492 research outputs found
Decompactifications and Massless D-Branes in Hybrid Models
A method of determining the mass spectrum of BPS D-branes in any phase limit
of a gauged linear sigma model is introduced. A ring associated to monodromy is
defined and one considers K-theory to be a module over this ring. A simple but
interesting class of hybrid models with Landau-Ginzburg fibres over CPn are
analyzed using special Kaehler geometry and D-brane probes. In some cases the
hybrid limit is an infinite distance in moduli space and corresponds to a
decompactification. In other cases the hybrid limit is at a finite distance and
acquires massless D-branes. An example studied appears to correspond to a novel
theory of supergravity with an SU(2) gauge symmetry where the gauge and
gravitational couplings are necessarily tied to each other.Comment: PDF-LaTeX, 34 pages, 2 mps figure
An Approach to the Cosmological Constant Problem(s)
We propose an approach to explaining why naive large quantum fluctuations are
not the right estimate for the cosmological constant. We argue that the
universe is in a superposition of many vacua, in such a way that the resulting
fluctuations are suppressed by level repulsion to a very small value. The
approach combines several aspects of string theory and the early history of the
universe, and is only valid if several assumptions hold true. The approach may
also explain why the effective cosmological constant reamins small as the
universe evolves though several phase transitions. It provides a non-anthropic
mechansim leading to a small, non-zero cosmological constant.Comment: Talk given at Rencontres de Moriond, 2004 by G.L. Kan
Recent Efforts in the Computation of String Couplings:
We review recent advances towards the computation of string couplings.
Duality symmetry, mirror symmetry, Picard-Fuchs equations, etc. are some of the
tools.Comment: Talk hold at the `International Conference on Modern Problems in
Quantum Field Theorie, Strings and Quantum Gravity', Kiev, June 1992, 18
page
Excitation of a Kaluza-Klein mode by parametric resonance
In this paper we investigate a parametric resonance phenomenon of a
Kaluza-Klein mode in a -dimensional generalized Kaluza-Klein theory. As the
origin of the parametric resonance we consider a small oscillation of a scale
of the compactification around a today's value of it. To make our arguments
definite and for simplicity we consider two classes of models of the
compactification: those by () and those by (, ). For these models we show that
parametric resonance can occur for the Kaluza-Klein mode. After that, we give
formulas of a creation rate and a number of created quanta of the Kaluza-Klein
mode due to the parametric resonance, taking into account the first and the
second resonance band. By using the formulas we calculate those quantities for
each model of the compactification. Finally we give conditions for the
parametric resonance to be efficient and discuss cosmological implications.Comment: 36 pages, Latex file, Accepted for publication in Physical Review
M-theory moduli spaces and torsion-free structures
Motivated by the description of M-theory compactifications to
four-dimensions given by Exceptional Generalized Geometry, we propose a way to
geometrize the M-theory fluxes by appropriately relating the compactification
space to a higher-dimensional manifold equipped with a torsion-free structure.
As a non-trivial example of this proposal, we construct a bijection from the
set of -structures on an eight-dimensional -bundle to the set
of -structures on the base space, fully characterizing the
-torsion clases when the total space is equipped with a torsion-free
-structure. Finally, we elaborate on how the higher-dimensional
manifold and its moduli space of torsion-free structures can be used to obtain
information about the moduli space of M-theory compactifications.Comment: 24 pages. Typos fixed. Minor clarifications adde
Effective actions on the squashed three-sphere
The effective actions of a scalar and massless spin-half field are determined
as functions of the deformation of a symmetrically squashed three-sphere. The
extreme oblate case is particularly examined as pertinant to a high temperature
statistical mechanical interpretation that may be relevant for the holographic
principle. Interpreting the squashing parameter as a temperature, we find that
the effective `free energies' on the three-sphere are mixtures of thermal
two-sphere scalars and spinors which, in the case of the spinor on the
three-sphere, have the `wrong' thermal periodicities. However the free energies
do have the same leading high temperature forms as the standard free energies
on the two-sphere. The next few terms in the high-temperature expansion are
also explicitly calculated and briefly compared with the Taub-Bolt-AdS bulk
result.Comment: 23 pages, JyTeX. Conclusion slightly amended, one equation and minor
misprints correcte
EEG characterization of the Alzheimer's disease continuum by means of multiscale entropies
Alzheimer's disease (AD) is a neurodegenerative disorder with high prevalence, known for its highly disabling symptoms. The aim of this study was to characterize the alterations in the irregularity and the complexity of the brain activity along the AD continuum. Both irregularity and complexity can be studied applying entropy-based measures throughout multiple temporal scales. In this regard, multiscale sample entropy (MSE) and refined multiscale spectral entropy (rMSSE) were calculated from electroencephalographic (EEG) data. Five minutes of resting-state EEG activity were recorded from 51 healthy controls, 51 mild cognitive impaired (MCI) subjects, 51 mild AD patients (ADMIL), 50 moderate AD patients (ADMOD), and 50 severe AD patients (ADSEV). Our results show statistically significant differences (p-values < 0.05, FDR-corrected Kruskal-Wallis test) between the five groups at each temporal scale. Additionally, average slope values and areas under MSE and rMSSE curves revealed significant changes in complexity mainly for controls vs. MCI, MCI vs. ADMIL and ADMOD vs. ADSEV comparisons (p-values < 0.05, FDR-corrected Mann-Whitney U-test). These findings indicate that MSE and rMSSE reflect the neuronal disturbances associated with the development of dementia, and may contribute to the development of new tools to track the AD progression.This research was supported by European Commission and European Regional Development Fund (FEDER) under project “Análisis y correlación entre el genoma completo y la actividad cerebral para la ayuda en el diagnóstico de la enfermedad de Alzheimer” (Cooperation Programme Interreg V-A Spain-Portugal, POCTEP 2014-2020); by “Ministerio de Ciencia, Innovación y Universidades” and FEDER under projects PGC2018-098214-A-I00 and DPI2017-84280-R; and by “Fundação para a Ciência e a Tecnologia/Ministério da Ciência, Tecnologia e Inovação” and FEDER under projects POCI-01-0145-FEDER-007274 and UID/MAT/00144/2013
Toric Elliptic Fibrations and F-Theory Compactifications
The 102581 flat toric elliptic fibrations over P^2 are identified among the
Calabi-Yau hypersurfaces that arise from the 473800776 reflexive 4-dimensional
polytopes. In order to analyze their elliptic fibration structure, we describe
the precise relation between the lattice polytope and the elliptic fibration.
The fiber-divisor-graph is introduced as a way to visualize the embedding of
the Kodaira fibers in the ambient toric fiber. In particular in the case of
non-split discriminant components, this description is far more accurate than
previous studies. The discriminant locus and Kodaria fibers groups of all
102581 elliptic fibrations are computed. The maximal gauge group is SU(27),
which would naively be in contradiction with 6-dimensional anomaly
cancellation.Comment: 40 pages, 14 figures, 3 table
F-theory on Genus-One Fibrations
We argue that M-theory compactified on an arbitrary genus-one fibration, that
is, an elliptic fibration which need not have a section, always has an F-theory
limit when the area of the genus-one fiber approaches zero. Such genus-one
fibrations can be easily constructed as toric hypersurfaces, and various
and models are presented as examples. To each
genus-one fibration one can associate a -function on the base as well as
an representation which together define the IIB axio-dilaton
and 7-brane content of the theory. The set of genus-one fibrations with the
same -function and representation, known as the
Tate-Shafarevich group, supplies an important degree of freedom in the
corresponding F-theory model which has not been studied carefully until now.
Six-dimensional anomaly cancellation as well as Witten's zero-mode count on
wrapped branes both imply corrections to the usual F-theory dictionary for some
of these models. In particular, neutral hypermultiplets which are localized at
codimension-two fibers can arise. (All previous known examples of localized
hypermultiplets were charged under the gauge group of the theory.) Finally, in
the absence of a section some novel monodromies of Kodaira fibers are allowed
which lead to new breaking patterns of non-Abelian gauge groups.Comment: 53 pages, 9 figures, 6 tables. v2: references adde
On the Classification of Quasihomogeneous Functions
We give a criterion for the existence of a non-degenerate quasihomogeneous
polynomial in a configuration, i.e. in the space of polynomials with a fixed
set of weights, and clarify the relation of this criterion to the necessary
condition derived from the formula for the Poincar\'e polynomial. We further
prove finiteness of the number of configurations for a given value of the
singularity index. For the value 3 of this index, which is of particular
interest in string theory, a constructive version of this proof implies an
algorithm for the calculation of all non-degenerate configurations.Comment: 12 page
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