1,176 research outputs found
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
Computational Strategies in Lattice QCD
Lectures given at the Summer School on "Modern perspectives in lattice QCD",
Les Houches, August 3-28, 2009Comment: Latex source, 72 pages, 23 figures; v2: misprints corrected, minor
text change
Improved Semileptonic Form Factor Calculations in Lattice QCD
We investigate the computational efficiency of two stochastic based
alternatives to the Sequential Propagator Method used in Lattice QCD
calculations of heavy-light semileptonic form factors. In the first method, we
replace the sequential propagator, which couples the calculation of two of the
three propagators required for the calculation, with a stochastic propagator so
that the calculations of all three propagators are independent. This method is
more flexible than the Sequential Propagator Method but introduces stochastic
noise. We study the noise to determine when this method becomes competitive
with the Sequential Propagator Method, and find that for any practical
calculation it is competitive with or superior to the Sequential Propagator
Method. We also examine a second stochastic method, the so-called ``one-end
trick", concluding it is relatively inefficient in this context. The
investigation is carried out on two gauge field ensembles, using the
non-perturbatively improved Wilson-Sheikholeslami-Wohlert action with N_f=2
mass-degenerate sea quarks. The two ensembles have similar lattice spacings but
different sea quark masses. We use the first stochastic method to extract
-improved, matched lattice results for the semileptonic form
factors on the ensemble with lighter sea quarks, extracting f_+(0)
Overlap Valence on 2+1 Flavor Domain Wall Fermion Configurations with Deflation and Low-mode Substitution
The overlap fermion propagator is calculated on 2+1 flavor domain wall
fermion gauge configurations on 16^3 x 32, 24^3 x 64 and 32^3 x 64 lattices.
With HYP smearing and low eigenmode deflation, it is shown that the inversion
of the overlap operator can be expedited by ~ 20 times for the 16^3 x 32
lattice and ~ 80 times for the 32^3 x 64 lattice. Through the study of
hyperfine splitting, we found that the O(m^2a^2) error is small and these
dynamical fermion lattices can adequately accommodate quark mass up to the
charm quark. The low energy constant \Delta_{mix} which characterizes the
discretization error of the pion made up of a pair of sea and valence quarks in
this mixed action approach is calculated via the scalar correlator with
periodic and anti-periodic boundary conditions. It is found to be small which
shifts a 300 MeV pion mass by ~ 10 to 19 MeV on these sets of lattices. We have
studied the signal-to-noise issue of the noise source for the meson and baryon.
It is found that the many-to-all meson and baryon correlators with Z_3 grid
source and low eigenmode substitution is efficient in reducing errors for the
correlators of both mesons and baryons. With 64-point Z_3 grid source and
low-mode substitution, it can reduce the statistical errors of the light quark
(m_{\pi} ~ 200 - 300 MeV) meson and nucleon correlators by a factor of ~ 3-4 as
compared to the point source. The Z_3 grid source itself can reduce the errors
of the charmonium correlators by a factor of ~ 3.Comment: 30 pages, 18 figures, replaced with the version to be published in
PR
Spectral representation of lattice gluon and ghost propagators at zero temperature
We consider the analytic continuation of Euclidean propagator data obtained
from 4D simulations to Minkowski space. In order to perform this continuation,
the common approach is to first extract the K\"all\'en-Lehmann spectral density
of the field. Once this is known, it can be extended to Minkowski space to
yield the Minkowski propagator. However, obtaining the K\"all\'en-Lehmann
spectral density from propagator data is a well known ill-posed numerical
problem. To regularize this problem we implement an appropriate version of
Tikhonov regularization supplemented with the Morozov discrepancy principle. We
will then apply this to various toy model data to demonstrate the conditions of
validity for this method, and finally to zero temperature gluon and ghost
lattice QCD data. We carefully explain how to deal with the IR singularity of
the massless ghost propagator. We also uncover the numerically different
performance when using two ---mathematically equivalent--- versions of the
K\"all\'en-Lehmann spectral integral.Comment: 33 pages, 18 figure
Static-light hadrons on a dynamical anisotropic lattice
We present preliminary results for the static-light meson and baryon spectra
for QCD. The study is performed on an anisotropic lattice and uses a
new all-to-all propagator method allowing us to determine particle masses to a
high precision.Comment: 6 pages, Contribution to Lattice2005, PoS styl
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