106 research outputs found
Preasymptotic effects in beauty decays
Large preasymptotic effects in beauty decays have been found using
heavy-quark and SU(3) symmetry, as well as experimental data on charmed
hyperons. Contrary to rather uniform beauty-meson lifetimes, a much larger
spread of beauty-baryon lifetimes is predicted. However, it is highly unlikely
that, theoretically, the ratio, which at
present deviates more than from the experimental result, can be
lowered below 0.9.Comment: 6 pages, talk given by B. Melic at The 3rd International Conference
on B Physics and CP Violation, Taipei, Taiwan, December 3-7, 1999, to appear
in the Proceedings, two figures adde
Inclusive decays and lifetimes of doubly charmed baryons
The analysis of singly charmed hadrons has been extended to the case of
doubly charmed baryons, , and .
Doubly charmed baryons are described as a system containing a heavy
-diquark and a light quark, similarly as in a heavy-light meson. This leads
to preasymptotic effects in semileptonic and nonleptonic decays which are
essentially proportional to the meson wave function. Interplay between
preasymptotic effects in semileptonic and/or nonleptonic decay rates leads to
very clear predictions for semileptonic branching ratios and lifetimes of
doubly charmed baryons.Comment: 14 pages latex file, 4 figures, 1 tabl
Asymptotic behavior of Cardassian cosmologies with exponential potentials
In this paper we analyze the asymptotic behavior of Cardassian cosmological
models filled with a perfect fluid and a scalar field with an exponential
potential. Cardassian cosmologies arise from modifications of the Friedmann
equation, and among the different proposals within that framework we will
choose those of the form with . We construct two
three dimensional dynamical systems arising from the evolution equations,
respectively adapted for studying the high and low energy limits. Using
standard dynamical systems techniques we find the fixed points and characterize
the solutions they represent. We pay especial attention to the properties
inherent to the modifications and compare with the (standard) unmodified
scenario. Among other interesting results, we find there are no late-time
tracking attractors.Comment: 9 pages, 8 figures, revtex
Wikipedias: Collaborative web-based encyclopedias as complex networks
Wikipedia is a popular web-based encyclopedia edited freely and
collaboratively by its users. In this paper we present an analysis of
Wikipedias in several languages as complex networks. The hyperlinks pointing
from one Wikipedia article to another are treated as directed links while the
articles represent the nodes of the network. We show that many network
characteristics are common to different language versions of Wikipedia, such as
their degree distributions, growth, topology, reciprocity, clustering,
assortativity, path lengths and triad significance profiles. These
regularities, found in the ensemble of Wikipedias in different languages and of
different sizes, point to the existence of a unique growth process. We also
compare Wikipedias to other previously studied networks.Comment: v3: 9 pages, 12 figures, Change of title, few paragraphs and two
figures. Accepted for publication in Phys. Rev.
Dark energy transition between quintessence and phantom regimes - an equation of state analysis
The dark energy transition between quintessence () and phantom ()
regimes (the crossing of the cosmological constant boundary) is studied using
the dark energy equation of state. Models characterized by this type of
transition are explicitly constructed and their equation of state is found to
be {\em implicitly} defined. The behavior of the more general models with the
implicitly defined equation of state, obtained by the generalization of the
explicitly constructed models, is studied to gain insight into the necessary
conditions for the occurrence of the transition, as well as to investigate the
mechanism behind the transition. It is found that the parameters of the
generalized models need to satisfy special conditions for the transition to
happen and that the mechanism behind the transition is the cancellation of the
contribution of the cosmological constant boundary. The aspects of the behavior
of the generalized models which are not related to the transition are briefly
discussed and the role of the implicitly defined dark energy equation of state
in the description of the dark energy evolution is emphasized.Comment: v1: 9 pages, 6 figures. v2: references added. v3: minor changes.
Version accepted for publication in Phys. Rev.
"Expansion" around the vacuum: how far can we go from Lambda?
The cosmological constant (), i.e., the energy density stored in the
true vacuum state of all existing fields in the Universe, is the simplest and
the most natural possibility to describe the current cosmic acceleration.
However, despite its observational successes, such a possibility exacerbates
the well known problem, requiring a natural explanation for its
small, but nonzero, value. In this paper we discuss how different our Universe
may be from the CDM model by studying observational aspects of a kind
of "expansion" around the vacuum given by the equation of (EOS)
. In different parameter regimes such a
parametrization is capable of describing both quintessence-like and
phantom-like dark energy, transient acceleration, and various (non)singular
possibilities for the final destiny of the Universe, including singularities at
finite values of the scale factor, the so-called "Big Rip", as well as sudden
future singularities. By using some of the most recent cosmological
observations we show that if the functional form of the dark energy EOS has
additional parameters very little can be said about their values from the
current observational results, which postpones, until the arrival of more
precise observational data, a definitive answer to the question posed above.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
Properties of singularities in (phantom) dark energy universe
The properties of future singularities are investigated in the universe
dominated by dark energy including the phantom-type fluid. We classify the
finite-time singularities into four classes and explicitly present the models
which give rise to these singularities by assuming the form of the equation of
state of dark energy. We show the existence of a stable fixed point with an
equation of state and numerically confirm that this is actually a
late-time attractor in the phantom-dominated universe. We also construct a
phantom dark energy scenario coupled to dark matter that reproduces singular
behaviors of the Big Rip type for the energy density and the curvature of the
universe. The effect of quantum corrections coming from conformal anomaly can
be important when the curvature grows large, which typically moderates the
finite-time singularities.Comment: 17 pages, 6 figures, references are added, version to appear in
Physical Review
Solving for Micro- and Macro- Scale Electrostatic Configurations Using the Robin Hood Algorithm
We present a novel technique by which highly-segmented electrostatic
configurations can be solved. The Robin Hood method is a matrix-inversion
algorithm optimized for solving high density boundary element method (BEM)
problems. We illustrate the capabilities of this solver by studying two
distinct geometry scales: (a) the electrostatic potential of a large volume
beta-detector and (b) the field enhancement present at surface of electrode
nano-structures. Geometries with elements numbering in the O(10^5) are easily
modeled and solved without loss of accuracy. The technique has recently been
expanded so as to include dielectrics and magnetic materials.Comment: 40 pages, 20 figure
- âŠ