577 research outputs found
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 () 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
Parameters of the lowest order chiral Lagrangian from fermion eigenvalues
Recent advances in Random Matrix Theory enable one to determine the
pseudoscalar decay constant from the response of eigenmodes of quenched
fermions to an imaginary isospin chemical potential. We perform a pilot test of
this idea, from simulations with two flavors of dynamical overlap fermions.Comment: 11 pages, 7 figures, Revte
Renormalization of Bilinear Quark Operators for Overlap Fermions
We present non-perturbative renormalization constants of fermionic bilinears
on the lattice in the quenched approximation at beta=6.1 using an overlap
fermion action with hypercubic(HYP)-blocked links. We consider the effects of
the exact zero modes of the Dirac operator and find they are important in
calculating the renormalization constants of the scalar and pseudoscalar
density. The results are given in the RI' and MS bar schemes and compared to
the perturbative calculations.Comment: 14 pages, 13 figure
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
Another determination of the quark condensate from an overlap action
I use the technique of Hernandez, et al (hep-lat/0106011) to convert a recent
calculation of the lattice-regulated quark condensate from an overlap action to
a continuum-regulated number. I find Sigma(MSbar)(mu = 2 GeV) = (282(6)
MeV)-cubed times (a-inverse/1766 MeV)-cubed from a calculation with the Wilson
gauge action at beta=5.9.Comment: 3 pages, Revtex, 1 postscript figure. References added. COLO-HEP-47
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
Chiral Corrections to Hyperon Axial Form Factors
We study the complete set of flavor changing hyperon axial current matrix
elements at small momentum transfer. Using partially quenched heavy baryon
chiral perturbation theory, we derive the chiral and momentum behavior of the
axial and induced pseudoscalar form factors. The meson pole contributions to
the latter posses a striking signal for chiral physics. We argue that the study
of hyperon axial matrix elements enables a systematic lattice investigation of
the efficacy of three flavor chiral expansions in the baryon sector. This can
be achieved by considering chiral corrections to SU(3) symmetry predictions,
and their partially quenched generalizations. In particular, despite the
presence of eight unknown low-energy constants, we are able to make
next-to-leading order symmetry breaking predictions for two linear combinations
of axial charges.Comment: 23 pages, 3 figures, typos corrected and a new NLO prediction adde
Chiral properties of two-flavor QCD in small volume and at large lattice spacing
We present results from simulations of two flavors of dynamical overlap
fermions on 8^4 lattices at three values of the sea quark mass and a lattice
spacing of about 0.16 fm. We measure the topological susceptibility and the
chiral condensate. A comparison of the low-lying spectrum of the overlap
operator with predictions from random matrix theory is made. To demonstrate the
effect of the dynamical fermions, we compare meson two-point functions with
quenched results. Algorithmic improvements over a previous publication and the
performance of the algorithm are discussed.Comment: 16 pages, 12 figure
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