3,507 research outputs found
Massless D-Branes on Calabi-Yau Threefolds and Monodromy
We analyze the link between the occurrence of massless B-type D-branes for
specific values of moduli and monodromy around such points in the moduli space.
This allows us to propose a classification of all massless B-type D-branes at
any point in the moduli space of Calabi-Yau's. This classification then
justifies a previous conjecture due to Horja for the general form of monodromy.
Our analysis is based on using monodromies around points in moduli space where
a single D-brane becomes massless to generate monodromies around points where
an infinite number become massless. We discuss the various possibilities within
the classification.Comment: 29 pages, LaTeX2e, 3 figures, author order fixe
D-branes on Orbifolds and Topology Change
We consider D-branes on an orbifold and investigate the moduli
space of the D-brane world-volume gauge theory by using toric geometry and
gauged linear sigma models. For , we find that there are five phases,
which are topologically distinct and connected by flops to each other. We also
verify that non-geometric phases are projected out for cases as
expected. Resolutions of non-isolated singularities are also investigated.Comment: 21 pages + 12 eps figures, LaTeX, minor correction
On the Geometric Interpretation of N = 2 Superconformal Theories
We clarify certain important issues relevant for the geometric interpretation
of a large class of N = 2 superconformal theories. By fully exploiting the
phase structure of these theories (discovered in earlier works) we are able to
clearly identify their geometric content. One application is to present a
simple and natural resolution to the question of what constitutes the mirror of
a rigid Calabi-Yau manifold. We also discuss some other models with unusual
phase diagrams that highlight some subtle features regarding the geometric
content of conformal theories.Comment: 25 pages, note adde
Small Volumes in Compactified String Theory
We discuss some of the classical and quantum geometry associated to the
degeneration of cycles within a Calabi-Yau compactification. In particular, we
focus on the definition and properties of quantum volume, especially as it
applies to identifying the physics associated to loci in moduli space where
nonperturbative effects become manifest. We discuss some unusual features of
quantum volume relative to its classical counterpart.Comment: 22 pages, 5 eps figures, harvma
More Dual String Pairs From Orbifolding
We construct more dual pairs of type II-heterotic strings in four dimensions
with spacetime supersymmetry. On the type II side the construction
utilizes the various possible choices of K3 automorphisms with fixed points
which transform the holomorphic two-form nontrivially, and rotation plus
translation on . The Calabi-Yau orbifolds so obtained have non-zero Euler
numbers, so quantum corrections exist on the type IIA strings. The heterotic
string (asymmetric) orbifold duals are found which depend on going to the
enhanced symmetry points. Some aspects of the construction are discussed
including the role of the singularity and the possibility of going beyond the
adiabatic argument. Many of these examples have also orientifold analogs.Comment: LaTex, 13 page
Enhanced Gauge Symmetries and K3 Surfaces
String-string duality dictates that type IIA strings compactified on a K3
surface acquire non-abelian gauge groups for certain values of the K3 moduli.
We argue that, contrary to expectation, the theories for which such enhanced
gauge symmetries appear are not orbifolds in the string sense. For a specific
example we show that a theory with enhanced gauge symmetry and an orbifold
theory have the same classical K3 surface as a target space but the value of
the ``B-field'' differs. This raises the possibility that the conformal field
theory associated to a string theory with an enhanced gauge group is badly
behaved in some way.Comment: 9 pages, LaTe
Enhanced Gauge Symmetries and Calabi-Yau Threefolds
We consider the general case of a type IIA string compactified on a
Calabi-Yau manifold which has a heterotic dual description. It is shown that
the nonabelian gauge symmetries which can appear nonperturbatively in the type
II string but which are understood perturbatively in the heterotic string are
purely a result of string-string duality in six dimensions. We illustrate this
with some examples.Comment: 11 pages, LaTe
Phase Transitions In M-Theory And F-Theory
Phase transitions are studied in -theory and -theory. In -theory
compactification to five dimensions on a Calabi-Yau, there are
topology-changing transitions similar to those seen in conformal field theory,
but the non-geometrical phases known in conformal field theory are absent. At
boundaries of moduli space where such phases might have been expected, the
moduli space ends, by a conventional or unconventional physical mechanism. The
unconventional mechanisms, which roughly involve the appearance of tensionless
strings, can sometimes be better understood in -theory.Comment: 30 pp, harvman
On the Ubiquity of K3 Fibrations in String Duality
We consider the general case of N=2 dual pairs of type IIA/heterotic string
theories in four dimensions. We show that if the type IIA string in this pair
can be viewed as having been compactified on a Calabi-Yau manifold in the usual
way then this manifold must be of the form of a K3 fibration. We also see how
the bound on the rank of the gauge group of the perturbative heterotic string
has a natural interpretation on the type IIA side.Comment: LateX, 16 pages, 1 figure, minor corrections, reference adde
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