6,774 research outputs found
Direct measurement of diurnal polar motion by ring laser gyroscopes
We report the first direct measurements of the very small effect of forced
diurnal polar motion, successfully observed on three of our large ring lasers,
which now measure the instantaneous direction of Earth's rotation axis to a
precision of 1 part in 10^8 when averaged over a time interval of several
hours. Ring laser gyroscopes provide a new viable technique for directly and
continuously measuring the position of the instantaneous rotation axis of the
Earth and the amplitudes of the Oppolzer modes. In contrast, the space geodetic
techniques (VLBI, SLR, GPS, etc.) contain no information about the position of
the instantaneous axis of rotation of the Earth, but are sensitive to the
complete transformation matrix between the Earth-fixed and inertial reference
frame. Further improvements of gyroscopes will provide a powerful new tool for
studying the Earth's interior.Comment: 5 pages, 4 figures, agu2001.cl
Quantification of depth of anesthesia by nonlinear time series analysis of brain electrical activity
We investigate several quantifiers of the electroencephalogram (EEG) signal
with respect to their ability to indicate depth of anesthesia. For 17 patients
anesthetized with Sevoflurane, three established measures (two spectral and one
based on the bispectrum), as well as a phase space based nonlinear correlation
index were computed from consecutive EEG epochs. In absence of an independent
way to determine anesthesia depth, the standard was derived from measured blood
plasma concentrations of the anesthetic via a pharmacokinetic/pharmacodynamic
model for the estimated effective brain concentration of Sevoflurane. In most
patients, the highest correlation is observed for the nonlinear correlation
index D*. In contrast to spectral measures, D* is found to decrease
monotonically with increasing (estimated) depth of anesthesia, even when a
"burst-suppression" pattern occurs in the EEG. The findings show the potential
for applications of concepts derived from the theory of nonlinear dynamics,
even if little can be assumed about the process under investigation.Comment: 7 pages, 5 figure
Detecting Determinism in High Dimensional Chaotic Systems
A method based upon the statistical evaluation of the differentiability of
the measure along the trajectory is used to identify in high dimensional
systems. The results show that the method is suitable for discriminating
stochastic from deterministic systems even if the dimension of the latter is as
high as 13. The method is shown to succeed in identifying determinism in
electro-encephalogram signals simulated by means of a high dimensional system.Comment: 8 pages (RevTeX 3 style), 5 EPS figures, submitted to Phys. Rev. E
(25 apr 2001
Beam Profile Measurements and Simulations of the PETRA Laser-Wire
The Laser-wire will be an essential diagnostic tool at the International
Linear Collider. It uses a finely focussed laser beam to measure the transverse
profile of electron bunches by detecting the Compton-scattered photons (or
degraded electrons) downstream of where the laser beam intersects the electron
beam. Such a system has been installed at the PETRA storage ring at DESY, which
uses a piezo-driven mirror to scan the laser-light across the electron beam.
Lat- est results of experimental data taking are presented and compared to
detailed simulations using the Geant4 based program BDSIM.Comment: 3 pagesm 4 figures. Submitted as a conference paper for the Particle
Accelerator Conference 2005 (PAC05
The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau
Variations of eclipse arrival times have recently been detected in several
post common envelope binaries consisting of a white dwarf and a main sequence
companion star. The generally favoured explanation for these timing variations
is the gravitational pull of one or more circumbinary substellar objects
periodically moving the center of mass of the host binary. Using the new
extreme-AO instrument SPHERE, we image the prototype eclipsing post-common
envelope binary V471 Tau in search of the brown dwarf that is believed to be
responsible for variations in its eclipse arrival times. We report that an
unprecedented contrast of 12.1 magnitudes in the H band at a separation of 260
mas was achieved, but resulted in a non-detection. This implies that there is
no brown dwarf present in the system unless it is three magnitudes fainter than
predicted by evolutionary track models, and provides damaging evidence against
the circumbinary interpretation of eclipse timing variations. In the case of
V471 Tau, a more consistent explanation is offered with the Applegate
mechanism, in which these variations are prescribed to changes in the
quadrupole moment within the main-sequence sta
Non-Perturbative Mass Renormalization in Quenched QED from the Worldline Variational Approach
Following Feynman's successful treatment of the polaron problem we apply the
same variational principle to quenched QED in the worldline formulation. New
features arise from the description of fermions by Grassmann trajectories, the
supersymmetry between bosonic and fermionic variables and the much more
singular structure of a renormalizable gauge theory like QED in 3+1 dimensions.
We take as trial action a general retarded quadratic action both for the
bosonic and fermionic degrees of freedom and derive the variational equations
for the corresponding retardation functions. We find a simple analytic,
non-perturbative, solution for the anomalous mass dimension gamma_m(alpha) in
the MS scheme. For small couplings we compare our result with recent four-loop
perturbative calculations while at large couplings we find that gamma_m(alpha)
becomes proportional to (alpha)^(1/2). The anomalous mass dimension shows no
obvious sign of the chiral symmetry breaking observed in calculations based on
the use of Dyson-Schwinger equations, however we find that a perturbative
expansion of gamma_m(alpha) diverges for alpha > 0.7934. Finally, we
investigate the behaviour of gamma_m(alpha) at large orders in perturbation
theory.Comment: 18 pages, 1 Figure, RevTeX; the manuscript has been substantially
revised and enlarged in order to make it selfcontained; accepted for
publication in Phys. Rev.
Shape Analysis of the Level Spacing Distribution around the Metal Insulator Transition in the Three Dimensional Anderson Model
We present a new method for the numerical treatment of second order phase
transitions using the level spacing distribution function . We show that
the quantities introduced originally for the shape analysis of eigenvectors can
be properly applied for the description of the eigenvalues as well. The
position of the metal--insulator transition (MIT) of the three dimensional
Anderson model and the critical exponent are evaluated. The shape analysis of
obtained numerically shows that near the MIT is clearly different
from both the Brody distribution and from Izrailev's formula, and the best
description is of the form , with
. This is in good agreement with recent analytical results.Comment: 14 pages in plain TeX, 6 figures upon reques
Nonlinear Volatility of River Flux Fluctuations
We study the spectral properties of the magnitudes of river flux increments,
the volatility. The volatility series exhibits (i) strong seasonal periodicity
and (ii) strongly power-law correlations for time scales less than one year. We
test the nonlinear properties of the river flux increment series by randomizing
its Fourier phases and find that the surrogate volatility series (i) has almost
no seasonal periodicity and (ii) is weakly correlated for time scales less than
one year. We quantify the degree of nonlinearity by measuring (i) the amplitude
of the power spectrum at the seasonal peak and (ii) the correlation power-law
exponent of the volatility series.Comment: 5 revtex pages, 6 page
Chiral symmetry breaking in dimensionally regularized nonperturbative quenched QED
In this paper we study dynamical chiral symmetry breaking in dimensionally
regularized quenched QED within the context of Dyson-Schwinger equations. In D
< 4 dimensions the theory has solutions which exhibit chiral symmetry breaking
for all values of the coupling. To begin with, we study this phenomenon both
numerically and, with some approximations, analytically within the rainbow
approximation in the Landau gauge. In particular, we discuss how to extract the
critical coupling alpha_c = pi/3 relevant in four dimensions from the D
dimensional theory. We further present analytic results for the chirally
symmetric solution obtained with the Curtis-Pennington vertex as well as
numerical results for solutions exhibiting chiral symmetry breaking. For these
we demonstrate that, using dimensional regularization, the extraction of the
critical coupling relevant for this vertex is feasible. Initial results for
this critical coupling are in agreement with cut-off based work within the
currently achievable numerical precision.Comment: 24 pages, including 5 figures; submitted to Phys. Rev.
- âŠ