214 research outputs found
Highly-improved lattice field-strength tensor
We derive an O(a^4)-improved lattice version of the continuum field-strength
tensor. Discretization errors are reduced via the combination of several clover
terms of various sizes, complemented by tadpole improvement. The resulting
improved field-strength tensor is used to construct O(a^4)-improved topological
charge and action operators. We compare the values attained by these operators
as we cool several configurations to self-duality with a previously defined
highly-improved action and assess the relative scale of the remaining
discretization errors.Comment: 22 pages, 7 postscript figure
Comparison of |Q|=1 and |Q|=2 gauge-field configurations on the lattice four-torus
It is known that exactly self-dual gauge-field configurations with
topological charge |Q|=1 cannot exist on the untwisted continuum 4-torus. We
explore the manifestation of this remarkable fact on the lattice 4-torus for
SU(3) using advanced techniques for controlling lattice discretization errors,
extending earlier work of De Forcrand et. al. for SU(2). We identify three
distinct signals for the instability of |Q|=1 configurations, and show that
these manifest themselves early in the cooling process, long before the
would-be instanton has shrunk to a size comparable to the lattice
discretization threshold. These signals do not appear for our |Q|=2
configurations. This indicates that these signals reflect the truly global
nature of the instability, rather than local discretization effects.
Monte-Carlo generated SU(3) gauge field configurations are cooled to the
self-dual limit using an O(a^4)-improved gauge action chosen to have small but
positive O(a^6) errors. This choice prevents lattice discretization errors from
destroying instantons provided their size exceeds the dislocation threshold of
the cooling algorithm. Lattice discretization errors are evaluated by comparing
the O(a^4)-improved gauge-field action with an O(a^4)-improved action
constructed from the square of an O(a^4)-improved lattice field-strength
tensor, thus having different O(a^6) discretization errors. The number of
action-density peaks, the instanton size and the topological charge of
configurations is monitored. We observe a fluctuation in the total topological
charge of |Q|=1 configurations, and demonstrate that the onset of this unusual
behavior corresponds with the disappearance of multiple-peaks in the action
density. At the same time discretization errors are minimal.Comment: 12 pages, 9 figures, submitted to Phys. Rev.
Topological Charge Evolution in the Markov-Chain of QCD
The topological charge is studied on lattices of large physical volume and
fine lattice spacing. We illustrate how a parity transformation on the SU(3)
link-variables of lattice gauge configurations reverses the sign of the
topological charge and leaves the action invariant. Random applications of the
parity transformation are proposed to traverse from one topological charge sign
to the other. The transformation provides an improved unbiased estimator of the
ensemble average and is essential in improving the ergodicity of the Markov
chain process.Comment: 4 pages, 2 figure
FLIC Fermions and Hadron Phenomenology
A pedagogical overview of the formulation of the Fat Link Irrelevant Clover
(FLIC) fermion action and its associated phenomenology is described. The
scaling analysis indicates FLIC fermions provide a new form of nonperturbative
O(a) improvement where near-continuum results are obtained at finite lattice
spacing. Spin-1/2 and spin-3/2, even and odd parity baryon resonances are
investigated in quenched QCD, where the nature of the Roper resonance and
Lambda(1405) are of particular interest. FLIC fermions allow efficient access
to the light quark-mass regime, where evidence of chiral nonanalytic behavior
in the Delta-baryon mass is observed.Comment: Invited plenary session talk at QNP 2002, International Conference on
Quark-Nuclear Physics, 9-14 June 2002, Forschungszentrum Julich, German
Spin-3/2 Nucleon and Delta Baryons in Lattice QCD
We present first results for masses of spin-3/2 N and Delta baryons in
lattice QCD using Fat-Link Irrelevant Clover (FLIC) fermions. Spin-3/2
interpolating fields providing overlap with both spin-3/2 and spin-1/2 states
are considered. In the isospin-1/2 sector, we observe, after appropriate spin
and parity projection, a strong signal for the J^P=3/2^- state together with a
weak but discernible signal for the 3/2^+ state with a mass splitting near that
observed experimentally. We also find good agreement between the 1/2^+/- masses
and earlier nucleon mass simulations with the standard spin-1/2 interpolating
field. For the isospin-3/2 Delta states, clear mass splittings are observed
between the various 1/2^+/- and 3/2^+/- channels, with the calculated level
orderings in good agreement with those observed empirically.Comment: 17 pages, 8 figures, 2 table
Particle Creation from Vacuum by Lorentz Violation
It is shown that the vacuum state in presence of Lorentz violation can be
followed by a particle-full universe that represents the current status of the
universe. In this model the modification in dispersion relation (Lorentz
violation) is picked up representing the regime of quantum gravity. The result
can be interpreted such that the existence of the particles is an evidence for
quantum effects of gravity in the past. It is concluded that only the vacuum
state is sufficient to appear the matter fields spontaneously after the process
of semi-classical analysis.Comment: 9 pages, 2 figure
Quantum gravity and the standard model
We show that a class of background independent models of quantum spacetime
have local excitations that can be mapped to the first generation fermions of
the standard model of particle physics. These states propagate coherently as
they can be shown to be noiseless subsystems of the microscopic quantum
dynamics. These are identified in terms of certain patterns of braiding of
graphs, thus giving a quantum gravitational foundation for the topological
preon model proposed by one of us.
These results apply to a large class of theories in which the Hilbert space
has a basis of states given by ribbon graphs embedded in a three-dimensional
manifold up to diffeomorphisms, and the dynamics is given by local moves on the
graphs, such as arise in the representation theory of quantum groups. For such
models, matter appears to be already included in the microscopic kinematics and
dynamics.Comment: 12 pages, 21 figures, improved presentation, results unchange
Excited Baryons in Lattice QCD
We present first results for the masses of positive and negative parity
excited baryons calculated in lattice QCD using an O(a^2)-improved gluon action
and a fat-link irrelevant clover (FLIC) fermion action in which only the
irrelevant operators are constructed with APE-smeared links. The results are in
agreement with earlier calculations of N^* resonances using improved actions
and exhibit a clear mass splitting between the nucleon and its chiral partner.
An correlation matrix analysis reveals two low-lying J^P=(1/2)^- states with a
small mass splitting. The study of different Lambda interpolating fields
suggests a similar splitting between the lowest two Lambda1/2^- octet states.
However, the empirical mass suppression of the Lambda^*(1405) is not evident in
these quenched QCD simulations, suggesting a potentially important role for the
meson cloud of the Lambda^*(1405) and/or a need for more exotic interpolating
fields.Comment: Correlation matrix analysis performed. Increased to 400
configurations. 22 pages, 13 figures, 15 table
Hadron Properties with FLIC Fermions
The Fat-Link Irrelevant Clover (FLIC) fermion action provides a new form of
nonperturbative O(a)-improvement in lattice fermion actions offering near
continuum results at finite lattice spacing. It provides computationally
inexpensive access to the light quark mass regime of QCD where chiral
nonanalytic behaviour associated with Goldstone bosons is revealed. The
motivation and formulation of FLIC fermions, its excellent scaling properties
and its low-lying hadron mass phenomenology are presented.Comment: 29 pages, 13 figures, 6 tables. Contribution to lecure notes in 2nd
Cairns Topical Workshop on Lattice Hadron Physics 2003 (LHP 2003), Cairns,
Australia, 22-30 Jul 200
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