854 research outputs found

### 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

### Instanton classical solutions of SU(3) fixed point actions on open lattices

We construct instanton-like classical solutions of the fixed point action of
a suitable renormalization group transformation for the SU(3) lattice gauge
theory. The problem of the non-existence of one-instantons on a lattice with
periodic boundary conditions is circumvented by working on open lattices. We
consider instanton solutions for values of the size (0.6-1.9 in lattice units)
which are relevant when studying the SU(3) topology on coarse lattices using
fixed point actions. We show how these instanton configurations on open
lattices can be taken into account when determining a few-couplings
parametrization of the fixed point action.Comment: 23 pages, LaTeX, 4 eps figures, epsfig.sty; some comments adde

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