3,442 research outputs found
Symplectically harmonic cohomology of nilmanifolds
This paper can be considered as an extension to our paper [On symplectically
harmonic forms on six-dimensional nilmanifolds, Comment. Math. Helv. 76 (2001),
n 1, 89-109]. Also, it contains a brief survey of recent results on
symplectically harmonic cohomology.Comment: 18 page
On the evolution of a large class of inhomogeneous scalar field cosmologies
The asymptotic behaviour of a family of inhomogeneous scalar field
cosmologies with exponential potential is studied. By introducing new variables
we can perform an almost complete analysis of the evolution of these
cosmologies. Unlike the homogeneous case (Bianchi type solutions), when k^2<2
the models do not isotropize due to the presence of the inhomogeneitiesComment: 23 pages, 1 figure. Submitted to Classical and Quantum Gravit
D-brane Inspired Fermion Mass Textures
In this paper, the issues of the quark mass hierarchies and the Cabbibo
Kobayashi Maskawa mixing are analyzed in a class of intersecting D-brane
configurations with Standard Model gauge symmetry. The relevant mass matrices
are constructed taking into account the constraints imposed by extra abelian
symmetries and anomaly cancelation conditions. Possible mass generating
mechanisms including perturbative as well as non-perturbative effects are
discussed and specific patterns of mass textures are found characterized by the
hierarchies of the scales where the various sources contribute. It is argued
that the Cholesky decomposition of the mass matrices is the most appropriate
way to determine the properties of these fermion mass patterns, while the
associated triangular mass matrix form provides a unified description of all
phenomenologically equivalent symmetric and non-symmetric mass matrices. An
elegant analytic formula is derived for the Cholesky triangular form of the
mass matrices where the entries are given as simple functions of the mass
eigenstates and the diagonalizing transformation entries. Finally, motivated by
the possibility of vanishing zero Yukawa mass entries in several D-brane and
F-theory constructions due to the geometry of the internal space, we analyse in
detail all possible texture-zeroes mass matrices within the proposed new
context. These new texture-zeroes are compared to those existing in the
literature while D-brane inspired cases are worked out in detail.Comment: 58 pages, 7 figure
Toward Realistic Intersecting D-Brane Models
We provide a pedagogical introduction to a recently studied class of
phenomenologically interesting string models, known as Intersecting D-Brane
Models. The gauge fields of the Standard-Model are localized on D-branes
wrapping certain compact cycles on an underlying geometry, whose intersections
can give rise to chiral fermions. We address the basic issues and also provide
an overview of the recent activity in this field. This article is intended to
serve non-experts with explanations of the fundamental aspects, and also to
provide some orientation for both experts and non-experts in this active field
of string phenomenology.Comment: 85 pages, 8 figures, Latex, Bibtex, v2: refs added, typos correcte
Standard-like Models as Type IIB Flux Vacua
We construct new semi-realistic Type IIB flux vacua on
orientifolds with three- and four- Standard Model (SM) families and up to three
units of quantized flux. The open-string sector is comprised of magnetized
D-branes and is T-dual to supersymmetric intersecting D6-brane constructions.
The SM sector contains magnetized D9-branes with negative D3-brane charge
contribution. There are large classes of such models and we present explicit
constructions for representative ones. In addition to models with one and two
units of quantized flux, we also construct the first three- and four-family
Standard-like models with supersymmetric fluxes, i.e. comprising three units of
quantized flux. Supergravity fluxes are due to the self-dual NS-NS and R-R
three-form field strength and they fix the toroidal complex structure moduli
and the dilaton. The supersymmetry conditions for the D-brane sector fix in
some models all three toroidal K\"ahler moduli. We also provide examples where
toroidal K\" ahler moduli are fixed by strong gauge dynamics on the ``hidden
sector'' D7-brane. Most of the models possess Higgs doublet pairs with Yukawa
couplings that can generate masses for quarks and leptons. The models have
(mainly right-) chiral exotics.Comment: 31 page
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