92 research outputs found
Single flavour filtering for RHMC in BQCD
Filtering algorithms for two degenerate quark flavours have advanced to the
point that, in 2+1 flavour simulations, the cost of the strange quark is
significant compared with the light quarks. This makes efficient filtering
algorithms for single flavour actions highly desirable, in particular when
considering 1+1+1 flavour simulations for QED+QCD. Here we discuss methods for
filtering the RHMC algorithm that are implemented within BQCD, an open-source
Fortran program for Hybrid Monte Carlo simulations.Comment: 8 pages, 3 figures, proceedings of the 35th International Symposium
on Lattice Field Theory, 18-24 June 2017, Granada, Spai
Improved Smoothing Algorithms for Lattice Gauge Theory
The relative smoothing rates of various gauge field smoothing algorithms are
investigated on -improved \suthree Yang--Mills gauge field
configurations. In particular, an -improved version of APE
smearing is motivated by considerations of smeared link projection and cooling.
The extent to which the established benefits of improved cooling carry over to
improved smearing is critically examined. We consider representative gauge
field configurations generated with an -improved gauge field
action on \1 lattices at and \2 lattices at
having lattice spacings of 0.165(2) fm and 0.077(1) fm respectively. While the
merits of improved algorithms are clearly displayed for the coarse lattice
spacing, the fine lattice results put the various algorithms on a more equal
footing and allow a quantitative calibration of the smoothing rates for the
various algorithms. We find the relative rate of variation in the action may be
succinctly described in terms of simple calibration formulae which accurately
describe the relative smoothness of the gauge field configurations at a
microscopic level
Efficient operators for studying higher partial waves
An extended multi-hadron operator is developed to extract the spectra of
irreducible representations in the finite volume. The irreducible
representations of the cubic group are projected using a coordinate-space
operator. The correlation function of this operator is computationally
efficient to extract lattice spectra. In particular, this new formulation only
requires propagator inversions from two distinct locations, at fixed physical
separation. We perform a proof-of-principle study on a lattice
volume with ~MeV by isolating the spectra of ,
and of the system with isospin-2 in the rest frame.Comment: 8 pages, 3 figures, Contribution to the conference Lattice201
Gluons, quarks, and the transition from nonperturbative to perturbative QCD
Lattice-based investigations of two fundamental QCD quantities are described,
namely the gluon and quark propagators in Landau gauge. We have studied the
Landau gauge gluon propagator using a variety of lattices with spacings from a
= 0.17 to 0.41 fm. We demonstrate that it is possible to obtain scaling
behavior over a very wide range of momenta and lattice spacings and to explore
the infinite volume and continuum limits. These results confirm that the Landau
gauge gluon propagator is infrared finite. We study the Landau gauge quark
propagator in quenched QCD using two forms of the O(a)-improved propagator and
we find good agreement between these. The extracted value of the infrared quark
mass in the chiral limit is found to be 300 +/- 30 MeV. We conclude that the
momentum regime where the transition from nonperturbative to perturbative QCD
occurs is Q^2 approx 4GeV^2.Comment: 8 pages, 6 figures, 1 table. Talk presented by AGW at the Workshop on
Lepton Scattering, Hadrons and QCD, March 26-April 5, 2001, CSSM, Adelaide,
Australia. To appear in the proceeding
Nucleon isovector structure functions in (2+1)-flavor QCD with domain wall fermions
We report on numerical lattice QCD calculations of some of the low moments of
the nucleon structure functions. The calculations are carried out with gauge
configurations generated by the RBC and UKQCD collaborations with (2+1)-flavors
of dynamical domain wall fermions and the Iwasaki gauge action (). The inverse lattice spacing is GeV, and two spatial
volumes of ((2.7{\rm fm})^3) and ((1.8 {\rm fm})^3) are used. The up and down
quark masses are varied so the pion mass lies between 0.33 and 0.67 GeV while
the strange mass is about 12 % heavier than the physical one. The structure
function moments we present include fully non-perturbatively renormalized
iso-vector quark momentum fraction, (_{u-d}), helicity fraction, (< x
>_{\Delta u - \Delta d}), and transversity, (_{\delta u - \delta d}), as
well as an unrenormalized twist-3 coefficient, (d_1). The ratio of the momentum
to helicity fractions, (_{u-d}/_{\Delta u - \Delta d}), does not show
dependence on the light quark mass and agrees well with the value obtained from
experiment. Their respective absolute values, fully renormalized, show
interesting trends toward their respective experimental values at the lightest
quark mass. A prediction for the transversity, (0.7 _{\delta u -\delta
d} < 1.1), in the (\bar{\rm MS}) scheme at 2 GeV is obtained. The twist-3
coefficient, (d_1), though yet to be renormalized, supports the perturbative
Wandzura-Wilczek relation.Comment: 14 pages, 22 figures
Improved determination of hadron matrix elements using the variational method
The extraction of hadron form factors in lattice QCD using the standard two-
and three-point correlator functions has its limitations. One of the most
commonly studied sources of systematic error is excited state contamination,
which occurs when correlators are contaminated with results from higher energy
excitations. We apply the variational method to calculate the axial vector
current gA and compare the results to the more commonly used summation and
two-exponential fit methods. The results demonstrate that the variational
approach offers a more efficient and robust method for the determination of
nucleon matrix elements.Comment: 7 pages, 6 figures, talk presented at Lattice 2015, PoS (LATTICE2015
The S Parameter in QCD from Domain Wall Fermions
We have computed the SU(2) Low Energy Constant l5 and the mass splitting
between charged and neutral pions from a lattice QCD simulation of nf = 2 + 1
flavors of Domain Wall Fermions at a scale of a-1 = 2.33GeV. Relating l5 to the
S parameter in QCD we obtain a value of S(mH=120GeV) = 0.42(7), in agreement
with previous determinations. Our result can be compared with the value of S
from electroweak precision data which constrains strongly interacting models of
new physics like Technicolor. This work in QCD serves as a test for the methods
to compute the S parameter with Domain Wall Fermions in theories beyond the
Standard Model. We also infer a value for the pion mass splitting in agreement
with experiment.Comment: 15 pages, 12 figure
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