27,895 research outputs found
A study to determine the flight characteristics and handling qualitites of variable geometry spacecraft. Volume 3: Low L/D concept with fold-down wings
A study was conducted to determine the flight characteristics and wing deployment transients for a variable geometry spacecraft concept having a hypersonic lift-to-drag ratio near 1.0, and employing fold-down wings. Unpowered flight conditions were considered throughout the study. The body of the spacecraft uses a trapezoidal cross section. The variable geometry wings, stowed in the sides of the vehicle, are deployed at transonic speeds
Extension Bundles and the Standard Model
We construct a new class of stable vector bundles suitable for heterotic
string compactifications. Using these we describe a novel way to derive the
fermionic matter content of the Standard Model from the heterotic string. For
this we compactify on an elliptically fibered Calabi-Yau threefold X with two
sections, the B-fibration, a variant of the ordinary Weierstrass fibration,
which allows X to carry a free involution. We construct rank five vector
bundles, invariant under this involution, such that turning on a Wilson line we
obtain the Standard Model gauge group and find various three generation models.
This rank five bundle is derived from a stable rank four bundle that arises as
an extension of bundles pulled-back from the base and twisted by suitable line
bundles. We also give an account of various previous results and put the
present construction into perspective.Comment: 31 pages, harvmac, references adde
Occurrence of normal and anomalous diffusion in polygonal billiard channels
From extensive numerical simulations, we find that periodic polygonal
billiard channels with angles which are irrational multiples of pi generically
exhibit normal diffusion (linear growth of the mean squared displacement) when
they have a finite horizon, i.e. when no particle can travel arbitrarily far
without colliding. For the infinite horizon case we present numerical tests
showing that the mean squared displacement instead grows asymptotically as t
log t. When the unit cell contains accessible parallel scatterers, however, we
always find anomalous super-diffusion, i.e. power-law growth with an exponent
larger than 1. This behavior cannot be accounted for quantitatively by a simple
continuous-time random walk model. Instead, we argue that anomalous diffusion
correlates with the existence of families of propagating periodic orbits.
Finally we show that when a configuration with parallel scatterers is
approached there is a crossover from normal to anomalous diffusion, with the
diffusion coefficient exhibiting a power-law divergence.Comment: 9 pages, 15 figures. Revised after referee reports: redrawn figures,
additional comments. Some higher quality figures available at
http://www.fis.unam.mx/~dsander
The Topology of Branching Universes
The purpose of this paper is to survey the possible topologies of branching
space-times, and, in particular, to refute the popular notion in the literature
that a branching space-time requires a non-Hausdorff topology
Entanglement Entropy of Random Fractional Quantum Hall Systems
The entanglement entropy of the and quantum Hall
states in the presence of short range random disorder has been calculated by
direct diagonalization. A microscopic model of electron-electron interaction is
used, electrons are confined to a single Landau level and interact with long
range Coulomb interaction. For very weak disorder, the values of the
topological entanglement entropy are roughly consistent with expected
theoretical results. By considering a broader range of disorder strengths, the
fluctuation in the entanglement entropy was studied in an effort to detect
quantum phase transitions. In particular, there is a clear signature of a
transition as a function of the disorder strength for the state.
Prospects for using the density matrix renormalization group to compute the
entanglement entropy for larger system sizes are discussed.Comment: 29 pages, 16 figures; fixed figures and figure captions; revised
fluctuation calculation
SU(5) Heterotic Standard Model Bundles
We construct a class of stable SU(5) bundles on an elliptically fibered
Calabi-Yau threefold with two sections, a variant of the ordinary Weierstrass
fibration, which admits a free involution. The bundles are invariant under the
involution, solve the topological constraint imposed by the heterotic anomaly
equation and give three generations of Standard Model fermions after symmetry
breaking by Wilson lines of the intermediate SU(5) GUT-group to the Standard
Model gauge group. Among the solutions we find some which can be perturbed to
solutions of the Strominger system. Thus these solutions provide a step toward
the construction of phenomenologically realistic heterotic flux
compactifications via non-Kahler deformations of Calabi-Yau geometries with
bundles. This particular class of solutions involves a rank two hidden sector
bundle and does not require background fivebranes for anomaly cancellation.Comment: 17 page
Quantifying Model Complexity via Functional Decomposition for Better Post-Hoc Interpretability
Post-hoc model-agnostic interpretation methods such as partial dependence
plots can be employed to interpret complex machine learning models. While these
interpretation methods can be applied regardless of model complexity, they can
produce misleading and verbose results if the model is too complex, especially
w.r.t. feature interactions. To quantify the complexity of arbitrary machine
learning models, we propose model-agnostic complexity measures based on
functional decomposition: number of features used, interaction strength and
main effect complexity. We show that post-hoc interpretation of models that
minimize the three measures is more reliable and compact. Furthermore, we
demonstrate the application of these measures in a multi-objective optimization
approach which simultaneously minimizes loss and complexity
Optical properties of the vibrations in charged C molecules
The transition strengths for the four infrared-active vibrations of charged
C molecules are evaluated in self-consistent density functional theory
using the local density approximation. The oscillator strengths for the second
and fourth modes are strongly enhanced relative to the neutral C
molecule, in good agreement with the experimental observation of ``giant
resonances'' for those two modes. Previous theory, based on a ``charged
phonon'' model, predicted a quadratic dependence of the oscillator strength on
doping, but this is not borne out in our calculations.Comment: 10 pages, RevTeX3.
Heterotic Standard Model Moduli
In previous papers, we introduced a heterotic standard model and discussed
its basic properties. The Calabi-Yau threefold has, generically, three Kahler
and three complex structure moduli. The observable sector of this vacuum has
the spectrum of the MSSM with one additional pair of Higgs-Higgs conjugate
fields. The hidden sector has no charged matter in the strongly coupled string
and only minimal matter for weak coupling. Additionally, the spectrum of both
sectors will contain vector bundle moduli. The exact number of such moduli was
conjectured to be small, but was not explicitly computed. In this paper, we
rectify this and present a formalism for computing the number of vector bundle
moduli. Using this formalism, the number of moduli in both the observable and
strongly coupled hidden sectors is explicitly calculated.Comment: 28 pages, LaTeX; v2: typos corrected, references added; v3:
clarifications, references adde
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