888 research outputs found
Local coherence and deflation of the low quark modes in lattice QCD
The spontaneous breaking of chiral symmetry in QCD is known to be linked to a
non-zero density of eigenvalues of the massless Dirac operator near the origin.
Numerical studies of two-flavour QCD now suggest that the low quark modes are
locally coherent to a certain extent. As a consequence, the modes can be
simultaneously deflated, using local projectors, with a total computational
effort proportional to the lattice volume (rather than its square). Deflation
has potentially many uses in lattice QCD. The technique is here worked out for
the case of quark propagator calculations, where large speed-up factors and a
flat scaling behaviour with respect to the quark mass are achieved.Comment: Plain TeX, 23 pages, 4 figures included; minor text modifications;
version published in JHE
One-Flavour Hybrid Monte Carlo with Wilson Fermions
The Wilson fermion determinant can be written as product of the determinants
of two hermitian positive definite matrices. This formulation allows to
simulate non-degenerate quark flavors by means of the hybrid Monte Carlo
algorithm. A major numerical difficulty is the occurrence of nested inversions.
We construct a Uzawa iteration scheme which treats the nested system within one
iterative process.Comment: 11 pages, to appear in proceedings of the workshop "Numerical
Challenges in Lattice QCD", Springer Verla
Dynamical overlap fermions, results with hybrid Monte-Carlo algorithm
We present first, exploratory results of a hybrid Monte-Carlo algorithm for
dynamical, n_f=2, four-dimensional QCD with overlap fermions. As expected, the
computational requirements are typically two orders of magnitude larger for the
dynamical overlap formalism than for the more conventional (Wilson or
staggered) formulations.Comment: 13 pages, 2 figure
Tolerance without clonal expansion: Self-antigen-expressing B cells program self-reactive T cells for future deletion
B cells have been shown in various animal models to induce immunological tolerance leading to reduced immune responses and protection from autoimmunity. We show that interaction of B cells with naive T cells results in T cell triggering accompanied by the expression of negative costimulatory molecules such as PD-1, CTLA-4, B and T lymphocyte attenuator, and CD5. Following interaction with B cells, T cells were not induced to proliferate, in a process that was dependent on their expression of PD-1 and CTLA-4, but not CD5. In contrast, the T cells became sensitive to Ag-induced cell death. Our results demonstrate that B cells participate in the homeostasis of the immune system by ablation of conventional self-reactive T cells
A Mutation in Amino Acid Permease AAP6 Reduces the Amino Acid Content of the Arabidopsis Sieve Elements but Leaves Aphid Herbivores Unaffected.
The aim of this study was to investigate the role of the amino acid permease gene AAP6 in regulating phloem amino
acid composition and then to determine the effects of this altered diet on aphid performance. A genotype of
Arabidopsis thaliana (L.) was produced in which the function of the amino acid permease gene AAP6 (At5g49630)
was abolished. Plants homozygous for the insertionally inactivated AAP6 gene had a significantly larger mean
rosette width than the wild type and a greater number of cauline leaves. Seeds from the aap6 mutant were also
significantly larger than those from the wild-type plants. Sieve element (SE) sap was collected by aphid stylectomy
and the amino acids derivatized, separated, and quantified using Capillary Electrophoresis with Laser Induced
Fluorescence (CE-LIF). In spite of the large variation across samples, the total amino acid concentration of SE sap of the aap6 mutant plants was significantly lower than that of the wild-type plants. The concentrations of lysine,
phenylalanine, leucine, and aspartic acid were all significantly lower in concentration in the aap6 mutant plants compared with wild-type plants. This is the first direct demonstration of a physiological role for an amino acid transporter in regulating SE composition in vivo. The amino acid availability in sieve element sap is thought to be the major limiting factor for aphid growth and reproduction. Despite the changes in their diet, the aphid Myzus persicae(Sulzer) displayed only small changes in feeding behaviour on mutant plants when measured using the Electronic Penetration Graph (EPG) technique. Salivation by the aphid into the SE (E1 phase) was increased on mutant plants but there was no significant effect on other feeding EPG behaviours, or in the rate of honeydew production.
Consistent with the small effect on aphid feeding behaviour, there was only a small effect of reduced sieve element amino acid concentration on aphid reproduction. The data are discussed in relation to the regulation of phloem
composition and the role of phloem amino acids in regulating aphid performance
A study of chiral symmetry in quenched QCD using the Overlap-Dirac operator
We compute fermionic observables relevant to the study of chiral symmetry in
quenched QCD using the Overlap-Dirac operator for a wide range of the fermion
mass. We use analytical results to disentangle the contribution from exact zero
modes and simplify our numerical computations. Details concerning the numerical
implementation of the Overlap-Dirac operator are presented.Comment: 24 pages revtex with 5 postscript figures included by eps
Improved Quenched QCD on Large Lattices - First Results
Continuing our investigations of quenched QCD with improved fermions we have
started simulations for lattice size 32^3 x 64 at beta=6.2. We present first
results for light hadron masses at kappa=0.13520, 0.13540, and 0.13555.
Moreover we compare our initial experiences on the T3E with those for
APE/Quadrics systems.Comment: 3 pages, Latex2e, 4 figures, espcrc2, epsfig and latexsym require
The Mass Spectrum of Light and Heavy Hadrons from Improved Lattice Actions
We use improved lattice actions for glue, light quarks and heavy quarks for
which we use lattice NRQCD to compute hadron masses. Our results are in good
agreement with experiment, except for charmed hadrons. It seems that charmed
quar ks are not well approximated as heavy quarks nor as light quarks.Comment: 14 pages +6 pages figures, plain-tex fil
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