9,027 research outputs found
Low-energy diffraction; a direct-channel point of view: the background
We argue that at low-energies, typical of the resonance region, the
contribution from direct-channel exotic trajectories replaces the Pomeron
exchange, typical of high energies. A dual model realizing this idea is
suggested. While at high energies it matches the Regge pole behavior, dominated
by a Pomeron exchange, at low energies it produces a smooth, structureless
behavior of the total cross section determined by a direct-channel nonlinear
exotic trajectory, dual to the Pomeron exchange.Comment: 6 pages, 1 figure. Talk presented at the Second International
"Cetraro" Workshop & NATO Advanced Research Workshop "Diffraction 2002",
Alushta, Crimea, Ukraine, August 31 - September 6, 200
Equilibrium orbit analysis in a free-electron laser with a coaxial wiggler
An analysis of single-electron orbits in combined coaxial wiggler and axial
guide magnetic fields is presented. Solutions of the equations of motion are
developed in a form convenient for computing orbital velocity components and
trajectories in the radially dependent wiggler. Simple analytical solutions are
obtained in the radially-uniform-wiggler approximation and a formula for the
derivative of the axial velocity with respect to Lorentz factor
is derived. Results of numerical computations are presented and the
characteristics of the equilibrium orbits are discussed. The third spatial
harmonic of the coaxial wiggler field gives rise to group orbits which
are characterized by a strong negative mass regime.Comment: 13 pages, 9 figures, to appear in phys. rev.
Computing with cells: membrane systems - some complexity issues.
Membrane computing is a branch of natural computing which abstracts computing models from the structure and the functioning of the living cell. The main ingredients of membrane systems, called P systems, are (i) the membrane structure, which consists of a hierarchical arrangements of membranes which delimit compartments where (ii) multisets of symbols, called objects, evolve according to (iii) sets of rules which are localised and associated with compartments. By using the rules in a nondeterministic/deterministic maximally parallel manner, transitions between the system configurations can be obtained. A sequence of transitions is a computation of how the system is evolving. Various ways of controlling the transfer of objects from one membrane to another and applying the rules, as well as possibilities to dissolve, divide or create membranes have been studied. Membrane systems have a great potential for implementing massively concurrent systems in an efficient way that would allow us to solve currently intractable problems once future biotechnology gives way to a practical bio-realization. In this paper we survey some interesting and fundamental complexity issues such as universality vs. nonuniversality, determinism vs. nondeterminism, membrane and alphabet size hierarchies, characterizations of context-sensitive languages and other language classes and various notions of parallelism
Experimental analysis of lateral impact on planar brittle material
The fragmentation of alumina and glass plates due to lateral impact is
studied. A few hundred plates have been fragmented at different impact
velocities and the produced fragments are analyzed. The method employed in this
work allows one to investigate some geometrical properties of the fragments,
besides the traditional size distribution usually studied in former
experiments. We found that, although both materials exhibit qualitative similar
fragment size distribution function, their geometrical properties appear to be
quite different. A schematic model for two-dimensional fragmentation is also
presented and its predictions are compared to our experimental results. The
comparison suggests that the analysis of the fragments' geometrical properties
constitutes a more stringent test of the theoretical models' assumptions than
the size distribution
Statistical Mechanics of Linear and Nonlinear Time-Domain Ensemble Learning
Conventional ensemble learning combines students in the space domain. In this
paper, however, we combine students in the time domain and call it time-domain
ensemble learning. We analyze, compare, and discuss the generalization
performances regarding time-domain ensemble learning of both a linear model and
a nonlinear model. Analyzing in the framework of online learning using a
statistical mechanical method, we show the qualitatively different behaviors
between the two models. In a linear model, the dynamical behaviors of the
generalization error are monotonic. We analytically show that time-domain
ensemble learning is twice as effective as conventional ensemble learning.
Furthermore, the generalization error of a nonlinear model features
nonmonotonic dynamical behaviors when the learning rate is small. We
numerically show that the generalization performance can be improved remarkably
by using this phenomenon and the divergence of students in the time domain.Comment: 11 pages, 7 figure
Dynamic instabilities of fracture under biaxial strain using a phase field model
We present a phase field model of the propagation of fracture under plane
strain. This model, based on simple physical considerations, is able to
accurately reproduce the different behavior of cracks (the principle of local
symmetry, the Griffith and Irwin criteria, and mode-I branching). In addition,
we test our model against recent experimental findings showing the presence of
oscillating cracks under bi-axial load. Our model again reproduces well
observed supercritical Hopf bifurcation, and is therefore the first simulation
which does so
Ensemble learning of linear perceptron; Online learning theory
Within the framework of on-line learning, we study the generalization error
of an ensemble learning machine learning from a linear teacher perceptron. The
generalization error achieved by an ensemble of linear perceptrons having
homogeneous or inhomogeneous initial weight vectors is precisely calculated at
the thermodynamic limit of a large number of input elements and shows rich
behavior. Our main findings are as follows. For learning with homogeneous
initial weight vectors, the generalization error using an infinite number of
linear student perceptrons is equal to only half that of a single linear
perceptron, and converges with that of the infinite case with O(1/K) for a
finite number of K linear perceptrons. For learning with inhomogeneous initial
weight vectors, it is advantageous to use an approach of weighted averaging
over the output of the linear perceptrons, and we show the conditions under
which the optimal weights are constant during the learning process. The optimal
weights depend on only correlation of the initial weight vectors.Comment: 14 pages, 3 figures, submitted to Physical Review
A low stray light, high current, low energy electron source
A design of an electron gun system is presented whose stray light emission is reduced by about three orders of magnitude compared to a regular low-energy electron diffraction gun. This is achieved by a combination of a BaO cathode run at rather low temperature and a 30° tandem parallel-plate analyzer used as an optical baffle. The system provides a high beam current of several microampers at 50 eV beam energy. The system can be used down to ∼10 eV
Theoretical Investigation of Laser Induced Desorption of Small Molecules from Oxide Surfaces: A First Principles Study
State resolved laser induced desorption of NO molecules from a NiO(100) surface is studied theoretically. A full potential energy surface for the excited state was constructed by means of ab initio cluster calculations in addition to the potential energy surface for the ground state. Multidimensional wave packet calculations on these two surfaces allow a detailed simulation of experimental observables, such as velocity distributions and desorption probabilities, on a full ab initio basis
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