917 research outputs found
Supersymmetric deformations of F1-NS5-branes and their exact CFT description
We consider certain classes of operators in the exact conformal field theory
SL(2,R) x SU(2) x U(1)^4 describing strings in an AdS(3) x S(3) x T4 geometry
supported by Neveu--Schwarz 3-form fluxes. This background arises in the
near-horizon limit of a system of NS5-branes wrapped on a 4-torus and F1-branes
smeared on the 4-torus when both types of branes are located at the same point
in their common transverse space. We find a class of operators that lead to
spacetime supersymmetric deformations. It is remarkable that most of these
operators are not chiral primary with respect to the N=2 superconformal algebra
on the wordsheet. A subset of these worldsheet conformal field theory
deformations admits an interpretation either as a geometric deformation of the
brane system or as a deformation of the distribution of the F1-branes, viewed
as smooth instantons, inside the wrapped NS5-brane worldvolume. The
2-dimensional conformal field theory, however, seems to lack operators
corresponding to arbitrary NS5-brane deformations, in contrast to pure
NS5-brane systems where all geometric deformations can be accounted for by
chiral primary operators.Comment: 30+1 pages, 1 table; v2 minor changes, version to appear in JHE
R^2 Corrections and Non-perturbative Dualities of N=4 String ground states
We compute and analyse a variety of four-derivative gravitational terms in
the effective action of six- and four-dimensional type II string ground states
with N=4 supersymmetry. In six dimensions, we compute the relevant perturbative
corrections for the type II string compactified on K3. In four dimensions we do
analogous computations for several models with (4,0) and (2,2) supersymmetry.
Such ground states are related by heterotic-type II duality or type II-type II
U-duality. Perturbative computations in one member of a dual pair give a
non-perturbative result in the other member. In particular, the exact CP-even
R^2 coupling on the (2,2) side reproduces the tree-level term plus NS 5-brane
instanton contributions on the (4,0) side. On the other hand, the exact CP-odd
coupling yields the one-loop axionic interaction a.R\wedge R together with a
similar instanton sum. In a subset of models, the expected breaking of the
SL(2,Z)_S S-duality symmetry to a \Gamma(2)_S subgroup is observed on the
non-perturbative thresholds. Moreover, we present a duality chain that provides
evidence for the existence of heterotic N=4 models in which N=8 supersymmetry
appears at strong coupling.Comment: Latex2e, 51 pages, 1 figur
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On reconciling macro and micro energy transport forecasts for strategic decision making in the tanker industry
We propose the use of hierarchical structures for forecasting freight earnings. Hierarchical time series approaches are applied in the dry-bulk and tanker markets to identify the most suitable strategy for forecasting freight rates. We argue that decision making for shipping practitioners should be based on forecasts of freight earnings at different hierarchical levels. In other words, different strategic shipping decisions such as operations management, choice of freight charter contract and type of investment should be matched with the appropriate level of forecasts of freight earnings that are aggregated at different macro-levels: operating route, vessel size and type of trade
Bose-Einstein condensation in linear sigma model at Hartree and large N approximation
The BEC of charged pions is investigated in the framework of O(4) linear
sigma model. By using Cornwall-Jackiw-Tomboulis formalism, we have derived the
gap equations for the effective masses of the mesons at finite temperature and
finite isospin density. The BEC is discussed in chiral limit and non-chiral
limit at Hartree approximation and also at large N approximation.Comment: 11 pages, 9 figure
Fast and broadband fiber dispersion measurement with dense wavelength sampling
We report on a method to obtain dispersion measurements from spectral-domain low-coherence interferograms which enables high accuracy (~ps/(nm·km)), broadband measurements and the determination of very dense (up to 20 points/nm over 500 nm) data sets for both dispersion and dispersion slope. The method exploits a novel phase extraction algorithm which allows the phase associated with each sampling point of the interferogram to be calculated and provides for very accurate results as well as a fast measurement capability, enabling close to real time measurements. The important issue of mitigating the measurement errors due to any residual dispersion of optical elements and to environmental fluctuations was also addressed. We performed systematic measurements on standard fibers which illustrate the accuracy and precision of the technique, and we demonstrated its general applicability to challenging problems by measuring a carefully selected set of microstructured fibers: a lead silicate W-type fiber with a flat, near-zero dispersion profile; a hollow core photonic bandgap fiber with strongly wavelength dependent dispersion and dispersion slope; a small core, highly birefringent index guiding microstructured fiber, for which polarization resolved measurements over an exceptionally wide (~1000 nm) wavelength interval were obtained
An Overview of the Use of the SimSphere Soil Vegetation Atmosphere Transfer (SVAT) Model for the Study of Land-Atmosphere Interactions
Soil Vegetation Atmosphere Transfer (SVAT) models consist of deterministic mathematical representations of the physical processes involved between the land surface and the atmosphere and of their interactions, at time-steps acceptable for the study of land surface processes. The present article provides a comprehensive and systematic review of one such SVAT model suitable for use in mesoscale or boundary layer studies, originally developed by [1]. This model, which has evolved significantly both architecturally and functionally since its foundation, has been widely applied in over thirty interdisciplinary science investigations, and it is currently used as a learning resource for students in a number of educational institutes globally. The present review is also regarded as very timely, since a variation of a method using this specific SVAT model along with satellite observations is currently being considered in a scheme being developed for the operational retrieval of soil surface moisture by the US National Polar-orbiting Operational Environmental Satellite System (NPOESS), in a series of satellites that are due to be launched from 2016 onwards
Comment on superluminality in general relativity
General relativity provides an appropriate framework for addressing the issue
of sub- or superluminality as an apparent effect. Even though a massless
particle travels on the light cone, its average velocity over a finite path
measured by different observers is not necessarily equal to the velocity of
light, as a consequence of the time dilation or contraction in gravitational
fields. This phenomenon occurs in either direction (increase or depletion)
irrespectively of the details and strength of the gravitational interaction.
Hence, it does not intrinsically guarantee superluminality, even when the
gravitational field is reinforced.Comment: 6 page
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