8,782 research outputs found
On the Short Distance Behavior of the Critical Ising Model Perturbed by a Magnetic Field
We apply here a recently developed approach to compute the short distance
corrections to scaling for the correlators of all primary operators of the
critical two dimensional Ising model in a magnetic field. The essence of the
method is the fact that if one deals with O.P.E. Wilson coefficients instead of
correlators, all order I.R. safe formulas can be obtained for the perturbative
expansion with respect to magnetic field. This approach yields in a natural way
the expected fractional powers of the magnetic field, that are clearly absent
in the naive perturbative expression for correlators. The technique of the
Mellin transform have been used to compute the I.R. behavior of the regularized
integrals. As a corollary of our results, by comparing the existing numerical
data for the lattice model we give an estimate of the Vacuum Expectation Value
of the energy operator, left unfixed by usual nonperturbative approaches
(Thermodynamic Bethe Ansatz).Comment: 19 pages, LATEX, 2 figure
Avalanche in the Valley (Fermions, Anomaly and Unitarity in High-Energy Electroweak Interactions)
Problems related to fermions, unitarity and chiral anomaly in high energy
electroweak interactions, are investigated. Particular attention is paid to the
correct functional integration over fermion fields in the background of
instanton-anti\-instanton type configurations. This leads to an expansion of
correlation functions in terms of a small parameter, , when the
instanton-antiinstanton separation () is large compared to their sizes
(). Applying such a method to widely discussed cases of fermion-number
violation in the electroweak theory, we conclude that there are no theoretical
basis for expecting anomalous cross sections to become observable at energies
in the TeV region.Comment: 11 pages + 1 figure (not included
On the c-theorem in more than two dimensions
Several pieces of evidence have been recently brought up in favour of the
c-theorem in four and higher dimensions, but a solid proof is still lacking. We
present two basic results which could be useful for this search: i) the values
of the putative c-number for free field theories in any even dimension, which
illustrate some properties of this number; ii) the general form of three-point
function of the stress tensor in four dimensions, which shows some physical
consequences of the c-number and of the other trace-anomaly numbers.Comment: Latex, 7 pages, 1 tabl
S- and X-band SAR data fusion
This paper investigates the benefits deriving from introducing a wavelet-transform-based fusion framework for multi-frequency Synthetic Aperture Radar (SAR) data. A specific application is considered in the assessment of the fused classification map derived and this is the discrimination of different kinds of oil in sea. S-band and X-band datasets, concurrently acquired from the same airborne platform, have here been used. The findings suggest that fusing S-band and X-band SAR data does improve the oil type discrimination between crude oil and diesel oil used in the exercise, although a more quantitative analysis should be conducted in the future to measure the degree of improvement
A Monte Carlo study of the three-dimensional XY universality class:Universal amplitude ratios
We simulate lattice models in the three-dimensional XY universality class in
the low and the high temperature phase. This allows us to compute a number of
universal amplitude ratios with unprecedented precision: R_{\Upsilon}=0.411(2),
R_B=2.83(1), R_{\xi}^+=0.3562(10) and R_{\xi}^-=0.850(5). These results can be
compared with those obtained from other theoretical methods, such as field
theoretic methods or the high temperature series expansion and also with
experimental results for the lambda-transition of He. In addition to the XY
model, we study the three-dimensional two-component model on the
simple cubic lattice. The parameter of the model is chosen such that
leading corrections to scaling are small.Comment: 28 pages 5 figure
New Optimization Methods for Converging Perturbative Series with a Field Cutoff
We take advantage of the fact that in lambda phi ^4 problems a large field
cutoff phi_max makes perturbative series converge toward values exponentially
close to the exact values, to make optimal choices of phi_max. For perturbative
series terminated at even order, it is in principle possible to adjust phi_max
in order to obtain the exact result. For perturbative series terminated at odd
order, the error can only be minimized. It is however possible to introduce a
mass shift in order to obtain the exact result. We discuss weak and strong
coupling methods to determine the unknown parameters. The numerical
calculations in this article have been performed with a simple integral with
one variable. We give arguments indicating that the qualitative features
observed should extend to quantum mechanics and quantum field theory. We found
that optimization at even order is more efficient that at odd order. We compare
our methods with the linear delta-expansion (LDE) (combined with the principle
of minimal sensitivity) which provides an upper envelope of for the accuracy
curves of various Pade and Pade-Borel approximants. Our optimization method
performs better than the LDE at strong and intermediate coupling, but not at
weak coupling where it appears less robust and subject to further improvements.
We also show that it is possible to fix the arbitrary parameter appearing in
the LDE using the strong coupling expansion, in order to get accuracies
comparable to ours.Comment: 10 pages, 16 figures, uses revtex; minor typos corrected, refs. adde
A SAR image-based tool for prompt and effective earthquake response
Recently, a new concept for detection of damaged infrastructure after earthquakes has been introduced, based on analysis of double reflection lines in SAR images. This paper describes the development of a processing step for extraction of double-reflection lines, and its implementation. In particular, an unsupervised bright line detector working on the ratio of pre- and post-event single look complex SAR data is introduced, and is demonstrated using COSMO-SkyMed SAR data from the 2009 L'Aquila earthquake
Self-energy and critical temperature of weakly interacting bosons
Using the exact renormalization group we calculate the momentum-dependent
self-energy Sigma (k) at zero frequency of weakly interacting bosons at the
critical temperature T_c of Bose-Einstein condensation in dimensions 3 <= D <
4. We obtain the complete crossover function interpolating between the critical
regime k << k_c, where Sigma (k) propto k^{2 - eta}, and the short-wavelength
regime k >> k_c, where Sigma (k) propto k^{2 (D-3)} in D> 3 and Sigma (k)
\propto ln (k/k_c) in D=3. Our approach yields the crossover scale k_c on the
same footing with a reasonable estimate for the critical exponent eta in D=3.
From our Sigma (k) we find for the interaction-induced shift of T_c in three
dimensions Delta T_c / T_c approx 1.23 a n^{1/3}, where a is the s-wave
scattering length and n is the density.Comment: 4 pages,1 figur
GRB970228 and the class of GRBs with an initial spikelike emission: do they follow the Amati relation?
On the basis of the recent understanding of GRB050315 and GRB060218, we
return to GRB970228, the first Gamma-Ray Burst (GRB) with detected afterglow.
We proposed it as the prototype for a new class of GRBs with "an occasional
softer extended emission lasting tenths of seconds after an initial spikelike
emission". Detailed theoretical computation of the GRB970228 light curves in
selected energy bands for the prompt emission are presented and compared with
observational BeppoSAX data. From our analysis we conclude that GRB970228 and
likely the ones of the above mentioned new class of GRBs are "canonical GRBs"
have only one peculiarity: they exploded in a galactic environment, possibly
the halo, with a very low value of CBM density. Here we investigate how
GRB970228 unveils another peculiarity of this class of GRBs: they do not
fulfill the "Amati relation". We provide a theoretical explanation within the
fireshell model for the apparent absence of such correlation for the GRBs
belonging to this new class.Comment: 5 pages, 3 figures, in the Proceedings of the "4th Italian-Sino
Workshop on Relativistic Astrophysics", held in Pescara, Italy, July 20-28,
2007, C.L. Bianco, S.-S. Xue, Editor
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