1,399 research outputs found
Self-regularization of projection methods with a posteriori discretization level choice for severely ill-posed problems
It is well known that projection schemes for certain linear ill-posed problems Aí = y can be regularized by a proper choice of the discretization level only, where no additional regularization is needed. The previous study of this self-regularization phenomenon was restricted to the case of so-called moderately ill-posed problems, i.e., when the singular values Ïí(A), í = 1,2,..., of the operator A tend to zero with polynomial rate. The main accomplishment of the present paper is a new strategy for a discretization level choice that provides optimal order accuracy also for severely ill-posed problems, i.e., when Ïí(A) tend to zero exponentially. The proposed strategy does not require a priori information regarding the solution smoothness and the exact rate of Ïí(A)
Towards the Standard Model spectrum from elliptic Calabi-Yau
We show that it is possible to construct supersymmetric three-generation
models of Standard Model gauge group in the framework of non-simply-connected
elliptically fibered Calabi-Yau, without section but with a bi-section. The
fibrations on a cover Calabi-Yau, where the model has 6 generations of SU(5)
and the bundle is given via the spectral cover description, use a different
description of the elliptic fibre which leads to more than one global section.
We present two examples of a possible cover Calabi-Yau with a free involution:
one is a fibre product of rational elliptic surfaces ; another example is
an elliptic fibration over a Hirzebruch surface. There we give the necessary
amount of chiral matter by turning on in the bundles a further parameter,
related to singularities of the fibration and the branching of the spectral
cover.Comment: 31 pages, 1 eps-figure, reference added, bundle parameters for dP9
model change
Aharonov-Casher oscillations of spin current through a multichannel mesoscopic ring
The Aharonov-Casher (AC) oscillations of spin current through a 2D ballistic
ring in the presence of Rashba spin-orbit interaction and external magnetic
field has been calculated using the semiclassical path integral method. For
classically chaotic trajectories the Fokker-Planck equation determining
dynamics of the particle spin polarization has been derived. On the basis of
this equation an analytic expression for the spin conductance has been obtained
taking into account a finite width of the ring arms carrying large number of
conducting channels. It was shown that the finite width results in a broadening
and damping of spin current AC oscillations. We found that an external magnetic
field leads to appearance of new nondiagonal components of the spin
conductance, allowing thus by applying a rather weak magnetic field to change a
direction of the transmitted spin current polarization.Comment: 16 pages, 6 figure
Quantum Mechanical Properties of Bessel Beams
Bessel beams are studied within the general framework of quantum optics. The
two modes of the electromagnetic field are quantized and the basic dynamical
operators are identified. The algebra of these operators is analyzed in detail;
it is shown that the operators that are usually associated to linear momentum,
orbital angular momentum and spin do not satisfy the algebra of the translation
and rotation group. In particular, what seems to be the spin is more similar to
the helicity. Some physical consequences of these results are examined.Comment: 17 pages, no figures. New versio
A QCD Calculation of the Interaction of Quarkonium with Nuclei
The interaction of quarkonium with nuclei is studied in the limit of QCD, where the binding energy is found to be exactly
computable. The dominant contribution to the interaction is from two-gluon
operators. The forward matrix elements of these two-gluon operators can be
determined from the QCD scale anomaly, and from deep inelastic scattering. We
apply our results to the and , treating the \qqbar
interaction as purely Coulombic. We find the binds in nuclear matter
with a binding energy of a few \mev, while for the binding is of
order 10 \mev. For the in particular we expect confinement effects
to produce large corrections to this result.Comment: (10 pages
Model Analysis of the Electroproduction Reaction on the Proton
Recent CLAS data on the electroproduction off protons at
1.3W1.57 GeV and 0.250.6 GeV have been analyzed using
a meson-baryon phenomenological model. By fitting nine 1-fold differential
cross section data for each and bin, the charged double pion
electroproduction mechanisms are identified from their manifestations in the
observables. We have extracted the cross sections from amplitudes of each of
the considered isobar channels as well as from their coherent sum. We also
obtained non-resonant partial wave amplitudes of all contributing isobar
channels which could be useful for advancing a complete coupled-channel
analysis of all meson electroproduction data.Comment: Experiment Numbers: E93-006, E94-005 Group: Hall
A search for J^{PC}=1^{-+} exotic mesons in the pi- pi- pi+ and pi- pi0 pi0 systems
A partial wave analysis (PWA) of the pi-pi-pi+ and pi-pi0pi0 systems produced
in the reaction pi- p -> (3pi)-p at 18 GeV/c was carried out using an isobar
model assumption. This analysis is based on 3.0M pi-pi0pi0 events and 2.6M
pi-pi-pi+ events and shows production of the a2(1320), pi2(1670) and \pi(1800)
mesons. An earlier analysis of 250K pi-pi-pi+ events from the same experiment
showed possible evidence for a J^{PC}=1^{-+}$ exotic meson with a mass of 1.6
GeV/c^2 decaying into rho pi. In this analysis of a higher statistics sample of
the (3pi)- system in two charged modes we find no evidence of an exotic meson.Comment: 4 pages, 5 figures, added comment about the negative reflectivity
exotic wave
Cooling Torsional Nanomechanical Vibration by Spin-Orbit Interactions
We propose and study a spin-orbit interaction based mechanism to actively
cool down the torsional vibration of a nanomechanical resonator made by
semiconductor materials. We show that the spin-orbit interactions of electrons
can induce a coherent coupling between the electron spins and the torsional
modes of nanomechanical vibration. This coherent coupling leads to an active
cooling for the torsional modes via the dynamical thermalization of the
resonator and the spin ensemble.Comment: 4 pages, 3 figure
Dissipative dynamics of topological defects in frustrated Heisenberg spin systems
We study the dynamics of topological defects of a frustrated spin system
displaying spiral order. As a starting point we consider the SO(3) nonlinear
sigma model to describe long-wavelength fluctuations around the noncollinear
spiral state. Besides the usual spin-wave magnetic excitations, the model
allows for topologically non-trivial static solutions of the equations of
motion, associated with the change of chirality (clockwise or counterclockwise)
of the spiral. We consider two types of these topological defects, single
vortices and vortex-antivortex pairs, and quantize the corresponding solutions
by generalizing the semiclassical approach to a non-Abelian field theory. The
use of the collective coordinates allows us to represent the defect as a
particle coupled to a bath of harmonic oscillators, which can be integrated out
employing the Feynman-Vernon path-integral formalism. The resulting effective
action for the defect indicates that its motion is damped due to the scattering
by the magnons. We derive a general expression for the damping coefficient of
the defect, and evaluate its temperature dependence in both cases, for a single
vortex and for a vortex-antivortex pair. Finally, we consider an application of
the model for cuprates, where a spiral state has been argued to be realized in
the spin-glass regime. By assuming that the defect motion contributes to the
dissipative dynamics of the charges, we can compare our results with the
measured inverse mobility in a wide range of temperature. The relatively good
agreement between our calculations and the experiments confirms the possible
relevance of an incommensurate spiral order for lightly doped cuprates.Comment: 22 pages, 7 figures, final published versio
Auditory-Motor Mapping Training in a More Verbal Child with Autism
We tested the effect of Auditory-Motor Mapping Training (AMMT), a novel, intonation-based treatment for spoken language originally developed for minimally verbal (MV) children with autism, on a more-verbal child with autism. We compared this childâs performance after 25 therapy sessions with that of: (1) a child matched on age, autism severity, and expressive language level who received 25 sessions of a non-intonation-based control treatment Speech Repetition Therapy (SRT); and (2) a matched pair of MV children (one of whom received AMMT; the other, SRT). We found a significant Time Ă Treatment effect in favor of AMMT for number of Syllables Correct and Consonants Correct per stimulus for both pairs of children, as well as a significant Time Ă Treatment effect in favor of AMMT for number of Vowels Correct per stimulus for the more-verbal pair. Magnitudes of the difference in post-treatment performance between AMMT and SRT, adjusted for Baseline differences, were: (a) larger for the more-verbal pair than for the MV pair; and (b) associated with very large effect sizes (Cohenâs d > 1.3) in the more-verbal pair. Results hold promise for the efficacy of AMMT for improving spoken language production in more-verbal children with autism as well as their MV peers and suggest hypotheses about brain function that are testable in both correlational and causal behavioral-imaging studies
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