130 research outputs found
Hawking Spectrum and High Frequency Dispersion
We study the spectrum of created particles in two-dimensional black hole
geometries for a linear, hermitian scalar field satisfying a Lorentz
non-invariant field equation with higher spatial derivative terms that are
suppressed by powers of a fundamental momentum scale . The preferred frame
is the ``free-fall frame" of the black hole. This model is a variation of
Unruh's sonic black hole analogy. We find that there are two qualitatively
different types of particle production in this model: a thermal Hawking flux
generated by ``mode conversion" at the black hole horizon, and a non-thermal
spectrum generated via scattering off the background into negative free-fall
frequency modes. This second process has nothing to do with black holes and
does not occur for the ordinary wave equation because such modes do not
propagate outside the horizon with positive Killing frequency. The horizon
component of the radiation is astonishingly close to a perfect thermal
spectrum: for the smoothest metric studied, with Hawking temperature
, agreement is of order at frequency
, and agreement to order persists out to
where the thermal number flux is ). The flux
from scattering dominates at large and becomes many orders of
magnitude larger than the horizon component for metrics with a ``kink", i.e. a
region of high curvature localized on a static worldline outside the horizon.
This non-thermal flux amounts to roughly 10\% of the total luminosity for the
kinkier metrics considered. The flux exhibits oscillations as a function of
frequency which can be explained by interference between the various
contributions to the flux.Comment: 32 pages, plain latex, 16 figures included using psfi
Nonperturbative Renormalization and the QCD Vacuum
We present a self consistent approach to Coulomb gauge Hamiltonian QCD which
allows one to relate single gluon spectral properties to the long range
behavior of the confining interaction. Nonperturbative renormalization is
discussed. The numerical results are in good agreement with phenomenological
and lattice forms of the static potential.Comment: 23 pages in RevTex, 4 postscript figure
On the Origin of the Outgoing Black Hole Modes
The question of how to account for the outgoing black hole modes without
drawing upon a transplanckian reservoir at the horizon is addressed. It is
argued that the outgoing modes must arise via conversion from ingoing modes. It
is further argued that the back-reaction must be included to avoid the
conclusion that particle creation cannot occur in a strictly stationary
background. The process of ``mode conversion" is known in plasma physics by
this name and in condensed matter physics as ``Andreev reflection" or ``branch
conversion". It is illustrated here in a linear Lorentz non-invariant model
introduced by Unruh. The role of interactions and a physical short distance
cutoff is then examined in the sonic black hole formed with Helium-II.Comment: 12 pages, plain latex, 2 figures included using psfig; Analogy to
``Andreev reflection" in superfluid systems noted, references and
acknowledgment added, format changed to shorten tex
Coulomb Gauge QCD, Confinement, and the Constituent Representation
Quark confinement and the genesis of the constituent quark model are examined
in nonperturbative QCD in Coulomb gauge. We employ a self-consistent method to
construct a quasiparticle basis and to determine the quasiparticle interaction.
The results agree remarkably well with lattice computations. They also
illustrate the mechanism by which confinement and constituent quarks emerge,
provide support for the Gribov-Zwanziger confinement scenario, clarify several
perplexing issues in the constituent quark model, and permit the construction
of an improved model of low energy QCD.Comment: 43 pages, 14 figures, revtex, uses psfig.st
ASTERIX and 2.0 Knowledge Management : exploring the appropriation of 2.0 KMS via the Myth of the Gaulish village
International audienceKnowledge Management Systems (KMS) in companies have profoundly changed in recent years. They have become KMS 2.0 that aim to transform the firm and are driven by a new relationship to knowledge in line with 2.0 organisations. These 2.0 KMS have implemented modes of organisation that disrupt those that previously guided firmsâ performance. This can sometimes lead to paradoxical organizational dysfunctions as witnessed by the difficulties faced by some traditionally hierarchical French companies. Through a case study of Constructor and a theoretical background on IS appropriation in organizations and myths in management, we show how the Asterix myth contributes to understanding how 2.0. KMS are appropriated in such companies. We find evidence of similarities regarding knowledge and Knowledge Management between the Asterixâ myth and the behaviours and practices concerning knowledge management within Constructor. As a result, the Asterixâ myth may be a relevant perspective for understanding the obstacles, advantages and appropriations of 2.0. KMS within French organizations
The nuclear envelope proteome differs notably between tissues
One hypothesis to explain how mutations in the same nuclear envelope proteins yield pathologies focused in distinct tissues is that as yet unidentified tissue-specific partners mediate the disease pathologies. The nuclear envelope proteome was recently determined from leukocytes and muscle. Here the same methodology is applied to liver and a direct comparison of the liver, muscle and leukocyte data sets is presented. At least 74 novel transmembrane proteins identified in these studies have been directly confirmed at the nuclear envelope. Within this set, RT-PCR, western blot and staining of tissue cryosections confirms that the protein complement of the nuclear envelope is clearly distinct from one tissue to another. Bioinformatics reveals similar divergence between tissues across the larger data sets. For proteins acting in complexes according to interactome data, the whole complex often exhibited the same tissue-specificity. Other tissue-specific nuclear envelope proteins identified were known proteins with functions in signaling and gene regulation. The high tissue specificity in the nuclear envelope likely underlies the complex disease pathologies and argues that all organelle proteomes warrant re-examination in multiple tissues
Cassava whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), in sub-Saharan African farming landscapes: a review of the factors determining abundance
Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a pest species complex that causes widespread damage to cassava, a staple food crop for millions of smallholder households in Sub-Saharan Africa. Species in the complex cause direct feeding damage to cassava and are the vectors of multiple plant viruses. Whilst significant work has gone into developing virus-resistant cassava cultivars, there has been little research effort aimed at understanding the ecology of these insect vectors. In this review we critically assess the knowledge base relating to factors that may lead to high population densities of Sub-Saharan African (SSA) Bemisia tabaci species in cassava production landscapes of East Africa. We focus first on empirical studies that have examined biotic or abiotic factors that may lead to high populations. We then identify knowledge gaps that need to be filled to deliver long-term sustainable solutions to manage both the vectors and the viruses that they transmit. We found that whilst many hypotheses have been put forward to explain the increases in abundance witnessed since the early 1990s, there are little available published data and these tend to have been collected in a piecemeal manner. The most critical knowledge gaps identified were: (i) understanding how cassava cultivars and alternative host plants impact B. tabaci population dynamics and its natural enemies; (ii) the impact of natural enemies in terms of reducing the frequency of outbreaks and (iii) the use and management of insecticides to delay or avoid the development of resistance. In addition, there are several fundamental methodologies that need to be developed and deployed in East Africa to address some of the more challenging knowledge gaps
Particle acceleration mechanisms
We review the possible mechanisms for production of non-thermal electrons
which are responsible for non-thermal radiation in clusters of galaxies. Our
primary focus is on non-thermal Bremsstrahlung and inverse Compton scattering,
that produce hard X-ray emission. We briefly review acceleration mechanisms and
point out that in most astrophysical situations, and in particular for the
intracluster medium, shocks, turbulence and plasma waves play a crucial role.
We consider two scenarios for production of non-thermal radiation. The first is
hard X-ray emission due to non-thermal Bremsstrahlung by nonrelativistic
particles. Non-thermal tails are produced by accelerating electrons from the
background plasma with an initial Maxwellian distribution. However, these tails
are accompanied by significant heating and they are present for a short time of
<10^6 yr, which is also the time that the tail will be thermalised. Such
non-thermal tails, even if possible, can only explain the hard X-ray but not
the radio emission which needs GeV or higher energy electrons. For these and
for production of hard X-rays by the inverse Compton model, we need the second
scenario where there is injection and subsequent acceleration of relativistic
electrons. It is shown that a steady state situation, for example arising from
secondary electrons produced from cosmic ray proton scattering by background
protons, will most likely lead to flatter than required electron spectra or it
requires a short escape time of the electrons from the cluster. An episodic
injection of relativistic electrons, presumably from galaxies or AGN, and/or
episodic generation of turbulence and shocks by mergers can result in an
electron spectrum consistent with observations but for only a short period of
less than one billion years.Comment: 22 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 11; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Search for the Xb and other hidden-beauty states in the Ï+ÏâÏ(1S) channel at ATLAS
This Letter presents a search for a hidden-beauty counterpart of the X(3872) in the mass ranges of 10.05â10.31 GeV and 10.40â11.00 GeV, in the channel XbâÏ+ÏâÏ(1S)(âÎŒ+ÎŒâ), using 16.2 fbâ1 of pp  collision data collected by the ATLAS detector at the LHC. No evidence for new narrow states is found, and upper limits are set on the product of the Xb cross section and branching fraction, relative to those of the Ï(2S), at the 95% confidence level using the CLS approach. These limits range from 0.8% to 4.0%, depending on mass. For masses above 10.1 GeV, the expected upper limits from this analysis are the most restrictive to date. Searches for production of the Ï(13DJ), , and states also reveal no significant signals
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