461 research outputs found
L’avaluació de xarxes que promouen l’educació per al desenvolupament sostenible : el cas de la xarxa europea support
En aquest treball es planteja desenvolupar una estratègia d’ avaluació que promogui la millora del funcionament de la Xarxa Comenius SUPPORT, dintre d’ un paradigma soci-crític que té en compte els valors comuns de la ESD compartits pels participants, les seves visions de la “qualitat”, les activitats concretes, i els productes obtinguts. L’avaluació de la Xarxa Comenius SUPPORT inclou activitats que promouen la reflexió i el canvi conjuntament amb els participants com, (a) establir els “Criteris de Qualitat”, que seran el centre de l’ estratègia d’ avaluació, (b) dissenyar instruments de recollida de dades, (c) elaborar documents per compartir. D’aquesta forma, s’espera obtenir unes reflexions sobre els problemes del funcionament d’ aquest tipus de xarxes, y el desenvolupament d’estratègies per a promoure l’avaluació formativa en aquests contextos per a la EDS
BFKL Pomeron, Reggeized gluons and Bern-Dixon-Smirnov amplitudes
After a brief review of the BFKL approach to Regge processes in QCD and in
supersymmetric (SUSY) gauge theories we propose a strategy for calculating the
next-to-next-to-leading order corrections to the BFKL kernel. They can be
obtained in terms of various cross-sections for Reggeized gluon interactions.
The corresponding amplitudes can be calculated in the framework of the
effective action for high energy scattering. In the case of N=4 SUSY it is also
possible to use the Bern-Dixon-Smirnov (BDS) ansatz. For this purpose the
analytic properties of the BDS amplitudes at high energies are investigated, in
order to verify their self-consistency. It is found that, for the number of
external particles being larger than five, these amplitudes, beyond one loop,
are not in agreement with the BFKL approach which predicts the existence of
Regge cuts in some physical channels.Comment: 41 pages, expanded version with many clarifications and new
references, conclusions unchanged. Note adde
Diffractive photoproduction at large momentum transfer in coherent hadron - hadron interactions at CERN LHC
The vector meson production in coherent hadron-hadron interactions at LHC
energies is studied assuming that the color singlet -channel exchange
carries large momentum transfer. We consider the non-forward solution of the
BFKL equation at high energy and large momentum transfer and estimate the
rapidity distribution and total cross section for the process , where can be a proton or a nucleus. We predict large rates,
which implies that the experimental identification can be feasible at the LHC.Comment: 10 pages, 5 figures, 1 table. Version to be published in Physical
Review
The next-to-leading order forward jet vertex in the small-cone approximation
We consider within QCD collinear factorization the process p+p to jet + jet
+X, where two forward high- jets are produced with a large separation in
rapidity (Mueller-Navelet jets). In this case the (calculable) hard
part of the reaction receives large higher-order corrections , which can be accounted for in the BFKL approach. In particular,
we calculate in the next-to-leading order the impact factor (vertex) for the
production of a forward high- jet, in the approximation of small aperture
of the jet cone in the pseudorapidity-azimuthal angle plane. The final
expression for the vertex turns out to be simple and easy to implement in
numerical calculations.Comment: 32 pages, 4 figures; a few comments and one reference added; a few
inessential misprints removed; version to appear on JHE
One-dimensional fermionic systems after interaction quenches and their description by bosonic field theories
We show that the dynamics of quenches in one dimension far off equilibrium
can be described by power laws, but with exponents differing from the fully
renormalized ones at lowest energies. Instead they depend on the initial state
and its excitation energy. Furthermore, we found that for quenches to strong
interactions unexpected similarities between systems in one and in infinite
dimensions occur, indicating the dominance of local processes.Comment: This is a distinctly revised version which is focussed on the
description of the dynamics by bosonization technique
Quantum quench dynamics of the sine-Gordon model in some solvable limits
In connection with the the thermalization problem in isolated quantum
systems, we investigate the dynamics following a quantum quench of the
sine-Gordon model in the Luther-Emery and the semiclassical limits. We consider
the quench from the gapped to the gapless phase as well as reversed one. By
obtaining analytic expressions for the one and two-point correlation functions
of the order parameter operator at zero-temperature, the manifestations of
integrability in the absence of thermalization in the sine-Gordon model are
studied. It is thus shown that correlations in the long time regime after the
quench are well described by a generalized Gibbs ensemble. We also consider the
case where the system is initially in contact with a reservoir at finite
temperature. The possible relevance of our results to current and future
experiments with ultracold atomic systems is also critically considered.Comment: 21 pages, no figures. To appear in New J. Phys
Resumming double logarithms in the QCD evolution of color dipoles
The higher-order perturbative corrections, beyond leading logarithmic
accuracy, to the BFKL evolution in QCD at high energy are well known to suffer
from a severe lack-of-convergence problem, due to radiative corrections
enhanced by double collinear logarithms. Via an explicit calculation of Feynman
graphs in light cone (time-ordered) perturbation theory, we show that the
corrections enhanced by double logarithms (either energy-collinear, or double
collinear) are associated with soft gluon emissions which are strictly ordered
in lifetime. These corrections can be resummed to all orders by solving an
evolution equation which is non-local in rapidity. This equation can be
equivalently rewritten in local form, but with modified kernel and initial
conditions, which resum double collinear logs to all orders. We extend this
resummation to the next-to-leading order BFKL and BK equations. The first
numerical studies of the collinearly-improved BK equation demonstrate the
essential role of the resummation in both stabilizing and slowing down the
evolution.Comment: 16 pages, 5 figure
Título: Super quartum librum Regum explanatio
Letra gótica.Colofón.Sign.: []2, a-z8, [r cuadrata]8, ?8, [u barrada]4.Texto a dos col.Port. a dos tintas.Il. xil. al principio del texto, con el incipit en tinta roja y esc. xil. episcopal en inicial
Next-to-leading and resummed BFKL evolution with saturation boundary
We investigate the effects of the saturation boundary on small-x evolution at
the next-to-leading order accuracy and beyond. We demonstrate that the
instabilities of the next-to-leading order BFKL evolution are not cured by the
presence of the nonlinear saturation effects, and a resummation of the higher
order corrections is therefore needed for the nonlinear evolution. The
renormalization group improved resummed equation in the presence of the
saturation boundary is investigated, and the corresponding saturation scale is
extracted. A significant reduction of the saturation scale is found, and we
observe that the onset of the saturation corrections is delayed to higher
rapidities. This seems to be related to the characteristic feature of the
resummed splitting function which at moderately small values of x possesses a
minimum.Comment: 34 page
Models of electron transport in single layer graphene
The main features of the conductivity of doped single layer graphene are
analyzed, and models for different scattering mechanisms are presented.Comment: 15 pages. Submitted to the Proceedings of the ULTI symposium on
Quantum Phenomena and Devices at Low Temperatures, Espoo, Finland, to be
published in the Journ. of Low. Temp. Phy
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