8,714 research outputs found
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
Group-Theoretical Aspects of Orbifold and Conifold GUTs
Motivated by the simplicity and direct phenomenological applicability of
field-theoretic orbifold constructions in the context of grand unification, we
set out to survey the immensely rich group-theoretical possibilities open to
this type of model building. In particular, we show how every maximal-rank,
regular subgroup of a simple Lie group can be obtained by orbifolding and
determine under which conditions rank reduction is possible. We investigate how
standard model matter can arise from the higher-dimensional SUSY gauge
multiplet. New model building options arise if, giving up the global orbifold
construction, generic conical singularities and generic gauge twists associated
with these singularities are considered. Viewed from the purely field-theoretic
perspective, such models, which one might call conifold GUTs, require only a
very mild relaxation of the constraints of orbifold model building. Our most
interesting concrete examples include the breaking of E_7 to SU(5) and of E_8
to SU(4)xSU(2)xSU(2) (with extra factor groups), where three generations of
standard model matter come from the gauge sector and the families are
interrelated either by SU(3) R-symmetry or by an SU(3) flavour subgroup of the
original gauge group.Comment: references adde
Gravity-Yang-Mills-Higgs unification by enlarging the gauge group
We revisit an old idea that gravity can be unified with Yang-Mills theory by
enlarging the gauge group of gravity formulated as gauge theory. Our starting
point is an action that describes a generally covariant gauge theory for a
group G. The Minkowski background breaks the gauge group by selecting in it a
preferred gravitational SU(2) subgroup. We expand the action around this
background and find the spectrum of linearized theory to consist of the usual
gravitons plus Yang-Mills fields charged under the centralizer of the SU(2) in
G. In addition, there is a set of Higgs fields that are charged both under the
gravitational and Yang-Mills subgroups. These fields are generically massive
and interact with both gravity and Yang-Mills sector in the standard way. The
arising interaction of the Yang-Mills sector with gravity is also standard.
Parameters such as the Yang-Mills coupling constant and Higgs mass arise from
the potential function defining the theory. Both are realistic in the sense
explained in the paper.Comment: 61 pages, no figures (v2) some typos correcte
Unification Scale, Proton Decay, And Manifolds Of G_2 Holonomy
Models of particle physics based on manifolds of holonomy are in most
respects much more complicated than other string-derived models, but as we show
here they do have one simplification: threshold corrections to grand
unification are particularly simple. We compute these corrections, getting
completely explicit results in some simple cases. We estimate the relation
between Newton's constant, the GUT scale, and the value of , and
explore the implications for proton decay. In the case of proton decay, there
is an interesting mechanism which (relative to four-dimensional SUSY GUT's)
enhances the gauge boson contribution to compared to other
modes such as or . Because of numerical
uncertainties, we do not know whether to intepret this as an enhancement of the
mode or a suppression of the others.Comment: 40 p
Local models for intersecting brane worlds
We describe the construction of configurations of D6-branes wrapped on
compact 3-cycles intersecting at points in non-compact Calabi-Yau threefolds.
Such constructions provide local models of intersecting brane worlds, and
describe sectors of four-dimensional gauge theories with chiral fermions. We
present several classes of non-compact manifolds with compact 3-cycles
intersecting at points, and discuss the rules required for model building with
wrapped D6-branes. The rules to build 3-cycles are simple, and allow easy
computation of chiral spectra, RR tadpoles and the amount of preserved
supersymmetry. We present several explicit examples of these constructions,
some of which have Standard Model like gauge group and three quark-lepton
generations. In some cases, mirror symmetry relates the models to other
constructions used in phenomenological D-brane model building, like D-branes at
singularities. Some simple N=1 supersymmetric configurations may lead to
relatively tractable G_2 manifolds upon lift to M-theory, which would be
non-compact but nevertheless yield four-dimensional chiral gauge field
theories.Comment: 52 pages, latex, 24 enclosed eps figures; v2. typo correcte
Introduction to Loop Quantum Gravity
This article is based on the opening lecture at the third quantum geometry
and quantum gravity school sponsored by the European Science Foundation and
held at Zakopane, Poland in March 2011. The goal of the lecture was to present
a broad perspective on loop quantum gravity for young researchers. The first
part is addressed to beginning students and the second to young researchers who
are already working in quantum gravity.Comment: 30 pages, 2 figures. arXiv admin note: substantial text overlap with
arXiv:gr-qc/041005
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