25,438 research outputs found
Constructing Gravitational Dimensions
It would be extremely useful to know whether a particular low energy
effective theory might have come from a compactification of a higher
dimensional space. Here, this problem is approached from the ground up by
considering theories with multiple interacting massive gravitons. It is
actually very difficult to construct discrete gravitational dimensions which
have a local continuum limit. In fact, any model with only nearest neighbor
interactions is doomed. If we could find a non-linear extension for the
Fierz-Pauli Lagrangian for a graviton of mass mg which does not break down
until the scale Lambda_2=(mg Mpl)^(1/2), this could be used to construct a
large class of models whose continuum limit is local in the extra dimension.
But this is shown to be impossible: a theory with a single graviton must break
down by Lambda_3 = (mg^2 Mpl)^(1/3). Next, we look at how the discretization
prescribed by the truncation of the KK tower of an honest extra diemsinon
rasies the scale of strong coupling. It dictates an intricate set of
interactions among various fields which conspire to soften the strongest
scattering amplitudes and allow for a local continuum limit. A number of
canditate symmetries associated with locality in the discretized dimension are
also discussed.Comment: 21 pages, 6 diagrams, 1 figur
A characterization of quasi-rational polygons
The aim of this paper is to study quasi-rational polygons related to the
outer billiard. We compare different notions introduced, and make a synthesis
of those.Comment: 15 pages, 9 figure
Web ontology representation and reasoning via fragments of set theory
In this paper we use results from Computable Set Theory as a means to
represent and reason about description logics and rule languages for the
semantic web.
Specifically, we introduce the description logic \mathcal{DL}\langle
4LQS^R\rangle(\D)--admitting features such as min/max cardinality constructs
on the left-hand/right-hand side of inclusion axioms, role chain axioms, and
datatypes--which turns out to be quite expressive if compared with
\mathcal{SROIQ}(\D), the description logic underpinning the Web Ontology
Language OWL. Then we show that the consistency problem for
\mathcal{DL}\langle 4LQS^R\rangle(\D)-knowledge bases is decidable by
reducing it, through a suitable translation process, to the satisfiability
problem of the stratified fragment of set theory, involving variables
of four sorts and a restricted form of quantification. We prove also that,
under suitable not very restrictive constraints, the consistency problem for
\mathcal{DL}\langle 4LQS^R\rangle(\D)-knowledge bases is
\textbf{NP}-complete. Finally, we provide a -translation of rules
belonging to the Semantic Web Rule Language (SWRL)
Discrete Gravitational Dimensions
We study the physics of a single discrete gravitational extra dimension using
the effective field theory for massive gravitons. We first consider a minimal
discretization with 4D gravitons on the sites and nearest neighbor hopping
terms. At the linear level, 5D continuum physics is recovered correctly, but at
the non-linear level the theory becomes highly non-local in the discrete
dimension. There is a peculiar UV/IR connection, where the scale of strong
interactions at high energies is related to the radius of the dimension. These
new effects formally vanish in the limit of zero lattice spacing, but do not do
so quickly enough to reproduce the continuum physics consistently in an
effective field theory up to the 5D Planck scale. Nevertheless, this model does
make sense as an effective theory up to energies parametrically higher than the
compactification scale. In order to have a discrete theory that appears local
in the continuum limit, the lattice action must have interactions between
distant sites. We speculate on the relevance of these observations to the
construction of finite discrete theories of gravity in four dimensions.Comment: 5 pages, 4 diagrams. Important typos in some equations corrected;
conclusion s unchange
The Highest Redshift Relativistic Jets
We describe our efforts to understand large-scale (10's-100's kpc)
relativistic jet systems through observations of the highest-redshift quasars.
Results from a VLA survey search for radio jets in ~30 z>3.4 quasars are
described along with new Chandra observations of 4 selected targets.Comment: 5 pages, 2 figures, to appear in Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray, Eds. T.A. Rector and D.S. De Youn
Temporal behavior of two-wave-mixing in photorefractive InP:Fe versus temperature
The temporal response of two-wave-mixing in photorefractive InP:Fe under a dc
electric field at different temperatures has been studied. In particular, the
temperature dependence of the characteristic time constant has been studied
both theoretically and experimentally, showing a strongly decreasing time
constant with increasing temperature
Tensors Mesons in AdS/QCD
We explore tensor mesons in AdS/QCD focusing on f2 (1270), the lightest
spin-two resonance in QCD. We find that the f2 mass and the partial width for
f2 -> gamma gamma are in very good agreement with data. In fact, the
dimensionless ratio of these two quantities comes out within the current
experimental bound. The result for this ratio depends only on Nc and Nf, and
the quark and glueball content of the operator responsible for the f2; more
importantly, it does not depend on chiral symmetry breaking and so is both
independent of much of the arbitrariness of AdS/QCD and completely out of reach
of chiral perturbation theory. For comparison, we also explore f2 -> pi pi,
which because of its sensitivity to the UV corrections has much more
uncertainty. We also calculate the masses of the higher spin resonances on the
Regge trajectory of the f2, and find they compare favorably with experiment.Comment: 21 pages, 1 figure; Li's correcte
An Integrated XRF/XRD Instrument for Mars Exobiology and Geology Experiments
By employing an integrated x-ray instrument on a future Mars mission, data obtained will greatly augment those returned by Viking; details characterizing the past and present environment on Mars and those relevant to the possibility of the origin and evolution of life will be acquired. A combined x-ray fluorescence/x-ray diffraction (XRF/XRD) instrument was breadboarded and demonstrated to accommodate important exobiology and geology experiment objectives outlined for MESUR and future Mars missions. Among others, primary objectives for the exploration of Mars include the intense study of local areas on Mars to establish the chemical, mineralogical, and petrological character of different components of the surface material; to determine the distribution, abundance, and sources and sinks of volatile materials, including an assessment of the biologic potential, now and during past epoches; and to establish the global chemical and physical characteristics of the Martian surface. The XRF/XRD breadboard instrument identifies and quantifies soil surface elemental, mineralogical, and petrological characteristics and acquires data necessary to address questions on volatile abundance and distribution. Additionally, the breadboard is able to characterize the biogenic element constituents of soil samples providing information on the biologic potential of the Mars environment. Preliminary breadboard experiments confirmed the fundamental instrument design approach and measurement performance
In-flight calibration of the Herschel-SPIRE instrument
SPIRE, the Spectral and Photometric Imaging REceiver, is the Herschel Space Observatory's submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 μm, and an imaging Fourier-transform spectrometer (FTS) covering 194–671 μm (447-1550 GHz). In this paper we describe the initial approach taken to the absolute calibration of the SPIRE instrument using a combination of the emission from the Herschel telescope itself and the modelled continuum emission from solar system objects and other astronomical targets. We present the photometric, spectroscopic and spatial accuracy that is obtainable in data processed through the “standard” pipelines. The overall photometric accuracy at this stage of the mission is estimated as 15% for the photometer and between 15 and 50% for the spectrometer. However, there remain issues with the photometric accuracy of the spectra of low flux sources in the longest wavelength part of the SPIRE spectrometer band. The spectrometer wavelength accuracy is determined to be better than 1/10th of the line FWHM. The astrometric accuracy in SPIRE maps is found to be 2 arcsec when the latest calibration data are used. The photometric calibration of the SPIRE instrument is currently determined by a combination of uncertainties in the model spectra of the astronomical standards and the data processing methods employed for map and spectrum calibration. Improvements in processing techniques and a better understanding of the instrument performance will lead to the final calibration accuracy of SPIRE being determined only by uncertainties in the models of astronomical standards
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