38 research outputs found
The strong-coupling limit of minimal lattice Landau gauge
We study the gluon and ghost propagators of lattice Landau gauge in the
strong coupling limit in pure SU(2) lattice gauge theory to find
evidence of the conformal infrared behaviour of these propagators as predicted
by a variety of functional continuum methods for asymptotically small momenta
. In the strong-coupling limit, this same
behaviour is obtained for the larger values of (in units of the
lattice spacing ), where it is otherwise swamped by the gauge field
dynamics. Deviations for are well parametrized by a transverse
gluon mass . Perhaps unexpectedly, these deviations are thus no
finite-volume effect but persist in the infinite-volume limit. They furthermore
depend on the definition of gauge fields on the lattice, while the asymptotic
conformal behaviour does not.Comment: 7 pages; talk presented at the XXVI International Symposium on
Lattice Field Theory, July 14-19 2008, Williamsburg, Virginia, US
Spectroscopy of four-dimensional supersymmetric SU(3) Yang-Mills theory
Supersymmetric gauge theories are an important building block for extensions
of the standard model. As a first step towards Super-QCD we investigate the
pure gauge sector with gluons and gluinos on the lattice, in particular the low
energy mass spectrum: meson-like gluinoballs, gluino-glueballs and pure
glueballs. We report on some first calculations performed with clover improved
Wilson fermions on rather small lattices. The supersymmetric continuum limit
and particle masses are discussed and compared to predictions from effective
field theory.Comment: 8 pages, 3 figures, talk presented at the 35th International
Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai
Fractional electric charge and quark confinement
Owing to their fractional electric charges, quarks are blind to
transformations that combine a color center phase with an appropriate
electromagnetic one. Such transformations are part of a global -like
center symmetry of the Standard Model that is lost when quantum chromodynamics
(QCD) is treated as an isolated theory. This symmetry and the corresponding
topological defects may be relevant to non-perturbative phenomena such as quark
confinement, much like center symmetry and ordinary center vortices are in pure
SU() gauge theories. Here we report on our investigations of an analogous
symmetry in a 2-color model with dynamical Wilson quarks carrying half-integer
electric charge.Comment: Conference proceedings for the XXIX International Symposium on
Lattice Field Theory, 201
Role of center vortices in chiral symmetry breaking in SU(3) gauge theory
We study the behavior of the AsqTad quark propagator in Landau gauge on SU(3)
Yang-Mills gauge configurations under the removal of center vortices. In SU(2)
gauge theory, center vortices have been observed to generate chiral symmetry
breaking and dominate the infrared behavior of the quark propagator. In
contrast, we report a weak dependence on the vortex content of the gauge
configurations, including the survival of dynamical mass generation on
configurations with vanishing string tension.Comment: 8 pages, 9 figure
SU(3) Landau gauge gluon and ghost propagators using the logarithmic lattice gluon field definition
We study the Landau gauge gluon and ghost propagators of SU(3) gauge theory,
employing the logarithmic definition for the lattice gluon fields and
implementing the corresponding form of the Faddeev-Popov matrix. This is
necessary in order to consistently compare lattice data for the bare
propagators with that of higher-loop numerical stochastic perturbation theory
(NSPT). In this paper we provide such a comparison, and introduce what is
needed for an efficient lattice study. When comparing our data for the
logarithmic definition to that of the standard lattice Landau gauge we clearly
see the propagators to be multiplicatively related. The data of the associated
ghost-gluon coupling matches up almost completely. For the explored lattice
spacings and sizes discretization artifacts, finite-size and Gribov-copy
effects are small. At weak coupling and large momentum, the bare propagators
and the ghost-gluon coupling are seen to be approached by those of higher-order
NSPT.Comment: 18 pages, 19 figures, 5 table
Lattice Landau gauge via Stereographic Projection
The complete cancellation of Gribov copies and the Neuberger 0/0 problem of
lattice BRST can be avoided in modified lattice Landau gauge. In compact U(1),
where the problem is a lattice artifact, there remain to be Gribov copies but
their number is exponentially reduced. Moreover, there is no cancellation of
copies there as the sign of the Faddeev-Popov determinant is positive. Applied
to the maximal Abelian subgroup this avoids the perfect cancellation amongst
the remaining Gribov copies for SU(N) also. In addition, based on a definition
of gauge fields on the lattice as stereographically-projected link variables,
it provides a framework for gauge fixed Monte-Carlo simulations. This will
include all Gribov copies in the spirit of BRST. Their average is not zero, as
demonstrated explicitly in simple models. This might resolve present
discrepancies between gauge-fixed lattice and continuum studies of QCD Green's
functions.Comment: 5 pages, talk presented at the 8th Conference Quark Confinement and
the Hadron Spectrum, September 1-6, 2008, Mainz, German
Modified Lattice Landau Gauge
We propose a modified lattice Landau gauge based on stereographically
projecting the link variables on the circle S^1 -> R for compact U(1) or the
3-sphere S^3 -> R^3 for SU(2) before imposing the Landau gauge condition. This
can reduce the number of Gribov copies exponentially and solves the Gribov
problem in compact U(1) where it is a lattice artifact. Applied to the maximal
Abelian subgroup this might be just enough to avoid the perfect cancellation
amongst the Gribov copies in a lattice BRST formulation for SU(N), and thus to
avoid the Neuberger 0/0 problem. The continuum limit of the Landau gauge
remains unchanged.Comment: 7 pages, 2 figures, for the proceedings of the XXV International
Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg,
German
Direct determinations of the nucleon and pion terms at nearly physical quark masses
We present a high statistics study of the pion and nucleon light and strange
quark sigma terms using dynamical non-perturbatively improved clover
fermions with a range of pion masses down to MeV and several
volumes, up to , and lattice spacings, fm,
enabling a study of finite volume and discretisation effects for MeV. Systematics are found to be reasonably under control. For the nucleon
we obtain MeV and MeV, or equivalently
in terms of the quark fractions, , and
, where the errors include estimates of both the systematic
and statistical uncertainties. These values, together with perturbative
matching in the heavy quark limit, lead to ,
and . In addition, through the use of the
(inverse) Feynman-Hellmann theorem our results for are shown
to be consistent with the nucleon masses determined in the analysis. For the
pion we implement a method which greatly reduces excited state contamination to
the scalar matrix elements from states travelling across the temporal boundary.
This enables us to demonstrate the Gell-Mann-Oakes-Renner expectation
over our range of pion masses.Comment: 31 pages, 18 figures, v2, small changes to text and figure