127,717 research outputs found
A Direct Multigrid Poisson Solver for Oct-Tree Adaptive Meshes
We describe a finite-volume method for solving the Poisson equation on
oct-tree adaptive meshes using direct solvers for individual mesh blocks. The
method is a modified version of the method presented by Huang and Greengard
(2000), which works with finite-difference meshes and does not allow for shared
boundaries between refined patches. Our algorithm is implemented within the
FLASH code framework and makes use of the PARAMESH library, permitting
efficient use of parallel computers. We describe the algorithm and present test
results that demonstrate its accuracy.Comment: 10 pages, 6 figures, accepted by the Astrophysical Journal; minor
revisions in response to referee's comments; added char
Finite width effects and gauge cancellations in W- and Z-boson production in framework of modified perturbation theory
The processes of production and subsequent decay of W- and Z-bosons in collisions are considered in a recently proposed modified perturbation
theory (PT), based on a direct expansion of probabilities instead of
amplitudes. In such an approach the non-integrable singularities in the phase
space, which are intrinsic in the conventional PT, appear as singularities in
the coupling constant (with subsequent compensation by the decay factors of
unstable particles). In the present paper the systematic investigation of the
modified PT is carried out. The results are compared with the results of the
conventional approach, based on calculation of the amplitude with Dyson
resummation. A solution to the problem of the loss of one-loop PT order in the
resonance region is found. On the basis of this solution the proof of gauge
cancellations in any order of the modified PT is given. A simple generalization
of the fermion-loop scheme is proposed which provides a complete description of
W-pair production in next-to-leading order approximation.Comment: 25 pages, 2 figures; minor changes in the text, version to appear in
Eur.Phys.J.
Correction Factors for Reactions involving Quark-Antiquark Annihilation or Production
In reactions with production or annihilation, initial-
and final-state interactions give rise to large corrections to the lowest-order
cross sections. We evaluate the correction factor first for low relative
kinetic energies by studying the distortion of the relative wave function. We
then follow the procedure of Schwinger to interpolate this result with the
well-known perturbative QCD vertex correction factors at high energies, to
obtain an explicit semi-empirical correction factor applicable to the whole
range of energies. The correction factor predicts an enhancement for
in color-singlet states and a suppression for color-octet states, the effect
increasing as the relative velocity decreases. Consequences on dilepton
production in the quark-gluon plasma, the Drell-Yan process, and heavy quark
production processes are discussed.Comment: 25 pages (REVTeX), includes 2 uuencoded compressed postscript figure
Extending CKKW-merging to One-Loop Matrix Elements
We extend earlier schemes for merging tree-level matrix elements with parton
showers to include also merging with one-loop matrix elements. In this paper we
make a first study on how to include one-loop corrections, not only for events
with a given jet multiplicity, but simultaneously for several different jet
multiplicities. Results are presented for the simplest non-trivial case of
hadronic events at LEP as a proof-of-concept
Extreme Value Statistics and Traveling Fronts: Various Applications
An intriguing connection between extreme value statistics and traveling
fronts has been found recently in a number of diverse problems. In this brief
review we outline a few such problems and consider their various applications.Comment: A brief review (6 pages, 2 figures) to appear in Physica A as part of
the proceedings of Statphys-Kolkata IV (2002
Stabilization of the Electroweak Vacuum by a Scalar Threshold Effect
We show how a heavy scalar singlet with a large vacuum expectation value can
evade the potential instability of the Standard Model electroweak vacuum. The
quartic interaction between the heavy scalar singlet and the Higgs doublet
leads to a positive tree-level threshold correction for the Higgs quartic
coupling, which is very effective in stabilizing the potential. We provide
examples, such as the see-saw, invisible axion and unitarized Higgs inflation,
where the proposed mechanism is automatically implemented in well-defined
ranges of Higgs masses.Comment: 18 pages, 5 figure
Reconstructing the Forest of Lineage Trees of Diverse Bacterial Communities Using Bio-inspired Image Analysis
Cell segmentation and tracking allow us to extract a plethora of cell
attributes from bacterial time-lapse cell movies, thus promoting computational
modeling and simulation of biological processes down to the single-cell level.
However, to analyze successfully complex cell movies, imaging multiple
interacting bacterial clones as they grow and merge to generate overcrowded
bacterial communities with thousands of cells in the field of view,
segmentation results should be near perfect to warrant good tracking results.
We introduce here a fully automated closed-loop bio-inspired computational
strategy that exploits prior knowledge about the expected structure of a
colony's lineage tree to locate and correct segmentation errors in analyzed
movie frames. We show that this correction strategy is effective, resulting in
improved cell tracking and consequently trustworthy deep colony lineage trees.
Our image analysis approach has the unique capability to keep tracking cells
even after clonal subpopulations merge in the movie. This enables the
reconstruction of the complete Forest of Lineage Trees (FLT) representation of
evolving multi-clonal bacterial communities. Moreover, the percentage of valid
cell trajectories extracted from the image analysis almost doubles after
segmentation correction. This plethora of trustworthy data extracted from a
complex cell movie analysis enables single-cell analytics as a tool for
addressing compelling questions for human health, such as understanding the
role of single-cell stochasticity in antibiotics resistance without losing site
of the inter-cellular interactions and microenvironment effects that may shape
it
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