193 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
Locality and Translations in Braided Ribbon Networks
An overview of microlocality in braided ribbon networks is presented.
Following this, a series of definitions are presented to explore the concept of
microlocality and the topology of ribbon networks. Isolated substructure of
ribbon networks are introduced, and a theorem is proven that allows them to be
relocated. This is followed by a demonstration of microlocal translations.
Additionally, an investigation into macrolocality and the implications of
invariants in braided ribbon networks are presented.Comment: 12 pages, 12 figure
Cooling for instantons and the Wrath of Nahm
The dynamics of instantons and anti-instantons in lattice QCD can be studied
by analysing the action and topological charge of configurations as they
approach a self-dual or anti-self-dual state, i.e. a state in which S/S_0=|Q|.
We use cooling to reveal the semi-classical structure of the configurations we
study. Improved actions which eliminate discretization errors up to and
including O(a^4) are used to stabilise instantons as we cool for several
thousand sweeps. An analogously improved lattice version of the continuum
field-strength tensor is used to construct a topological charge free from
O(a^4) discretization errors. Values of the action and topological charge
obtained with these improved operators approach mutually-consistent integer
values to within a few parts in 10^4 after several hundred cooling sweeps.
Analysis of configurations with |Q| \approx 1 and |Q| \approx 2 supports the
hypothesis that a self-dual |Q|=1 configuration cannot exist on the 4-torus.Comment: 5 pages, 4 figures, talk presented at the workshop on Lattice Hadron
Physics, Cairns Australia, July 200
Quantitative comparison of filtering methods in lattice QCD
We systematically compare filtering methods used to extract topological
excitations (like instantons, calorons, monopoles and vortices) from lattice
gauge configurations, namely APE-smearing and spectral decompositions based on
lattice Dirac and Laplace operators. Each of these techniques introduces
ambiguities, which can invalidate the interpretation of the results. We show,
however, that all these methods, when handled with care, reveal very similar
topological structures. Hence, these common structures are free of ambiguities
and faithfully represent infrared degrees of freedom in the QCD vacuum. As an
application we discuss an interesting power-law for the clusters of filtered
topological charge.Comment: 6 pages, 18 plots in 5 figures; final version as published in EPJ A;
section 4 was adde
Establishing Social Work Practices in England: The Early Evidence
Social Work Practices (SWPs) were established in England in 2009 to deliver social work services to looked after children and care leavers. The introduction of independent social work-led organisations generated controversy focused on issues such as the privatisation of children's services and social workers' conditions of employment. This paper reports early findings from the evaluation of four of these pilots, drawing on interviews with children and young people, staff, and local authority and national stakeholders. The SWPs assumed a variety of organisational forms. The procurement process was demanding, with protracted negotiations over matters such as budgetary control and providing a round-the-clock service. Start-up was facilitated by an established relationship between the SWP provider and the local authority. Once operational, SWPs continued to rely on local authorities for various functions; in most cases, local authorities retained control of placement budgets. Levels of consultation and choice offered to children and young people regarding the move to an SWP varied considerably. Children's understanding about SWPs was generally low except in the pilot where most children retained their original social worker. These early findings show some dilution of the original SWP model, while the pilots' diversity allows the benefits of particular models to emerge
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
Electromagnetic Form Factors with FLIC fermions
The Fat-Link Irrelevant Clover (FLIC) fermion action provides a new form of
nonperturbative O(a) improvement and allows efficient access to the light
quark-mass regime. FLIC fermions enable the construction of the
nonperturbatively O(a)-improved conserved vector current without the
difficulties associated with the fine tuning of the improvement coefficients.
The simulations are performed with an O(a^2) mean-field improved
plaquette-plus-rectangle gluon action on a 20^3 x 40 lattice with a lattice
spacing of 0.128 fm, enabling the first simulation of baryon form factors at
light quark masses on a large volume lattice.
Magnetic moments, electric charge radii and magnetic radii are extracted from
these form factors, and show interesting chiral nonanalytic behavior in the
light quark mass regime.Comment: Presented by J.Zanotti at the Workshop on Lattice Hadron Physics,
Cairns, Australia, 2003. 7pp, 8 figure
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
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