68 research outputs found
Partial Flavor Symmetry Restoration for Chiral Staggered Fermions
We study the leading discretization errors for staggered fermions by first
constructing the continuum effective Lagrangian including terms of O(a^2), and
then constructing the corresponding effective chiral Lagrangian. The terms of
O(a^2) in the continuum effective Lagrangian completely break the SU(4) flavor
symmetry down to the discrete subgroup respected by the lattice theory. We
find, however, that the O(a^2) terms in the potential of the chiral Lagrangian
maintain an SO(4) subgroup of SU(4). It follows that the leading discretization
errors in the pion masses are SO(4) symmetric, implying three degeneracies
within the seven lattice irreducible representations. These predictions hold
also for perturbatively improved versions of the action. These degeneracies are
observed, to a surprising degree of accuracy, in existing data. We argue that
the SO(4) symmetry does not extend to the masses and interactions of other
hadrons (vector mesons, baryons, etc), nor to higher order in a^2. We show how
it is possible that, for physical quark masses of O(a^2), the new SO(4)
symmetry can be spontaneously broken, leading to a staggered analogue of the
Aoki-phase of Wilson fermions. This does not, however, appear to happen for
presently studied versions of the staggered action.Comment: 26 pages, 2 figures (using psfig). Version to appear in PRD
(clarifications added to introduction and section 6; typos corrected;
references updated
One-loop matching coefficients for improved staggered bilinears
We calculate one-loop matching factors for bilinear operators composed of
improved staggered fermions. We compare the results for different improvement
schemes used in the recent literature, all of which involve the use of smeared
links. These schemes aim to reduce, though not completely eliminate, O(a^2)
discretization errors. We find that all these improvement schemes substantially
reduce the size of matching factors compared to unimproved staggered fermions.
The resulting corrections are comparable to, or smaller than, those found with
Wilson and domain-wall fermions. In the best case (``Fat-7'' and mean-field
improved HYP links) the corrections are 10 % or smaller at 1/a = 2 GeV.Comment: 13 pages, 1 figure (misleading sentence in sec. II removed; version
to appear in Physical Review D
Enhanced structure-function relationship in glaucoma with an anatomically and geometrically accurate neuroretinal rim measurement
yesPurpose: To evaluate the structure–function relationship between disc margin–based rim area (DM-RA) obtained with confocal scanning laser tomography (CSLT), Bruch's membrane opening–based horizontal rim width (BMO-HRW), minimum rim width (BMO-MRW), peripapillary retinal nerve fiber layer thickness (RNFLT) obtained with spectral-domain optical coherence tomography (SD-OCT), and visual field sensitivity.
Methods: We examined 151 glaucoma patients with CSLT, SD-OCT, and standard automated perimetry on the same day. Optic nerve head (ONH) and RNFL with SD-OCT were acquired relative to a fixed coordinate system (acquired image frame [AIF]) and to the eye-specific fovea-BMO center (FoBMO) axis. Visual field locations were mapped to ONH and RNFL sectors with fixed Garway-Heath (VFGH) and patient-specific (VFPS) maps customized for various biometric parameters.
Results: Globally and sectorally, the structure–function relationships between DM-RA and VFGH, BMO-HRWAIF and VFGH, and BMO-HRWFoBMO and VFPS were equally weak. The R2 for the relationship between DM-RA and VFGH ranged from 0.1% (inferonasal) to 11% (superotemporal) whereas that between BMO-HRWAIF and VFGH ranged from 0.1% (nasal) to 10% (superotemporal). Relatively stronger global and sectoral structure–function relationships with BMO-MRWAIF and with BMO-MRWFoBMO were obtained. The R2 between BMO-MRWAIF and VFGH ranged from 5% (nasal) to 30% (superotemporal), whereas that between BMO-MRWFoBMO and VFPS ranged from 5% (nasal) to 25% (inferotemporal). The structure–function relationship with RNFLT was not significantly different from that with BMO-MRW, regardless of image acquisition method.
Conclusions: The structure–function relationship was enhanced with BMO-MRW compared with the other neuroretinal rim measurements, due mainly to its geometrically accurate properties
Chiral Lagrangian Parameters for Scalar and Pseudoscalar Mesons
The results of a high-statistics study of scalar and pseudoscalar meson
propagators in quenched lattice QCD are presented. For two values of lattice
spacing, ( fm) and 5.9 ( fm), we
probe the light quark mass region using clover improved Wilson fermions with
the MQA pole-shifting ansatz to treat the exceptional configuration problem.
The quenched chiral loop parameters and are determined
from a study of the pseudoscalar hairpin correlator. From a global fit to the
meson correlators, estimates are obtained for the relevant chiral Lagrangian
parameters, including the Leutwyler parameters and . Using the
parameters obtained from the singlet and nonsinglet pseudoscalar correlators,
the quenched chiral loop effect in the nonsinglet scalar meson correlator is
studied. By removing this QCL effect from the lattice correlator, we obtain the
mass and decay constant of the ground state scalar, isovector meson .Comment: 36 pages, 12 figures, LaTe
The Leptonic Decay Constants of Mesons and the Lattice Resolution
We present a high statistics study of the leptonic decay constant of
heavy pseudoscalar mesons using propagating heavy Wilson quarks within the
quenched approximation, on lattices covering sizes from about 0.7~fm to 2~fm.
Varying between 5.74 and 6.26 we observe a sizeable dependence of
when one uses the quark field normalization that was suggested by
Kronfeld and Mackenzie, compared with the weaker dependence observed for the
standard relativistic norm. The two schemes come into agreement when one
extrapolates to . The extrapolations needed to reach the
continuum quantity introduce large errors and lead to the value
~GeV in the quenched approximation. This suggests that much more
effort will be needed to obtain an accurate lattice prediction for .Comment: 11 pages Latex + 5 tables + 8 postscript figures, unix shell archive,
DESY preprint DESY 93-17
New results on cut-off effects in spectroscopy with the fixed point action
Our study on the cut-off effects in quenched light hadron spectroscopy and
pion scattering length with the fixed point action is extended by results
obtained at a lattice spacing a=0.102 fm in a box of size L=1.8 fm. The cut-off
effects are small, but clearly seen as the resolution is increased from a=0.153
fm to a=0.102 fm. In the quark mass region where the errors are small and under
control, our results on the APE plot lie close to the extrapolated numbers of
the CP-PACS Collaboration.Comment: 15 pages, 11 figures, reference correcte
The nucleon's strange electromagnetic and scalar matrix elements
Quenched lattice QCD simulations and quenched chiral perturbation theory are
used together for this study of strangeness in the nucleon. Dependences of the
matrix elements on strange quark mass, valence quark mass and momentum transfer
are discussed in both the lattice and chiral frameworks. The combined results
of this study are in good agreement with existing experimental data and
predictions are made for upcoming experiments. Possible future refinements of
the theoretical method are suggested.Comment: 24 pages, 9 figure
Searching for chiral logs in the static-light decay constant
Using the clover fermion action in unquenched QCD with pion masses as low as
420 MeV, we look for evidence for chiral logs in the static-light decay
constant. There is some evidence for a chiral log term, if the original static
theory of Eichten and Hill is used. However, the more precise data from the
static action of the ALPHA collaboration do not show any evidence for
non-linear dependence of the static-light decay constant on the light quark
mass. We make some comments on the connection between chiral perturbation
theory for decay constants of the pion and static-light meson
Lattice Calculation of Heavy-Light Decay Constants with Two Flavors of Dynamical Quarks
We present results for , , , and their ratios in
the presence of two flavors of light sea quarks (). We use Wilson light
valence quarks and Wilson and static heavy valence quarks; the sea quarks are
simulated with staggered fermions. Additional quenched simulations with
nonperturbatively improved clover fermions allow us to improve our control of
the continuum extrapolation. For our central values the masses of the sea
quarks are not extrapolated to the physical , masses; that is, the
central values are "partially quenched." A calculation using "fat-link clover"
valence fermions is also discussed but is not included in our final results. We
find, for example,
MeV, , MeV, and , where in each case the first error is
statistical and the remaining three are systematic: the error within the
partially quenched approximation, the error due to the missing strange
sea quark and to partial quenching, and an estimate of the effects of chiral
logarithms at small quark mass. The last error, though quite significant in
decay constant ratios, appears to be smaller than has been recently suggested
by Kronfeld and Ryan, and Yamada. We emphasize, however, that as in other
lattice computations to date, the lattice quark masses are not very light
and chiral log effects may not be fully under control.Comment: Revised version includes an attempt to estimate the effects of chiral
logarithms at small quark mass; central values are unchanged but one more
systematic error has been added. Sections III E and V D are completely new;
some changes for clarity have also been made elsewhere. 82 pages; 32 figure
Kaon B Parameter in Quenched QCD
I calculate the kaon B-parameter with a lattice simulation in quenched
approximation. The lattice simulation uses an action possessing exact lattice
chiral symmetry, an overlap action. Computations are performed at two lattice
spacings, about 0.13 and 0.09 fm (parameterized by Wilson gauge action
couplings beta=5.9 and 6.1) with nearly the same physical volumes and quark
masses. I describe particular potential difficulties which arise due to the use
of such a lattice action in finite volume. My results are consistent with other
recent lattice determinations using domain-wall fermions.Comment: 23 pages, Revtex, 16 postscript figure
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