1,711 research outputs found
On an easy transition from operator dynamics to generating functionals by Clifford algebras
Clifford geometric algebras of multivectors are treated in detail. These
algebras are build over a graded space and exhibit a grading or multivector
structure. The careful study of the endomorphisms of this space makes it clear,
that opposite Clifford algebras have to be used also. Based on this
mathematics, we give a fully Clifford algebraic account on generating
functionals, which is thereby geometric. The field operators are shown to be
Clifford and opposite Clifford maps. This picture relying on geometry does not
need positivity in principle. Furthermore, we propose a transition from
operator dynamics to corresponding generating functionals, which is based on
the algebraic techniques. As a calculational benefit, this transition is
considerable short compared to standard ones. The transition is not injective
(unique) and depends additionally on the choice of an ordering. We obtain a
direct and constructive connection between orderings and the explicit form of
the functional Hamiltonian. These orderings depend on the propagator of the
theory and thus on the ground state. This is invisible in path integral
formulations. The method is demonstrated within two examples, a non-linear
spinor field theory and spinor QED. Antisymmetrized and normal-ordered
functional equations are derived in both cases.Comment: 23p., 76kB, plain LaTeX, [email protected]
The search for planetary mass companions to field brown dwarfs with HST/NICMOS
We present the results of a high-resolution spectral differential imaging
survey of 12 nearby, relatively young field L dwarfs (<1 Gyr) carried out with
HST/NICMOS to search for planetary mass companions at small physical
separations from their host. The survey resolved two brown dwarf binaries: the
L dwarf system Kelu-1AB and the newly discovered L/T transition system 2MASS
J031059+164815AB. For both systems common proper motion has already been
confirmed in follow-up observations which have been published elsewhere. The
derived separations of the binaries are smaller than 6 AU and consistent with
previous brown dwarf binary statistics. Their mass ratios of q > 0.8 confirm
the preference for equal mass systems similar to a large number of other
surveys. Furthermore, we found tentative evidence for a companion to the L4
dwarf 2MASS W033703-175807, straddling the brown dwarf/planetary mass boundary
and revealing an uncommonly low mass ratio system (q ~ 0.2) compared to the
vast majority of previously found brown dwarf binaries. With a derived minimum
mass of 10 - 15 Mjup, a planetary nature of the secondary cannot be ruled out
yet. However, it seems more likely to be a very low mass brown dwarf secondary
at the border of the spectral T/Y transition regime, primarily due to its
similarities to recently found very cool T dwarfs. This would make it one of
the closest resolved brown dwarf binaries (0.087" 0.015", corresponding
to 2.52 0.44 AU at a distance of 29 pc) with the coolest (Teff ~ 600-630
K) and least massive companion to any L or T dwarf.Comment: 33 pages, 8 figures, 2 tables, accepted for publication by Ap
Resolving the L/T transition binary SDSS J2052-1609 AB
Binaries provide empirical key constraints for star formation theories, like
the overall binary fraction, mass ratio distribution and the separation
distribution. They play a crucial role to calibrate the output of theoretical
models, like absolute magnitudes, colors and effective temperature depending on
mass, metallicity and age. We present first results of our on-going
high-resolution imaging survey of late type brown dwarfs. The survey aims at
resolving tight brown dwarf binary systems to better constrain the T dwarf
binary fraction. We intent to follow-up the individual binaries to determine
orbital parameters. Using NACO at the VLT we performed AO-assisted
near-infrared observations of SDSS J2052-1609. High-spatial resolution images
of the T1 dwarf were obtained in H and Ks filters. We resolved SDSS J2052-1609
into a binary system with a separation of 0.101" \pm 0.001". Archival data from
HST/NICMOS taken one year previous to our observations proves the components to
be co-moving. Using the flux ratio between the components we infer J, H and Ks
magnitudes for the resolved system. From the near-IR colors we estimate
spectral types of T1 +1 -4 and T2.5 \pm 1 for component A and B, respectively.
A first estimate of the total system mass yields Mtot > 78 Mjup, assuming a
circular orbit.Comment: 5 pages, 3 figures, 3 tables, accepted for publication by A&
Sampling properties of random graphs: the degree distribution
We discuss two sampling schemes for selecting random subnets from a network:
Random sampling and connectivity dependent sampling, and investigate how the
degree distribution of a node in the network is affected by the two types of
sampling. Here we derive a necessary and sufficient condition that guarantees
that the degree distribution of the subnet and the true network belong to the
same family of probability distributions. For completely random sampling of
nodes we find that this condition is fulfilled by classical random graphs; for
the vast majority of networks this condition will, however, not be met. We
furthermore discuss the case where the probability of sampling a node depends
on the degree of a node and we find that even classical random graphs are no
longer closed under this sampling regime. We conclude by relating the results
to real {\it E.coli} protein interaction network data.Comment: accepted for publication in Phys.Rev.
Atomistic modelling of large-scale metal film growth fronts
We present simulations of metallization morphologies under ionized sputter
deposition conditions, obtained by a new theoretical approach. By means of
molecular dynamics simulations using a carefully designed interaction
potential, we analyze the surface adsorption, reflection, and etching reactions
taking place during Al physical vapor deposition, and calculate their relative
probability. These probabilities are then employed in a feature-scale
cellular-automaton simulator, which produces calculated film morphologies in
excellent agreement with scanning-electron-microscopy data on ionized sputter
deposition.Comment: RevTeX 4 pages, 2 figure
Probability Models for Degree Distributions of Protein Interaction Networks
The degree distribution of many biological and technological networks has
been described as a power-law distribution. While the degree distribution does
not capture all aspects of a network, it has often been suggested that its
functional form contains important clues as to underlying evolutionary
processes that have shaped the network. Generally, the functional form for the
degree distribution has been determined in an ad-hoc fashion, with clear
power-law like behaviour often only extending over a limited range of
connectivities. Here we apply formal model selection techniques to decide which
probability distribution best describes the degree distributions of protein
interaction networks. Contrary to previous studies this well defined approach
suggests that the degree distribution of many molecular networks is often
better described by distributions other than the popular power-law
distribution. This, in turn, suggests that simple, if elegant, models may not
necessarily help in the quantitative understanding of complex biological
processes.
PV-0327 Patient-specific motion management and adaptive respiratory gating in Pancreatic SBRT
Digitalitzat per Artypla
Avaliação de variedades de milheto para produção de forragem no litoral Sul do Rio Grande do Sul.
bitstream/item/30293/1/boletim-38.pd
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