66,634 research outputs found
Approximate solutions for the single soliton in a Skyrmion-type model with a dilaton scalar field
We consider the analytical properties of the single-soliton solution in a
Skyrmion-type Lagrangian that incorporates the scaling properties of quantum
chromodynamics (QCD) through the coupling of the chiral field to a scalar field
interpreted as a bound state of gluons. The model was proposed in previous
works to describe the Goldstone pions in a dense medium, being also useful for
studying the properties of nuclear matter and the in-medium properties of
mesons and nucleons. Guided by an asymptotic analysis of the Euler-Lagrange
equations, we propose approximate analytical representations for the single
soliton solution in terms of rational approximants exponentially localized.
Following the Pad\'e method, we construct a sequence of approximants from the
exact power series solutions near the origin. We find that the convergence of
the approximate representations to the numerical solutions is considerably
improved by taking the expansion coefficients as free parameters and then
minimizing the mass of the Skyrmion using our ans\"atze for the fields. We also
perform an analysis of convergence by computation of physical quantities
showing that the proposed analytical representations are very useful useful for
phenomenological calculations.Comment: 13 pages, 3 eps figures, version to be published in Phys.Rev.
A knowledge based software engineering environment testbed
The Carnegie Group Incorporated and Boeing Computer Services Company are developing a testbed which will provide a framework for integrating conventional software engineering tools with Artifical Intelligence (AI) tools to promote automation and productivity. The emphasis is on the transfer of AI technology to the software development process. Experiments relate to AI issues such as scaling up, inference, and knowledge representation. In its first year, the project has created a model of software development by representing software activities; developed a module representation formalism to specify the behavior and structure of software objects; integrated the model with the formalism to identify shared representation and inheritance mechanisms; demonstrated object programming by writing procedures and applying them to software objects; used data-directed and goal-directed reasoning to, respectively, infer the cause of bugs and evaluate the appropriateness of a configuration; and demonstrated knowledge-based graphics. Future plans include introduction of knowledge-based systems for rapid prototyping or rescheduling; natural language interfaces; blackboard architecture; and distributed processin
Candidate High Redshift and Primeval Galaxies in Hubble Deep Field South
We present the results of colour selection of candidate high redshift
galaxies in Hubble Deep Field South (HDF-S) using the Lyman dropout scheme. The
HDF-S data we discuss were taken in a number of different filters extending
from the near--UV (F300W) to the infrared (F222M) in two different fields. This
allows us to select candidates with redshifts from z~3 to z~12. We find 15
candidate z~3 objects (F300W dropouts), 1 candidate z~4 object (F450W dropout)
and 16 candidate z5 objects (F606W dropouts) in the ~ 4.7 arcmin^2 WFPC-2
field, 4 candidate z~6 (optical dropouts) and 1 candidate z~8 (F110W dropout)
in the 0.84 arcmin^2 NICMOS-3 field. No F160W dropouts are found (z~12). We
compare our selection technique with existing data for HDF-North and discuss
alternative interpretations of the objects. We conclude that there are a number
of lower redshift interlopers in the selections, including one previously
identified object (Treu et al. 1998), and reject those objects most likely to
be foreground contaminants. Even after this we conclude that the F606W dropout
list is likely to still contain substantial foreground contamination. The lack
of candidate very high redshift UV-luminous galaxies supports earlier
conclusions by Lanzetta et al. (1998). We discuss the morphologies and
luminosity functions of the high redshift objects, and their cosmological
implications.Comment: Accepted for publication in MNRA
A Systems-Based Approach to the Identification of User/Infrastructure Interdependencies as a Precursor to Identifying Opportunities to Improve Infrastructure Project Value/Cost Ratios
The bulk of the investment needed for infrastructure renewal in the United Kingdom will have to come from private
sector investors, who will require attractive value/cost ratios. Government recognises infrastructure interdependencies
can help deliver these, but returns remain uncertain. New business models are required to overcome this problem,
which take account of enterprise-centred infrastructure interdependencies (interdependencies between social and
economic enterprises and the infrastructures they use). The complex and closely coupled nature of enterprise and
infrastructure systems can stand in the way of identifying these interdependencies; however, model-based systems
engineering techniques offer a framework for dealing with this complexity. This paper describes research that the
iBUILD project is doing to develop a methodology for modelling the interdependencies between infrastructure and
the enterprises that use it, as a precursor to identifying opportunities to improve infrastructure project value/cost
ratios. The methodology involves: identifying the suite of policy, strategy and operational documents relating to the
enterprise-of-interest; eliciting system data from the documents and integrating it to create an enterprise system
model; and, generating N2 diagrams from the model to identify the interdependencies
An optimal system design process for a Mars roving vehicle
The problem of determining the optimal design for a Mars roving vehicle is considered. A system model is generated by consideration of the physical constraints on the design parameters and the requirement that the system be deliverable to the Mars surface. An expression which evaluates system performance relative to mission goals as a function of the design parameters only is developed. The use of nonlinear programming techniques to optimize the design is proposed and an example considering only two of the vehicle subsystems is formulated and solved
Towards understanding Regge trajectories in holographic QCD
We reassess a work done by Migdal on the spectrum of low-energy vector mesons
in QCD in the light of the AdS-QCD correspondence. Recently, a tantalizing
parallelism was suggested between Migdal's work and a family of holographic
duals of QCD. Despite the intriguing similarities, both approaches face a major
drawback: the spectrum is in conflict with well-tested Regge scaling. However,
it has recently been shown that holographic duals can be modified to accomodate
Regge behavior. Therefore, it is interesting to understand whether Regge
behavior can also be achieved in Migdal's approach. In this paper we
investigate this issue. We find that Migdal's approach, which is based on a
modified Pade approximant, is closely related to the issue of quark-hadron
duality breakdown in QCD.Comment: 17 pages, 1 figure. Typos fixed, references added, improved
discussion. Minor changes to match the journal versio
From microscopic to macroscopic descriptions of cell\ud migration on growing domains
Cell migration and growth are essential components of the development of multicellular organisms. The role of various cues in directing cell migration is widespread, in particular, the role of signals in the environment in the control of cell motility and directional guidance. In many cases, especially in developmental biology, growth of the domain also plays a large role in the distribution of cells and, in some cases, cell or signal distribution may actually drive domain growth. There is a ubiquitous use of partial differential equations (PDEs) for modelling the time evolution of cellular density and environmental cues. In the last twenty years, a lot of attention has been devoted to connecting macroscopic PDEs with more detailed microscopic models of cellular motility, including models of directional sensing and signal transduction pathways. However, domain growth is largely omitted in the literature. In this paper, individual-based models describing cell movement and domain growth are studied, and correspondence with a macroscopic-level PDE describing the evolution of cell density is demonstrated. The individual-based models are formulated in terms of random walkers on a lattice. Domain growth provides an extra mathematical challenge by making the lattice size variable over time. A reaction-diffusion master equation formalism is generalised to the case of growing lattices and used in the derivation of the macroscopic PDEs
Vector Meson Dominance as a first step in a systematic approximation: the pion vector form factor
Pade Approximants can be used to go beyond Vector Meson Dominance in a
systematic approximation. We illustrate this fact with the case of the pion
vector form factor and extract values for the first two coefficients of its
Taylor expansion. Pade Approximants are shown to be a useful and simple tool
for incorporating high-energy information, allowing an improved determination
of these Taylor coefficients.Comment: 13 pages, 7 figure
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