5,188 research outputs found
Low Dirac Eigenmodes and the Topological and Chiral Structure of the QCD Vacuum
Several lattice calculations which probe the chiral and topological structure
of QCD are discussed. The results focus attention on the low-lying eigenmodes
of the Dirac operator in typical gauge field configurations.Comment: Talk presented at the DPF2000 Conferenc
Smoothed Particle Hydrodynamics in cosmology: a comparative study of implementations
We analyse the performance of twelve different implementations of Smoothed
Particle Hydrodynamics (SPH) using seven tests designed to isolate key
hydrodynamic elements of cosmological simulations which are known to cause the
SPH algorithm problems. In order, we consider a shock tube, spherical adiabatic
collapse, cooling flow model, drag, a cosmological simulation, rotating
cloud-collapse and disc stability. In the implementations special attention is
given to the way in which force symmetry is enforced in the equations of
motion. We study in detail how the hydrodynamics are affected by different
implementations of the artificial viscosity including those with a
shear-correction modification. We present an improved first-order
smoothing-length update algorithm that is designed to remove instabilities that
are present in the Hernquist and Katz (1989) algorithm.
For all tests we find that the artificial viscosity is the most important
factor distinguishing the results from the various implementations. The second
most important factor is the way force symmetry is achieved in the equation of
motion. Most results favour a kernel symmetrization approach. The exact method
by which SPH pressure forces are included has comparatively little effect on
the results. Combining the equation of motion presented in Thomas and Couchman
(1992) with a modification of the Monaghan and Gingold (1983) artificial
viscosity leads to an SPH scheme that is both fast and reliable.Comment: 30 pages, 26 figures and 9 tables included. Submitted to MNRAS.
Postscript version available at
ftp://phobos.astro.uwo.ca/pub/etittley/papers/sphtest.ps.g
The kunitz domain protein BLI-5 plays a functionally conserved role in cuticle formation in a diverse range of nematodes
The cuticle of parasitic nematodes performs many critical functions and is essential for proper development and for protection from the host immune response. The biosynthesis, assembly, modification and turnover of this exoskeleton have been most extensively studied in the free-living nematode, Caenorhabditis elegans, where it represents a complex multi-step process involving a whole suite of enzymes. The biosynthesis of the cuticle has an additional level of complexity, as many of the enzymes also require additional proteins to aid their activation and selective inhibition. Blister-5 (BLI-5) represents a protein with a kunitz-type serine protease interacting domain and is involved in cuticle collagen biosynthesis in C. elegans, through its interaction with subtilisin-like processing enzymes (such as BLI-4). Mutation of the bli-5 gene causes blistering of the collagenous adult cuticle. Homologues of BLI-5 have been identified in several parasitic species that span different nematode clades. In this study, we molecularly and biochemically characterize BLI-5 homologues from the clade V nematodes C. elegans and Haemonchus contortus and from the clade III filarial nematode Brugia malayi. The nematode BLI-5 orthologues possess a shared domain structure and perform similar in vitro and in vivo functions, performing important proteolytic enzyme functions. The results demonstrate that the bli-5 genes from these diverse parasitic nematodes are able to complement a C. elegansbli-5 mutant and thereby support the use of the C. elegans model system to examine gene function in the experimentally less-amenable parasitic species
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
A New Method for the High-Precision Assessment of Tumor Changes in Response to Treatment
Imaging demonstrates that preclinical and human tumors are heterogeneous,
i.e. a single tumor can exhibit multiple regions that behave differently during
both normal development and also in response to treatment. The large variations
observed in control group tumors can obscure detection of significant
therapeutic effects due to the ambiguity in attributing causes of change. This
can hinder development of effective therapies due to limitations in
experimental design, rather than due to therapeutic failure. An improved method
to model biological variation and heterogeneity in imaging signals is
described. Specifically, Linear Poisson modelling (LPM) evaluates changes in
apparent diffusion co-efficient (ADC) before and 72 hours after radiotherapy,
in two xenograft models of colorectal cancer. The statistical significance of
measured changes are compared to those attainable using a conventional t-test
analysis on basic ADC distribution parameters. When LPMs were applied to
treated tumors, the LPMs detected highly significant changes. The analyses were
significant for all tumors, equating to a gain in power of 4 fold (i.e.
equivelent to having a sample size 16 times larger), compared with the
conventional approach. In contrast, highly significant changes are only
detected at a cohort level using t-tests, restricting their potential use
within personalised medicine and increasing the number of animals required
during testing. Furthermore, LPM enabled the relative volumes of responding and
non-responding tissue to be estimated for each xenograft model. Leave-one-out
analysis of the treated xenografts provided quality control and identified
potential outliers, raising confidence in LPM data at clinically relevant
sample sizes
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