10,254 research outputs found

### Stability Walls in Heterotic Theories

We study the sub-structure of the heterotic Kahler moduli space due to the
presence of non-Abelian internal gauge fields from the perspective of the
four-dimensional effective theory. Internal gauge fields can be supersymmetric
in some regions of the Kahler moduli space but break supersymmetry in others.
In the context of the four-dimensional theory, we investigate what happens when
the Kahler moduli are changed from the supersymmetric to the non-supersymmetric
region. Our results provide a low-energy description of supersymmetry breaking
by internal gauge fields as well as a physical picture for the mathematical
notion of bundle stability. Specifically, we find that at the transition
between the two regions an additional anomalous U(1) symmetry appears under
which some of the states in the low-energy theory acquire charges. We compute
the associated D-term contribution to the four-dimensional potential which
contains a Kahler-moduli dependent Fayet-Iliopoulos term and contributions from
the charged states. We show that this D-term correctly reproduces the expected
physics. Several mathematical conclusions concerning vector bundle stability
are drawn from our arguments. We also discuss possible physical applications of
our results to heterotic model building and moduli stabilization.Comment: 37 pages, 4 figure

### Yukawa Textures From Heterotic Stability Walls

A holomorphic vector bundle on a Calabi-Yau threefold, X, with h^{1,1}(X)>1
can have regions of its Kahler cone where it is slope-stable, that is, where
the four-dimensional theory is N=1 supersymmetric, bounded by "walls of
stability". On these walls the bundle becomes poly-stable, decomposing into a
direct sum, and the low energy gauge group is enhanced by at least one
anomalous U(1) gauge factor. In this paper, we show that these additional
symmetries can strongly constrain the superpotential in the stable region,
leading to non-trivial textures of Yukawa interactions and restrictions on
allowed masses for vector-like pairs of matter multiplets. The Yukawa textures
exhibit a hierarchy; large couplings arise on the stability wall and some
suppressed interactions "grow back" off the wall, where the extended U(1)
symmetries are spontaneously broken. A number of explicit examples are
presented involving both one and two stability walls, with different
decompositions of the bundle structure group. A three family standard-like
model with no vector-like pairs is given as an example of a class of SU(4)
bundles that has a naturally heavy third quark/lepton family. Finally, we
present the complete set of Yukawa textures that can arise for any holomorphic
bundle with one stability wall where the structure group breaks into two
factors.Comment: 53 pages, 4 figures and 13 table

### Stabilizing the Complex Structure in Heterotic Calabi-Yau Vacua

In this paper, we show that the presence of gauge fields in heterotic
Calabi-Yau compacitifications causes the stabilisation of some, or all, of the
complex structure moduli of the Calabi-Yau manifold while maintaining a
Minkowski vacuum. Certain deformations of the Calabi-Yau complex structure,
with all other moduli held fixed, can lead to the gauge bundle becoming
non-holomorphic and, hence, non-supersymmetric. This leads to an F-term
potential which stabilizes the corresponding complex structure moduli. We use
10- and 4-dimensional field theory arguments as well as a derivation based
purely on algebraic geometry to show that this picture is indeed correct. An
explicit example is presented in which a large subset of complex structure
moduli is fixed. We demonstrate that this type of theory can serve as the
hidden sector in heterotic vacua and can co-exist with realistic particle
physics.Comment: 17 pages, Late

### Fluid-Induced Propulsion of Rigid Particles in Wormlike Micellar Solutions

In the absence of inertia, a reciprocal swimmer achieves no net motion in a
viscous Newtonian fluid. Here, we investigate the ability of a reciprocally
actuated particle to translate through a complex fluid that possesses a network
using tracking methods and birefringence imaging. A geometrically polar
particle, a rod with a bead on one end, is reciprocally rotated using magnetic
fields. The particle is immersed in a wormlike micellar (WLM) solution that is
known to be susceptible to the formation of shear bands and other localized
structures due to shear-induced remodeling of its microstructure. Results show
that the nonlinearities present in this WLM solution break time-reversal
symmetry under certain conditions, and enable propulsion of an artificial
"swimmer." We find three regimes dependent on the Deborah number (De): net
motion towards the bead-end of the particle at low De, net motion towards the
rod-end of the particle at intermediate De, and no appreciable propulsion at
high De. At low De, where the particle time-scale is longer then the fluid
relaxation time, we believe that propulsion is caused by an imbalance in the
fluid first normal stress differences between the two ends of the particle
(bead and rod). At De~1, however, we observe the emergence of a region of
network anisotropy near the rod using birefringence imaging. This anisotropy
suggests alignment of the micellar network, which is "locked in" due to the
shorter time-scale of the particle relative to the fluid

### Heterotic Line Bundle Standard Models

In a previous publication, arXiv:1106.4804, we have found 200 models from
heterotic Calabi-Yau compactifications with line bundles, which lead to
standard models after taking appropriate quotients by a discrete symmetry and
introducing Wilson lines. In this paper, we construct the resulting standard
models explicitly, compute their spectrum including Higgs multiplets, and
analyze some of their basic properties. After removing redundancies we find
about 400 downstairs models, each with the precise matter spectrum of the
supersymmetric standard model, with one, two or three pairs of Higgs doublets
and no exotics of any kind. In addition to the standard model gauge group, up
to four Green-Schwarz anomalous U(1) symmetries are present in these models,
which constrain the allowed operators in the four-dimensional effective
supergravity. The vector bosons associated to these anomalous U(1) symmetries
are massive. We explicitly compute the spectrum of allowed operators for each
model and present the results, together with the defining data of the models,
in a database of standard models accessible at
http://www-thphys.physics.ox.ac.uk/projects/CalabiYau/linebundlemodels/index.html.
Based on these results we analyze elementary phenomenological properties. For
example, for about 200 models all dimension four and five proton decay
violating operators are forbidden by the additional U(1) symmetries.Comment: 55 pages, Latex, 3 pdf figure

### Aerosol physical properties in the stratosphere (APPS) radiometer design

The measurement concepts and radiometer design developed to obtain earth-limb spectral radiance measurements for the Aerosol Physical Properties in the Stratosphere (APPS) measurement program are presented. The measurements made by a radiometer of this design can be inverted to yield vertical profiles of Rayleigh scatterers, ozone, nitrogen dioxide, aerosol extinction, and aerosol physical properties, including a Junge size-distribution parameter, and a real and imaginary index of refraction. The radiometer design provides the capacity for remote sensing of stratospheric constituents from space on platforms such as the space shuttle and satellites, and therefore provides for global measurements on a daily basis

### Tree-Independent Dual-Tree Algorithms

Dual-tree algorithms are a widely used class of branch-and-bound algorithms.
Unfortunately, developing dual-tree algorithms for use with different trees and
problems is often complex and burdensome. We introduce a four-part logical
split: the tree, the traversal, the point-to-point base case, and the pruning
rule. We provide a meta-algorithm which allows development of dual-tree
algorithms in a tree-independent manner and easy extension to entirely new
types of trees. Representations are provided for five common algorithms; for
k-nearest neighbor search, this leads to a novel, tighter pruning bound. The
meta-algorithm also allows straightforward extensions to massively parallel
settings.Comment: accepted in ICML 201

### Stabilizing All Geometric Moduli in Heterotic Calabi-Yau Vacua

We propose a scenario to stabilize all geometric moduli - that is, the
complex structure, Kahler moduli and the dilaton - in smooth heterotic
Calabi-Yau compactifications without Neveu-Schwarz three-form flux. This is
accomplished using the gauge bundle required in any heterotic compactification,
whose perturbative effects on the moduli are combined with non-perturbative
corrections. We argue that, for appropriate gauge bundles, all complex
structure and a large number of other moduli can be perturbatively stabilized -
in the most restrictive case, leaving only one combination of Kahler moduli and
the dilaton as a flat direction. At this stage, the remaining moduli space
consists of Minkowski vacua. That is, the perturbative superpotential vanishes
in the vacuum without the necessity to fine-tune flux. Finally, we incorporate
non-perturbative effects such as gaugino condensation and/or instantons. These
are strongly constrained by the anomalous U(1) symmetries which arise from the
required bundle constructions. We present a specific example, with a consistent
choice of non-perturbative effects, where all remaining flat directions are
stabilized in an AdS vacuum.Comment: 24 pages, 2 figure

### Numerical Hermitian Yang-Mills Connections and Vector Bundle Stability in Heterotic Theories

A numerical algorithm is presented for explicitly computing the gauge
connection on slope-stable holomorphic vector bundles on Calabi-Yau manifolds.
To illustrate this algorithm, we calculate the connections on stable monad
bundles defined on the K3 twofold and Quintic threefold. An error measure is
introduced to determine how closely our algorithmic connection approximates a
solution to the Hermitian Yang-Mills equations. We then extend our results by
investigating the behavior of non slope-stable bundles. In a variety of
examples, it is shown that the failure of these bundles to satisfy the
Hermitian Yang-Mills equations, including field-strength singularities, can be
accurately reproduced numerically. These results make it possible to
numerically determine whether or not a vector bundle is slope-stable, thus
providing an important new tool in the exploration of heterotic vacua.Comment: 52 pages, 15 figures. LaTex formatting of figures corrected in
version 2

### Heterotic Compactification, An Algorithmic Approach

We approach string phenomenology from the perspective of computational
algebraic geometry, by providing new and efficient techniques for proving
stability and calculating particle spectra in heterotic compactifications. This
is done in the context of complete intersection Calabi-Yau manifolds in a
single projective space where we classify positive monad bundles. Using a
combination of analytic methods and computer algebra we prove stability for all
such bundles and compute the complete particle spectrum, including gauge
singlets. In particular, we find that the number of anti-generations vanishes
for all our bundles and that the spectrum is manifestly moduli-dependent.Comment: 36 pages, Late

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