8,457 research outputs found
Loop and Path Spaces and Four-Dimensional BF Theories: Connections, Holonomies and Observables
We study the differential geometry of principal G-bundles whose base space is
the space of free paths (loops) on a manifold M. In particular we consider
connections defined in terms of pairs (A,B), where A is a connection for a
fixed principal bundle P(M,G) and B is a 2-form on M. The relevant curvatures,
parallel transports and holonomies are computed and their expressions in local
coordinates are exhibited. When the 2-form B is given by the curvature of A,
then the so-called non-abelian Stokes formula follows.
For a generic 2-form B, we distinguish the cases when the parallel transport
depends on the whole path of paths and when it depends only on the spanned
surface. In particular we discuss generalizations of the non-abelian Stokes
formula. We study also the invariance properties of the (trace of the) holonomy
under suitable transformation groups acting on the pairs (A,B).
In this way we are able to define observables for both topological and
non-topological quantum field theories of the BF type. In the non topological
case, the surface terms may be relevant for the understanding of the
quark-confinement problem. In the topological case the (perturbative)
four-dimensional quantum BF-theory is expected to yield invariants of imbedded
(or immersed) surfaces in a 4-manifold M.Comment: TeX, 39 page
Pre-Big Bang Scenario on Self-T-Dual Bouncing Branes
We consider a new class of 5-dimensional dilatonic actions which are
invariant under T-duality transformations along three compact coordinates,
provided that an appropriate potential is chosen. We show that the invariance
remains when we add a boundary term corresponding to a moving 3-brane, and we
study the effects of the T-duality symmetry on the brane cosmological
equations. We find that T-duality transformations in the bulk induce scale
factor duality on the brane, together with a change of sign of the pressure of
the brane cosmological matter. However, in a remarkable analogy with the
Pre-Big Bang scenario, the cosmological equations are unchanged. Finally, we
propose a model where the dual phases are connected through a scattering of the
brane induced by an effective potential. We show how this model can realise a
smooth, non-singular transition between a pre-Big Bang superinflationary
Universe and a post-Big Bang accelerating Universe.Comment: 18 pages, minor typos corrected, Sec. 2 expanded with more details on
the self-T-dual background, Sec.4 and 5 revised accordingly. Version to
appear on JCA
Aspects of Quantum Gravity in Cosmology
We review some aspects of quantum gravity in the context of cosmology. In
particular, we focus on models with a phenomenology accessible to current and
near-future observations, as the early Universe might be our only chance to
peep through the quantum gravity realm.Comment: 15 pages, 1 figure. Invited review for Modern Physics Letter A.
Version 2: minor typos corrected, few references adde
Four-Dimensional Yang-Mills Theory as a Deformation of Topological BF Theory
The classical action for pure Yang--Mills gauge theory can be formulated as a
deformation of the topological theory where, beside the two-form field
, one has to add one extra-field given by a one-form which transforms
as the difference of two connections. The ensuing action functional gives a
theory that is both classically and quantistically equivalent to the original
Yang--Mills theory. In order to prove such an equivalence, it is shown that the
dependency on the field can be gauged away completely. This gives rise
to a field theory that, for this reason, can be considered as semi-topological
or topological in some but not all the fields of the theory. The symmetry group
involved in this theory is an affine extension of the tangent gauge group
acting on the tangent bundle of the space of connections. A mathematical
analysis of this group action and of the relevant BRST complex is discussed in
details.Comment: 74 pages, LaTeX, minor corrections; to be published in Commun. Math.
Phy
International financial flows, domestic banks, and the economic development of the periphery: Italy 1861-1913
This paper analyses the impact of different sources of financing (foreign capital, migrants’ remittances, and domestic banks intermediation) on economic development in Italy between 1861 and WWI. Existing literature has analysed the role of these channels of financial intermediation
separately, while this paper for the first time considers them in conjunction.
Using IRF from a Cholesky identification structure of a VAR model and relying on an original dataset that combines the most recent series of several financial and economic aggregates, this paper shows that both international capital and domestic saving had a significant impact on investment, while remittances
did not. Foreign capital was invested directly, but also via domestic banks, in particular the “German-style” universal banks. Finally, foreign and d
omestic capital had different attitudes towards the types of investment (construction vs. plant, machinery and transport equipment) and industries they financed. Combined together, these results shed a new light on the process of economic development of Italy and, more generally, of peripheral economies in the age of the international gold standard
Exploiting exciton-exciton interactions in semiconductor quantum dots for quantum-information processing
We propose an all-optical implementation of quantum-information processing in
semiconductor quantum dots, where electron-hole excitations (excitons) serve as
the computational degrees of freedom (qubits). We show that the strong dot
confinement leads to an overall enhancement of Coulomb correlations and to a
strong renormalization of the excitonic states, which can be exploited for
performing conditional and unconditional qubit operations.Comment: 5 pages revtex, 2 encapsulated postscript figures. Accepted for
publication in Phys. Rev. B (Rapid Communication
Assembléias de aranhas em manchas de cerrado amplamente separadas no Estado de São Paulo, Brasil
Spider Assemblages in widely-separated patches of cerrado in São Paulo State, BrazilAssembléias de aranhas em manchas de cerrado amplamente separadas no Estado de São Paulo, Brasil
'Backgating' model including self-heating for low-frequency dispersive effects in III-V FETs
A new approach is proposed which takes into account both traps and thermal phenomena for the modelling of deviations between static and dynamic drain current characteristics in III-V field effect transistors. The model is based on the well-known `backgating' concept and can easily be identified on the basis of conventional static drain current characteristics and small-signal, low-frequency S parameters. Experimental results confirm the accuracy of the proposed mode
Multi-objective optimization of the operation and maintenance assets of an offshore wind farm using genetic algorithms
The first author was funded by the Marie Curie Actions of
the European Union’s Seventh Framework Programme FP7/2007-
2013/ under REA grant agreement number 607656 (OceaNet
project) and by the industrial partner James Fisher Marine Services
Ltd. Mojo Maritime (JFMS group) have provided access to
Mermaid to support, and for integration with, this research. This
work is also funded by the EPSRC (UK) grant for the SuperGen
United Kingdom Centre for Marine Energy Research (UKCMER)
[grant number: EP/P008682/1]This is the author accepted manuscriptThis paper explores the use of genetic algorithms to optimize the operation and maintenance (O&M) assets of an offshore wind farm. Three different methods are implemented in order to demonstrate the approach. The optimization problem simultaneously considers both the reliability characteristics of the offshore wind turbines and the composition of the maintenance fleet, seeking to identify the optimal configurations for the strategic assets.
These are evaluated in order to minimize the operating costs of the offshore farm while maximizing both its reliability and availability.
The considerations used for the application of genetic algorithms as an effective way to support the assets management are described, and a case study to show the applicability of the approach is presented.
The variation of the economic performance indicators as a consequence of the optimization procedure are discussed, and the implementation of this method in a wider computational framework for the O&M assets improvement introduced.European CommissionMojo Ocean Dynamics Ltd. T/A Mojo Maritime LtdEPSRC (UK) grant for the SuperGen United Kingdom Centre for Marine Energy Research (UKCMER
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