Improving meson two-point functions by low-mode averaging

Some meson correlation functions have a large contribution from the low lying eigenmodes of the Dirac operator. The contribution of these eigenmodes can be averaged over all positions of the source. This can improve the signal in these channels significantly. We test the method for meson two-point functions.Comment: Talk given at Lattice2004(spectrum), Fermilab, June 21-26, 200

Comment on ``Evidence Against Instanton Dominance of Topological Charge Fluctuations in QCD''

We comment on the recent paper (hep-lat/0102003) by Horvath, Isgur, McCune, and Thacker, which concludes that the local chiral structure of fermionic eigenmodes is not consistent with instanton dominance. Our calculations, done with an overlap action, suggest the opposite conclusion.Comment: 5 pages, Revtex, 4 postscript figures. COLO-HEP-45

Perturbation Theory for Fat-link Fermion Actions

We discuss weak coupling perturbation theory for lattice actions in which the fermions couple to smeared gauge links. The normally large integrals that appear in lattice perturbation theory are drastically reduced. Even without detailed calculation, it is easy to determine to good accuracy the scale of the logarithms that appear in cases where an anomalous dimension is present. We describe several 1-loop examples for fat-link Wilson and clover fermions. including the additive mass shift, the relation between the lattice and MSbar quark masses, and the axial current renormalization factor ($Z_A$) for light-light and static-light currents.Comment: LATTICE99(Improvement and Renormalization); 3 pages, no figure

Time Dependence of Nucleon Correlation Functions in Chiral Perturbation Theory

We consider corrections to nucleon correlation functions arising from times that are far from the asymptotic limit. For such times, the single nucleon state is contaminated by the pion-nucleon and pion-delta continuum. We use heavy baryon chiral perturbation theory to derive the spectral representation of the nucleon two-point function. Finite time corrections to the axial current correlation function are also derived. Pion-nucleon excited state contributions drive the axial correlator upward, while contributions from the interference of pion-delta and pion-nucleon states drive the axial correlator downward. Our results can be compared qualitatively to optimized nucleon correlators calculated in lattice QCD, because the chiral corrections characterize only low-energy excitations above the ground state. We show that improved nucleon operators can lead to an underestimation of the nucleon axial charge.Comment: 12 pages, 4 figure

Structure of the QCD Vacuum As Seen By Lattice Simulations

This talk is a review of our studies of instantons and their properties as seen in our lattice simulations of SU(2) gauge theory. We have measured the topological susceptibility and the size distribution of instantons in the QCD vacuum. We have also investigated the properties of quarks moving in instanton background field configurations, where the sizes and locations of the instantons are taken from simulations of the full gauge theory. By themselves, these multi-instanton configurations do not confine quarks, but they induce chiral symmetry breaking.Comment: 18 pages, LaTeX, 8 figures, uses epsf, Talk given at YKIS9

Are magnetic monopoles hadrons?

The charges of magnetic monopoles are constrained to a multiple of $2\pi$ times the inverse of the elementary unit electric charge. In the standard model, quarks have fractional charge, raising the question of whether the basic magnetic monople unit is a multiple of $2 \pi/e$ or three times that. A simple lattice construction shows how a magnetic monopole of the lower strength is possible if it interacts with gluonic fields as well. Such a monopole is thus a hadron. This is consistent with the construction of magnetic monopoles in grand unified theories.Comment: Poster presented at Lattice2004(topology), Fermilab, June 21-26, 2004. 3 pages, 5 figure

SU(4) lattice gauge theory with decuplet fermions: Schr\"odinger functional analysis

We complete a program of study of SU(N) gauge theories coupled to two flavors of fermions in the two-index symmetric representation by performing numerical simulations in SU(4). The beta function, defined and calculated via the Schr\"odinger functional, runs more slowly than the two-loop perturbative result. The mass anomalous dimension levels off in strong coupling at a value of about 0.45, rendering this theory unsuitable for walking technicolor. A large-N comparison of this data with results from SU(2) and SU(3) reveals striking regularities.Comment: 15 pages, 15 figure