626 research outputs found
Instanton approach to the Langevin motion of a particle in a random potential
We develop an instanton approach to the non-equilibrium dynamics in
one-dimensional random environments. The long time behavior is controlled by
rare fluctuations of the disorder potential and, accordingly, by the tail of
the distribution function for the time a particle needs to propagate along the
system (the delay time). The proposed method allows us to find the tail of the
delay time distribution function and delay time moments, providing thus an
exact description of the long-time dynamics. We analyze arbitrary environments
covering different types of glassy dynamics: dynamics in a short-range random
field, creep, and Sinai's motion.Comment: 4 pages, 1 figur
Assessing students’ engagement: A review of instruments with psychometric Qualities
Este E-Book reúne um conjunto de investigações apresentadas no “I Congresso Internacional Envolvimento dos Alunos na Escola: Perspetivas da Psicologia e Educação” (ICIEAE), organizado no âmbito do “Projeto PTDC/CPE-CED/114362/2009 - Envolvimento dos Alunos na Escola: Diferenciação e Promoção” (EAE-DP), financiado pela Fundação para a Ciência e a Tecnologia (FCT).Framework: Students Engagement in School (SES) has been occupying a central
position in the discussions regarding factors of academic success and school
dropout. A considerable amount of literature on this concept exists. Although its
conceptualization varies according to authors and the theoretical framework they
have adopted, there is a wide agreement concerning its multidimensional nature. Key
dimensions of students engagement in school (i.e., cognitive, affective, behavioral
and, more recently, agentic) have been described and empirically validated. Purpose:This study aimed to review the literature on assessment of students engagement in
school through a focus on the psychometric characteristics of several instruments.
Methodology: The present paper focuses on self-report measures which are
multidimensional. These instruments were validated on heterogeneous samples.
Results: Twelve self-report measures designed to measure the students engagement
in school were referred, along with four other instruments targeting teachers’
perspective as well as observational measures. Conclusions: Various measures
stem from different theoretical perspectives and were developed with different types
of samples. Conceptual variations often expressed in the number of dimensions
considered and in items content variability suggest limitations when comparing
psychometric indings of different studies. Suggestions: Studies on instruments
we reviewed in the present paper suggest the need for further research on the
multidimensionality of school engagement construct. Research should go beyond
investigation of differential and predictive validity of measures. Thus, there is little
evidence regarding the validity of engagement in school measures, when investigation
of effects of specific intervention programs is aimed or validity of their use in quasiexperimental
studies with useful applications in the field of education
The WKB Approximation without Divergences
In this paper, the WKB approximation to the scattering problem is developed
without the divergences which usually appear at the classical turning points. A
detailed procedure of complexification is shown to generate results identical
to the usual WKB prescription but without the cumbersome connection formulas.Comment: 13 pages, TeX file, to appear in Int. J. Theor. Phy
Space Symmetries and Quantum Behavior of Finite Energy Configurations in SU(2)-Gauge Theory
The quantum properties of localized finite energy solutions to classical
Euler-Lagrange equations are investigated using the method of collective
coordinates. The perturbation theory in terms of inverse powers of the coupling
constant is constructed, taking into account the conservation laws of
momentum and angular momentum (invariance of the action with respect to the
group of motion M(3) of 3-dimensional Euclidean space) rigorously in every
order of perturbation theory.Comment: LaTex, 17 pages typos correcte
The X-ray Emissions from the M87 Jet: Diagnostics and Physical Interpretation
We reanalyze the deep Chandra observations of the M87 jet, first examined by
Wilson & Yang (2002). By employing an analysis chain that includes image
deconvolution, knots HST-1 and I are fully separated from adjacent emission. We
find slight but significant variations in the spectral shape, with values of
ranging from . We use VLA radio observations, as well
as HST imaging and polarimetry data, to examine the jet's broad-band spectrum
and inquire as to the nature of particle acceleration in the jet. As shown in
previous papers, a simple continuous injection model for synchrotron-emitting
knots, in which both the filling factor, , of regions within which
particles are accelerated and the energy spectrum of the injected particles are
constant, cannot account for the X-ray flux or spectrum. Instead, we propose
that is a function of position and energy and find that in the inner
jet, , and
in knots A and B, , where is the emitted photon energy and and is the
emitting electron energy. In this model, the index of the injected electron
energy spectrum () is at all locations in
the jet, as predicted by models of cosmic ray acceleration by ultrarelativistic
shocks. There is a strong correlation between the peaks of X-ray emission and
minima of optical percentage polarization, i.e., regions where the jet magnetic
field is not ordered. We suggest that the X-ray peaks coincide with shock waves
which accelerate the X-ray emitting electrons and cause changes in the
direction of the magnetic field; the polarization is thus small because of beam
averaging.Comment: Accepted for publication in ApJ; 21 pages, 9 figures, 2 tables;
abstract shortened for astro-ph; Figures 1, 7 and 8 at reduced resolutio
Inelastic Processes in the Collision of Relativistic Highly Charged Ions with Atoms
A general expression for the cross sections of inelastic collisions of fast
(including relativistic) multicharged ions with atoms which is based on the
genelazition of the eikonal approximation is derived. This expression is
applicable for wide range of collision energy and has the standard
nonrelativistic limit and in the ultrarelativistic limit coincides with the
Baltz's exact solution ~\cite{art13} of the Dirac equation. As an application
of the obtained result the following processes are calculated: the excitation
and ionization cross sections of hydrogenlike atom; the single and double
excitation and ionization of heliumlike atom; the multiply ionization of neon
and argon atoms; the probability and cross section of K-vacancy production in
the relativistic collision. The simple analytic formulae
for the cross sections of inelastic collisions and the recurrence relations
between the ionization cross sections of different multiplicities are also
obtained. Comparison of our results with the experimental data and the results
of other calculations are given.Comment: 25 pages, latex, 7 figures avialable upon request,submitted to PR
Semiclassical treatment of logarithmic perturbation theory
The explicit semiclassical treatment of logarithmic perturbation theory for
the nonrelativistic bound states problem is developed. Based upon
-expansions and suitable quantization conditions a new procedure for
deriving perturbation expansions for the one-dimensional anharmonic oscillator
is offered. Avoiding disadvantages of the standard approach, new handy
recursion formulae with the same simple form both for ground and exited states
have been obtained. As an example, the perturbation expansions for the energy
eigenvalues of the harmonic oscillator perturbed by are
considered.Comment: 6 pages, LATEX 2.09 using IOP style
Noise storm continua: power estimates for electron acceleration
We use a generic stochastic acceleration formalism to examine the power
() input to nonthermal electrons that cause
noise storm continuum emission. The analytical approach includes the derivation
of the Green's function for a general second-order Fermi process, and its
application to obtain the particular solution for the nonthermal electron
distribution resulting from the acceleration of a Maxwellian source in the
corona. We compare with the power observed in noise
storm radiation. Using typical values for the various parameters, we find that
, yielding an efficiency
estimate in the range 10^{-10} \lsim \eta
\lsim 10^{-6} for this nonthermal acceleration/radiation process. These
results reflect the efficiency of the overall process, starting from electron
acceleration and culminating in the observed noise storm emission.Comment: Accepted for publication in Solar Physic
Cosmological Production of Vector Bosons and Cosmic Microwave Background Radiation
The intensive cosmological creation of vector W, Z- bosons in the
cosmological model with the relative units is considered. Field theoretical
models are studied, which predict that the CMB radiation and the baryon matter
in the universe can be products of decay and annihilation processes of these
primordial bosons.Comment: 31 pages, 1 figur
Logarithmic perturbation theory for quasinormal modes
Logarithmic perturbation theory (LPT) is developed and applied to quasinormal
modes (QNMs) in open systems. QNMs often do not form a complete set, so LPT is
especially convenient because summation over a complete set of unperturbed
states is not required. Attention is paid to potentials with exponential tails,
and the example of a Poschl-Teller potential is briefly discussed. A numerical
method is developed that handles the exponentially large wavefunctions which
appear in dealing with QNMs.Comment: 24 pages, 4 Postscript figures, uses ioplppt.sty and epsfig.st
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