossing of the lattice tree-level expression. We find good agreement between our improved quark propagators when using the appropriate nonperturbative improvement coefficients and hybrid tree-level correction. We also present a simple extrapolation of the quark mass function to the chiral limit.Comment: 12 pages, 18 figures, RevTeX4. Some clarifications and corrections. Final version, to appear in Phys.Rev.

Avoiding the Landau-pole in perturbative QCD

We propose an alternative perturbative expansion for QCD. All scheme and scale dependence is reduced to one free parameter. Fixing this parameter with a fastest apparent convergence criterion gives sensible results in the whole energy region. We apply the expansion to the calculation of the zero flavor triple gluon vertex, the quark gluon vertex, the gluon propagator and the ghost propagator. A qualitative agreement with the corresponding lattice results is found.Comment: 18 pages, 8 figure

Charmonium Spectrum on dynamical anisotropic lattices

We present a first study of the charmonium spectrum on N_f=2 dynamical, anisotropic lattices. We take advantage of all-to-all quark propagators to build spatially extended interpolating operators to increase the overlap with states not easily accessible with point propagators such as radially excited states of eta_c, psi, and chi_c, D-waves and hybrid states.Comment: 9 pages, 7 figures, Lattice 2005 Conferenc