3,015 research outputs found
Effects of non-equilibrated topological charge distributions on pseudoscalar meson masses and decay constants
We study the effects of failure to equilibrate the squared topological charge
on lattice calculations of pseudoscalar masses and decay constants. The
analysis is based on chiral perturbation theory calculations of the dependence
of these quantities on the QCD vacuum angle . For the light-light
partially quenched case, we rederive the known chiral perturbation theory
results of Aoki and Fukaya, but using the nonperturbatively-valid chiral theory
worked out by Golterman, Sharpe and Singleton, and by Sharpe and Shoresh. We
then extend these calculations to heavy-light mesons. Results when staggered
taste-violations are important are also presented. The derived dependence
is compared to that of simulations using the MILC collaboration's ensembles of
lattices with four flavors of HISQ dynamical quarks. We find agreement, albeit
with large statistical errors. These results can be used to correct for the
leading effects of unequilibrated , or to make estimates of the systematic
error coming from the failure to equilibrate . In an appendix, we show
that the partially quenched chiral theory may be extended beyond a lower bound
on valence masses discovered by Sharpe and Shoresh. Subtleties occurring when a
sea-quark mass vanishes are discussed in another appendix.Comment: 46 pages, 5 figures; added section on the effect of staggered taste
violations and made other improvements for clarity. Version to be published
in Phys. Rev.
Bayesian Analysis of Many-Pole Fits of Hadron Propagators in Lattice QCD
We use Bayes' probability theorem to analyze many-pole fits of hadron
propagators. An alternative method of estimating values and uncertainties of
the fit parameters is offered, which has certain advantages over the
conventional methods. The probability distribution of the parameters of a fit
is calculated. The relative probability of various models is calculated.Comment: 4 pages, 6 figures, Latex with espcrc2.sty, uuencoded compressed tar
file contains 7 Latex files: 1 with the paper and 6 with the figures. Talk
presented at LATTICE96(spectrum
The strange quark condensate in the nucleon in 2+1 flavor QCD
We calculate the "strange quark content of the nucleon", ,
which is important for interpreting the results of some dark matter detection
experiments. The method is to evaluate quark-line disconnected correlations on
the MILC lattice ensembles, which include the effects of dynamical strange
quarks. After continuum and chiral extrapolations, the result is <N |s s_bar
|N> = 0.69 +- 0.07(statistical) +- 0.09(systematic), in the modified minimal
subtraction scheme (2 GeV), or for the renormalization scheme invariant form,
m_s partial{M_N}/partial{m_s} = 59(6)(8) MeV.Comment: Added figures and references, especially for fit range choice. Other
changes for clarity. Version to appear in publicatio
The Quenched Continuum Limit
We show that all current formalisms for quarks in lattice QCD are consistent
in the quenched continuum limit, as they should be. We improve on previous
extrapolations to this limit, and the understanding of lattice systematic
errors there, by using a constrained fit including both leading and sub-leading
dependence on a.Comment: Poster presented at Lattice2004(spectrum), Fermilab, June 21-26, 200
Comparison of clustering techniques for residential load profiles in South Africa
This work compares techniques for clustering metered residential energy consumption data to construct representative daily load profiles in South Africa. The input data captures a population with high variability across temporal, geographic, social and economic dimensions. Different algorithms, normalisation and pre-binning techniques are evaluated to determine their effect on producing a good clustering structure. A Combined Index is developed as a relative score to ease the comparison of experiments across different metrics. The study shows that normalisation, specifically unit norm and the zero-one scaler, produce the best clusters. Pre-binning appears to improve clustering structures as a whole, but its effect on individual experiments remains unclear. Like several previous studies, the k-means algorithm produces the best results. To our knowledge this is the first work that rigorously compares state of the art cluster analysis techniques in the residential energy domain in a developing country context
High density QCD with static quarks
We study lattice QCD in the limit that the quark mass and chemical potential
are simultaneously made large, resulting in a controllable density of quarks
which do not move. This is similar in spirit to the quenched approximation for
zero density QCD. In this approximation we find that the deconfinement
transition seen at zero density becomes a smooth crossover at any nonzero
density, and that at low enough temperature chiral symmetry remains broken at
all densities.Comment: LaTeX, 18 pages, uses epsf.sty, postscript figures include
Third type of domain wall in soft magnetic nanostrips
Magnetic domain walls (DWs) in nanostructures are low-dimensional objects
that separate regions with uniform magnetisation. Since they can have different
shapes and widths, DWs are an exciting playground for fundamental research, and
became in the past years the subject of intense works, mainly focused on
controlling, manipulating, and moving their internal magnetic configuration. In
nanostrips with in-plane magnetisation, two DWs have been identified: in thin
and narrow strips, transverse walls are energetically favored, while in thicker
and wider strips vortex walls have lower energy. The associated phase diagram
is now well established and often used to predict the low-energy magnetic
configuration in a given magnetic nanostructure. However, besides the
transverse and vortex walls, we find numerically that another type of wall
exists in permalloy nanostrips. This third type of DW is characterised by a
three-dimensional, flux closure micromagnetic structure with an unusual length
and three internal degrees of freedom. Magnetic imaging on
lithographically-patterned permalloy nanostrips confirms these predictions and
shows that these DWs can be moved with an external magnetic field of about 1mT.
An extended phase diagram describing the regions of stability of all known
types of DWs in permalloy nanostrips is provided.Comment: 19 pages, 7 figure
- …