16,523 research outputs found
A BPS Skyrme model and baryons at large Nc
Within the class of field theories with the field contents of the Skyrme
model, one submodel can be found which consists of the square of the baryon
current and a potential term only. For this submodel, a Bogomolny bound exists
and the static soliton solutions saturate this bound. Further, already on the
classical level, this BPS Skyrme model reproduces some features of the liquid
drop model of nuclei. Here, we investigate the model in more detail and,
besides, we perform the rigid rotor quantization of the simplest Skyrmion (the
nucleon). In addition, we discuss indications that the viability of the model
as a low energy effective field theory for QCD is further improved in the limit
of a large number of colors N_c.Comment: latex, 23 pages, 1 figure, a numerical error in section 3.2
corrected; matches published versio
Exact String Solutions in 2+1-Dimensional De Sitter Spacetime
Exact and explicit string solutions in de Sitter spacetime are found. (Here,
the string equations reduce to a sinh-Gordon model). A new feature without flat
spacetime analogy appears: starting with a single world-sheet, several (here
two) strings emerge. One string is stable and the other (unstable) grows as the
universe grows. Their invariant size and energy either grow as the expansion
factor or tend to constant. Moreover, strings can expand (contract) for large
(small) universe radius with a different rate than the universe.Comment: 11 pages, Phyzzx macropackage used, PAR-LPTHE 92/32. Revised version
with a new understanding of the previous result
Self-pulsing dynamics of ultrasound in a magnetoacoustic resonator
A theoretical model of parametric magnetostrictive generator of ultrasound is
considered, taking into account magnetic and magnetoacoustic nonlinearities.
The stability and temporal dynamics of the system is analized with standard
techniques revealing that, for a given set of parameters, the model presents a
homoclinic or saddle--loop bifurcation, which predicts that the ultrasound is
emitted in the form of pulses or spikes with arbitrarily low frequency.Comment: 5 pages, 5 figure
Deconvolution problems in x-ray absorption fine structure
A Bayesian method application to the deconvolution of EXAFS spectra is
considered. It is shown that for purposes of EXAFS spectroscopy, from the
infinitely large number of Bayesian solutions it is possible to determine an
optimal range of solutions, any one from which is appropriate. Since this
removes the requirement for the uniqueness of solution, it becomes possible to
exclude the instrumental broadening and the lifetime broadening from EXAFS
spectra. In addition, we propose several approaches to the determination of
optimal Bayesian regularization parameter. The Bayesian deconvolution is
compared with the deconvolution which uses the Fourier transform and optimal
Wiener filtering. It is shown that XPS spectra could be in principle used for
extraction of a one-electron absorptance. The amplitude correction factors
obtained after deconvolution are considered and discussed.Comment: 6 two-column pages, 5 eps figures; submitted to J. Phys.: Appl. Phy
Validation and Application of the Thermal Hydraulic System Code TRACE for Analysis of BWR Transients
Evolution of density perturbations in decaying vacuum cosmology: The case of non-zero perturbations in the cosmological term
We extend the results of a previous paper where a model of interacting dark
energy, with a cosmological term decaying linearly with the Hubble parameter,
is tested against the observed mass power spectrum. In spite of the agreement
with observations of type Ia supernovas, baryonic acoustic oscillations and the
cosmic microwave background, we had shown previously that no good concordance
is achieved if we include the mass power spectrum. However, our analysis was
based on the ad hoc assumption that the interacting cosmological term is
strictly homogeneous. Now we perform a more complete analysis, by perturbing
such a term. Although our conclusions are still based on a particular, scale
invariant choice of the primordial spectrum of dark energy perturbations, we
show that a cosmological term decaying linearly with the Hubble parameter is
indeed disfavored as compared to the standard model.Comment: Version accepted for publication in Physical Review
String Instabilities in Black Hole Spacetimes
We study the emergence of string instabilities in - dimensional black
hole spacetimes (Schwarzschild and Reissner - Nordstr\o m), and De Sitter space
(in static coordinates to allow a better comparison with the black hole case).
We solve the first order string fluctuations around the center of mass motion
at spatial infinity, near the horizon and at the spacetime singularity. We find
that the time components are always well behaved in the three regions and in
the three backgrounds. The radial components are {\it unstable}: imaginary
frequencies develop in the oscillatory modes near the horizon, and the
evolution is like , , near the spacetime
singularity, , where the world - sheet time , and the
proper string length grows infinitely. In the Schwarzschild black hole, the
angular components are always well - behaved, while in the Reissner - Nordstr\o
m case they develop instabilities inside the horizon, near where the
repulsive effects of the charge dominate over those of the mass. In general,
whenever large enough repulsive effects in the gravitational background are
present, string instabilities develop. In De Sitter space, all the spatial
components exhibit instability. The infalling of the string to the black hole
singularity is like the motion of a particle in a potential
where depends on the spacetime
dimensions and string angular momentum, with for Schwarzschild and
for Reissner - Nordstr\o m black holes. For the
string ends trapped by the black hole singularity.Comment: 26pages, Plain Te
Geometric description of BTZ black holes thermodynamics
We study the properties of the space of thermodynamic equilibrium states of
the Ba\~nados-Teitelboim-Zanelli (BTZ) black hole in (2+1)-gravity. We use the
formalism of geometrothermodynamics to introduce in the space of equilibrium
states a dimensional thermodynamic metric whose curvature is non-vanishing,
indicating the presence of thermodynamic interaction, and free of
singularities, indicating the absence of phase transitions. Similar results are
obtained for generalizations of the BTZ black hole which include a Chern-Simons
term and a dilatonic field. Small logarithmic corrections of the entropy turn
out to be represented by small corrections of the thermodynamic curvature,
reinforcing the idea that thermodynamic curvature is a measure of thermodynamic
interaction
Kink stability, propagation, and length scale competition in the periodically modulated sine-Gordon equation
We have examined the dynamical behavior of the kink solutions of the
one-dimensional sine-Gordon equation in the presence of a spatially periodic
parametric perturbation. Our study clarifies and extends the currently
available knowledge on this and related nonlinear problems in four directions.
First, we present the results of a numerical simulation program which are not
compatible with the existence of a radiative threshold, predicted by earlier
calculations. Second, we carry out a perturbative calculation which helps
interpret those previous predictions, enabling us to understand in depth our
numerical results. Third, we apply the collective coordinate formalism to this
system and demonstrate numerically that it accurately reproduces the observed
kink dynamics. Fourth, we report on a novel occurrence of length scale
competition in this system and show how it can be understood by means of linear
stability analysis. Finally, we conclude by summarizing the general physical
framework that arises from our study.Comment: 19 pages, REVTeX 3.0, 24 figures available from A S o
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