1,510 research outputs found
Particle production in field theories coupled to strong external sources I. Formalism and main results
We develop a formalism for particle production in a field theory coupled to a
strong time-dependent external source. An example of such a theory is the Color
Glass Condensate. We derive a formula, in terms of cut vacuum-vacuum Feynman
graphs, for the probability of producing a given number of particles. This
formula is valid to all orders in the coupling constant. The distribution of
multiplicities is non--Poissonian, even in the classical approximation. We
investigate an alternative method of calculating the mean multiplicity. At
leading order, the average multiplicity can be expressed in terms of retarded
solutions of classical equations of motion. We demonstrate that the average
multiplicity at {\it next-to-leading order} can be formulated as an initial
value problem by solving equations of motion for small fluctuation fields with
retarded boundary conditions. The variance of the distribution can be
calculated in a similar fashion. Our formalism therefore provides a framework
to compute from first principles particle production in proton-nucleus and
nucleus-nucleus collisions beyond leading order in the coupling constant and to
all orders in the source density. We also provide a transparent interpretation
(in conventional field theory language) of the well known
Abramovsky-Gribov-Kancheli (AGK) cancellations. Explicit connections are made
between the framework for multi-particle production developed here and the
framework of Reggeon field theory.Comment: 44 pages, 19 postscript figures, version to appear in Nucl. Phys.
Sulfur K-Edge XAS Studies of the Effect of DNA Binding on the [Fe_4S_4] Site in EndoIII and MutY
S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe_4S_4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe–S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron–thiolate and iron–sulfide bonds would stabilize the oxidized state of the [Fe_4S_4] clusters. The results are compared to those on previously studied [Fe_4S_4] model complexes, ferredoxin (Fd), and to new data on high-potential iron–sulfur protein (HiPIP). A limited decrease in covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe_4S_4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron–sulfur bonds. In EndoIII, this change in covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP-like redox behavior
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Shifts of criteria or neural timing? The assumptions underlying timing perception studies
In timing perception studies, the timing of one event is usually manipulated relative to another, and participants are asked to judge if the two events were synchronous, or to judge which of the two events occurred first. Responses are analyzed to determine a measure of central tendency, which is taken as an estimate of the timing at which the two events are perceptually synchronous. When these estimates do not coincide with physical synchrony, it is often assumed that the sensory signals are asynchronous, as though the transfer of information concerning one input has been accelerated or decelerated relative to the other. Here we show that, while this is a viable interpretation, it is equally plausible that such effects are driven by shifts in the criteria used to differentiate simultaneous from asynchronous inputs. Our analyses expose important ambiguities concerning the interpretation of simultaneity judgement data, which have hitherto been underappreciated
The triple-pomeron regime and the structure function of the pomeron in the diffractive deep inelastic scattering at very small x
Misprints and numerical coefficients corrected, a bit of phenomenology and
one figure added. The case for the linear evolution of the unitarized structure
functions made stronger.Comment: KFA-IKP(Th)-1993-17, Landau-16/93, 46 pages, 14 figures upon request
from N.Nikolaev, [email protected]
QCD evolution of the gluon density in a nucleus
The Glauber approach to the gluon density in a nucleus, suggested by A.
Mueller, is developed and studied in detail. Using the GRV parameterization for
the gluon density in a nucleon, the value as well as energy and
dependence of the gluon density in a nucleus is calculated. It is shown that
the shadowing corrections are under theoretical control and are essential in
the region of small . They change crucially the value of the gluon density
as well as the value of the anomalous dimension of the nuclear structure
function, unlike of the nucleon one. The systematic theoretical way to treat
the correction to the Glauber approach is developed and a new evolution
equation is derived and solved. It is shown that the solution of the new
evolution equation can provide a selfconsistent matching of ``soft" high energy
phenomenology with ``hard" QCD physics.Comment: 63 pages,psfig.sty,25 pictures in eps.file
Concept Study of a fast VTOL-UAV Technology-Demonstrator for MUM-T
The Manned Unmanned Teaming (MUM-T) of rotorcraft offers the potential to increase the effectivity and survivability of the combined tactical unit. Currently, commercially available Unmanned Aerial Vehicles (UAVs) with Vertical Take-Off and Landing (VTOL) capability are not specifically designed for fast forward flight and would slow down the entire tactical unit. This study presents the first results of the development of a technology-demonstrator with a maximum airspeed of at least 180 kt. The investigation of different VTOL-UAV concepts, the selection of a thrust-compound configuration and the first details of the predesign are described. Furthermore, the flight performance is analyzed with focus on maximum airspeed, power, endurance and range. The results show, that the proposed design of the VTOL-UAV is expected to fulfill the requirements
Nuclear Shadowing in Electro-Weak Interactions
Shadowing is a quantum phenomenon leading to a non-additivity of electroweak
cross sections on nucleons bound in a nucleus. It occurs due to destructive
interference of amplitudes on different nucleons. Although the current
experimental evidence for shadowing is dominated by charged-lepton nucleus
scattering, studies of neutrino nucleus scattering have recently begun and
revealed unexpected results.Comment: 77 pages, 57 figures. To be published in "Progress in Particle and
Nuclear Physics" 201
Electrochemistry of the [4Fe4S] Cluster in Base Excision Repair Proteins: Tuning the Redox Potential with DNA
Escherichia coli endonuclease III (EndoIII) and MutY are DNA glycosylases that contain [4Fe4S] clusters and that serve to maintain the integrity of the genome after oxidative stress. Electrochemical studies on highly oriented pyrolytic graphite (HOPG) revealed that DNA binding by EndoIII leads to a large negative shift in the midpoint potential of the cluster, consistent with stabilization of the oxidized [4Fe4S]^(3+) form. However, the smooth, hydrophobic HOPG surface is nonideal for working with proteins in the absence of DNA. In this work, we use thin film voltammetry on a pyrolytic graphite edge electrode to overcome these limitations. Improved adsorption leads to substantial signals for both EndoIII and MutY in the absence of DNA, and a large negative potential shift is retained with DNA present. In contrast, the EndoIII mutants E200K, Y205H, and K208E, which provide electrostatic perturbations in the vicinity of the cluster, all show DNA-free potentials within error of wild type; similarly, the presence of negatively charged poly-L-glutamate does not lead to a significant potential shift. Overall, binding to the DNA polyanion is the dominant effect in tuning the redox potential of the [4Fe4S] cluster, helping to explain why all DNA-binding proteins with [4Fe4S] clusters studied to date have similar DNA-bound potentials
Rapid automatic assessment of microvascular density in sidestream dark field images
The purpose of this study was to develop a rapid and fully automatic method for the assessment of microvascular density and perfusion in sidestream dark field (SDF) images. We modified algorithms previously developed by our group for microvascular density assessment and introduced a new method for microvascular perfusion assessment. To validate the new algorithm for microvascular density assessment, we reanalyzed a selection of SDF video clips (n = 325) from a study in intensive care patients and compared the results to (semi-)manually found microvascular densities. The method for microvascular perfusion assessment (temporal SDF image contrast analysis, tSICA) was tested in several video simulations and in one high quality SDF video clip where the microcirculation was imaged before and during circulatory arrest in a cardiac surgery patient. We found that the new method for microvascular density assessment was very rapid (<30 s/clip) and correlated excellently with (semi-)manually measured microvascular density. The new method for microvascular perfusion assessment (tSICA) was shown to be limited by high cell densities and velocities, which severely impedes the applicability of this method in real SDF images. Hence, here we present a validated method for rapid and fully automatic assessment of microvascular density in SDF images. The new method was shown to be much faster than the conventional (semi-)manual method. Due to current SDF imaging hardware limitations, we were not able to automatically detect microvascular perfusion
Spin Structure of the Proton from Polarized Inclusive Deep-Inelastic Muon-Proton Scattering
We have measured the spin-dependent structure function in inclusive
deep-inelastic scattering of polarized muons off polarized protons, in the
kinematic range and . A
next-to-leading order QCD analysis is used to evolve the measured
to a fixed . The first moment of at is .
This result is below the prediction of the Ellis-Jaffe sum rule by more than
two standard deviations. The singlet axial charge is found to be . In the Adler-Bardeen factorization scheme, is
required to bring in agreement with the Quark-Parton Model. A
combined analysis of all available proton and deuteron data confirms the
Bjorken sum rule.Comment: 33 pages, 22 figures, uses ReVTex and smc.sty. submitted to Physical
Review
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