12,277 research outputs found
Stochastic dynamics of adhesion clusters under shared constant force and with rebinding
Single receptor-ligand bonds have finite lifetimes, so that biological
systems can dynamically react to changes in their environment. In cell
adhesion, adhesion bonds usually act cooperatively in adhesion clusters.
Outside the cellular context, adhesion clusters can be probed quantitatively by
attaching receptors and ligands to opposing surfaces. Here we present a
detailed theoretical analysis of the stochastic dynamics of a cluster of
parallel bonds under shared constant loading and with rebinding. Analytical
solutions for the appropriate one-step master equation are presented for
special cases, while the general case is treated with exact stochastic
simulations. If the completely dissociated state is modeled as an absorbing
boundary, mean cluster lifetime is finite and can be calculated exactly. We
also present a detailed analysis of fluctuation effects and discuss various
approximations to the full stochastic description.Comment: Revtex, 29 pages, 23 postscript figures included (some with reduced
image quality
A Development Environment for Visual Physics Analysis
The Visual Physics Analysis (VISPA) project integrates different aspects of
physics analyses into a graphical development environment. It addresses the
typical development cycle of (re-)designing, executing and verifying an
analysis. The project provides an extendable plug-in mechanism and includes
plug-ins for designing the analysis flow, for running the analysis on batch
systems, and for browsing the data content. The corresponding plug-ins are
based on an object-oriented toolkit for modular data analysis. We introduce the
main concepts of the project, describe the technical realization and
demonstrate the functionality in example applications
Performance of the CMS Pixel Detector at an upgraded LHC
The CMS experiment will include a pixel detector for pattern recognition and
vertexing. It will consist of three barrel layers and two endcaps on each side,
providing three space-points up to a pseudoraditity of 2.1. Taking into account
the expected limitations of its performance in the LHC environment an 8-9 layer
pixel detector for an upgraded LHC is discussed.Comment: Contribution to the 10th European Symposium on Semiconductor
Detectors, June 12 - 16, 2005 in Wildbad Kreuth, Germany. 6 pages, 4 figures,
1 table. Referee's comments implemente
A generative framework for the study of delusions
Despite the ubiquity of delusional information processing in psychopathology and everyday life, formal characterizations of such inferences are lacking. In this article, we propose a generative framework that entails a computational mechanism which, when implemented in a virtual agent and given new information, generates belief updates (i.e., inferences about the hidden causes of the information) that resemble those seen in individuals with delusions. We introduce a particular form of Dirichlet process mixture model with a sampling-based Bayesian inference algorithm. This procedure, depending on the setting of a single parameter, preferentially generates highly precise (i.e. over-fitting) explanations, which are compartmentalized and thus can co-exist despite being inconsistent with each other. Especially in ambiguous situations, this can provide the seed for delusional ideation. Further, we show by simulation how the excessive generation of such over-precise explanations leads to new information being integrated in a way that does not lead to a revision of established beliefs. In all configurations, whether delusional or not, the inference generated by our algorithm corresponds to Bayesian inference. Furthermore, the algorithm is fully compatible with hierarchical predictive coding. By virtue of these properties, the proposed model provides a basis for the empirical study and a step toward the characterization of the aberrant inferential processes underlying delusions
Building CMS Pixel Barrel Detectur Modules
For the barrel part of the CMS pixel tracker about 800 silicon pixel detector
modules are required. The modules are bump bonded, assembled and tested at the
Paul Scherrer Institute. This article describes the experience acquired during
the assembly of the first ~200 modules.Comment: 5 pages, 7 figures, Vertex200
When it Pays to Rush: Interpreting Morphogen Gradients Prior to Steady-State
During development, morphogen gradients precisely determine the position of
gene expression boundaries despite the inevitable presence of fluctuations.
Recent experiments suggest that some morphogen gradients may be interpreted
prior to reaching steady-state. Theoretical work has predicted that such
systems will be more robust to embryo-to-embryo fluctuations. By analysing two
experimentally motivated models of morphogen gradient formation, we investigate
the positional precision of gene expression boundaries determined by
pre-steady-state morphogen gradients in the presence of embryo-to-embryo
fluctuations, internal biochemical noise and variations in the timing of
morphogen measurement. Morphogens that are direct transcription factors are
found to be particularly sensitive to internal noise when interpreted prior to
steady-state, disadvantaging early measurement, even in the presence of large
embryo-to-embryo fluctuations. Morphogens interpreted by cell-surface receptors
can be measured prior to steady-state without significant decrease in
positional precision provided fluctuations in the timing of measurement are
small. Applying our results to experiment, we predict that Bicoid, a
transcription factor morphogen in Drosophila, is unlikely to be interpreted
prior to reaching steady-state. We also predict that Activin in Xenopus and
Nodal in zebrafish, morphogens interpreted by cell-surface receptors, can be
decoded in pre-steady-state.Comment: 18 pages, 3 figure
Qualification Procedures of the CMS Pixel Barrel Modules
The CMS pixel barrel system will consist of three layers built of about 800
modules. One module contains 66560 readout channels and the full pixel barrel
system about 48 million channels. It is mandatory to test each channel for
functionality, noise level, trimming mechanism, and bump bonding quality.
Different methods to determine the bump bonding yield with electrical
measurements have been developed. Measurements of several operational
parameters are also included in the qualification procedure. Among them are
pixel noise, gains and pedestals. Test and qualification procedures of the
pixel barrel modules are described and some results are presented.Comment: 7 Pages, 7 Figures. Contribution to Pixel 2005, September 5-8, 2005,
Bonn, Germna
Radiation hardness of CMS pixel barrel modules
Pixel detectors are used in the innermost part of the multi purpose
experiments at LHC and are therefore exposed to the highest fluences of
ionising radiation, which in this part of the detectors consists mainly of
charged pions. The radiation hardness of all detector components has thoroughly
been tested up to the fluences expected at the LHC. In case of an LHC upgrade,
the fluence will be much higher and it is not yet clear how long the present
pixel modules will stay operative in such a harsh environment. The aim of this
study was to establish such a limit as a benchmark for other possible detector
concepts considered for the upgrade.
As the sensors and the readout chip are the parts most sensitive to radiation
damage, samples consisting of a small pixel sensor bump-bonded to a CMS-readout
chip (PSI46V2.1) have been irradiated with positive 200 MeV pions at PSI up to
6E14 Neq and with 21 GeV protons at CERN up to 5E15 Neq.
After irradiation the response of the system to beta particles from a Sr-90
source was measured to characterise the charge collection efficiency of the
sensor. Radiation induced changes in the readout chip were also measured. The
results show that the present pixel modules can be expected to be still
operational after a fluence of 2.8E15 Neq. Samples irradiated up to 5E15 Neq
still see the beta particles. However, further tests are needed to confirm
whether a stable operation with high particle detection efficiency is possible
after such a high fluence.Comment: Contribution to the 11th European Symposium on Semiconductor
Detectors June 7-11, 2009 Wildbad Kreuth, German
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