3,767 research outputs found
Dynamical stabilization of classical multi electron targets against autoionization
We demonstrate that a recently published quasiclassical M\oller type approach
[Geyer and Rost 2002, J. Phys. B 35 1479] can be used to overcome the problem
of autoionization, which arises in classical trajectory calculations for many
electron targets. In this method the target is stabilized dynamically by a
backward--forward propagation scheme. We illustrate this refocusing and present
total cross sections for single and double ionization of helium by electron
impact.Comment: LaTeX, 6 pages, 2 figures; submitted to J. Phys.
Parton distribution functions from nonlocal light-cone operators with definite twist
We introduce the chiral-even and chiral-odd quark distributions as forward
matrix elements of related bilocal quark operators with well-defined
(geometric) twist. Thereby, we achieve a Lorentz invariant classification of
these distributions which differ from the conventional ones by explicitly
taking into account the necessary trace terms. The relations between both kinds
of distribution functions are given and the mismatch between their different
definition of twist is discussed. Wandzura-Wilczek--like relations between the
conventional distributions (based on dynamical twist) are derived by means of
geometric twist distribution functions.Comment: 17 pages, REVTEX, Extended version, The Introduction has been
rewritten, Setion V "Wandzura-Wilczek--like relations" and App. B are added;
Sign errors are correcte
Superspace formulation of general massive gauge theories and geometric interpretation of mass-dependent BRST symmetries
A superspace formulation is proposed for the osp(1,2)-covariant Lagrangian
quantization of general massive gauge theories. The superalgebra os0(1,2) is
considered as subalgebra of sl(1,2); the latter may be considered as the
algebra of generators of the conformal group in a superspace with two
anticommuting coordinates. The mass-dependent (anti)BRST symmetries of proper
solutions of the quantum master equations in the osp(1,2)-covariant formalism
are realized in that superspace as invariance under translations combined with
mass-dependent special conformal transformations. The Sp(2) symmetry - in
particular the ghost number conservation - and the "new ghost number"
conservation are realized as invariance under symplectic rotations and
dilatations, respectively. The transformations of the gauge fields - and of the
full set of necessarily required (anti)ghost and auxiliary fields - under the
superalgebra sl(1,2) are determined both for irreducible and first-stage
reducible theories with closed gauge algebra.Comment: 35 pages, AMSTEX, precision of reference
Electron impact double ionization of helium from classical trajectory calculations
With a recently proposed quasiclassical ansatz [Geyer and Rost, J. Phys. B 35
(2002) 1479] it is possible to perform classical trajectory ionization
calculations on many electron targets. The autoionization of the target is
prevented by a M\o{}ller type backward--forward propagation scheme and allows
to consider all interactions between all particles without additional
stabilization. The application of the quasiclassical ansatz for helium targets
is explained and total and partially differential cross sections for electron
impact double ionization are calculated. In the high energy regime the
classical description fails to describe the dominant TS1 process, which leads
to big deviations, whereas for low energies the total cross section is
reproduced well. Differential cross sections calculated at 250 eV await their
experimental confirmation.Comment: LaTeX, 22 pages, 10 figures, submitted to J. Phys.
Comment on ``Precision measurement of the Casimir-Lifshitz force in a fluid''
Recently J.N. Munday and F. Capasso [Phys. Rev. A {\bf 75}, 060102(R) (2007);
arXiv:0705.3793] claimed that they have performed a precision measurement of
the Casimir force between a sphere and a plate coated with Au, both immersed in
ethanol. The measurement results were claimed to be consistent with the
Lifshitz theory. We demonstrate that the calculation of the Casimir force
between the smooth bodies following the authors prescription has a discrepancy
up to 25% with respect to authors result. We show also that the attractive
electrostatic force only due to the surface potential differences was
underestimated by a factor of 590 and the charge double layer interaction was
not taken into account. All this leads to the conclusion that the results of
this experiment are in fact uncertain.Comment: 5 pages, 1 figure, submitted to Physical Review A; corrections are
made in accordance to referee's suggestion
DVCS amplitude at tree level: Transversality, twist-3, and factorization
We study the virtual Compton amplitude in the generalized Bjorken region (q^2
-> Infinity, t small) in QCD by means of a light-cone expansion of the product
of e.m. currents in string operators in coordinate space. Electromagnetic gauge
invariance (transversality) is maintained by including in addition to the
twist-2 operators 'kinematical' twist-3 operators which appear as total
derivatives of twist-2 operators. The non-forward matrix elements of the
elementary twist-2 operators are parametrized in terms of two-variable spectral
functions (double distributions), from which twist-2 and 3 skewed distributions
are obtained through reduction formulas. Our approach is equivalent to a
Wandzura-Wilczek type approximation for the twist-3 skewed distributions. The
resulting Compton amplitude is manifestly transverse up to terms of order
t/q^2. We find that in this approximation the tensor amplitude for longitudinal
polarization of the virtual photon is finite, while the one for transverse
polarization contains a divergence already at tree level. However, this
divergence has zero projection on the polarization vector of the final photon,
so that the physical helicity amplitudes are finite.Comment: 34 pages, revtex, 1 eps figure included using epsf. Misprints
corrected, one reference adde
Characterization of neurophysiologic and neurocognitive biomarkers for use in genomic and clinical outcome studies of schizophrenia.
BackgroundEndophenotypes are quantitative, laboratory-based measures representing intermediate links in the pathways between genetic variation and the clinical expression of a disorder. Ideal endophenotypes exhibit deficits in patients, are stable over time and across shifts in psychopathology, and are suitable for repeat testing. Unfortunately, many leading candidate endophenotypes in schizophrenia have not been fully characterized simultaneously in large cohorts of patients and controls across these properties. The objectives of this study were to characterize the extent to which widely-used neurophysiological and neurocognitive endophenotypes are: 1) associated with schizophrenia, 2) stable over time, independent of state-related changes, and 3) free of potential practice/maturation or differential attrition effects in schizophrenia patients (SZ) and nonpsychiatric comparison subjects (NCS). Stability of clinical and functional measures was also assessed.MethodsParticipants (SZ n = 341; NCS n = 205) completed a battery of neurophysiological (MMN, P3a, P50 and N100 indices, PPI, startle habituation, antisaccade), neurocognitive (WRAT-3 Reading, LNS-forward, LNS-reorder, WCST-64, CVLT-II). In addition, patients were rated on clinical symptom severity as well as functional capacity and status measures (GAF, UPSA, SOF). 223 subjects (SZ n = 163; NCS n = 58) returned for retesting after 1 year.ResultsMost neurophysiological and neurocognitive measures exhibited medium-to-large deficits in schizophrenia, moderate-to-substantial stability across the retest interval, and were independent of fluctuations in clinical status. Clinical symptoms and functional measures also exhibited substantial stability. A Longitudinal Endophenotype Ranking System (LERS) was created to rank neurophysiological and neurocognitive biomarkers according to their effect sizes across endophenotype criteria.ConclusionsThe majority of neurophysiological and neurocognitive measures exhibited deficits in patients, stability over a 1-year interval and did not demonstrate practice or time effects supporting their use as endophenotypes in neural substrate and genomic studies. These measures hold promise for informing the "gene-to-phene gap" in schizophrenia research
New approach to the thermal Casimir force between real metals
The new approach to the theoretical description of the thermal Casimir force
between real metals is presented. It uses the plasma-like dielectric
permittivity that takes into account the interband transitions of core
electrons. This permittivity precisely satisfies the Kramers-Kronig relations.
The respective Casimir entropy is positive and vanishes at zero temperature in
accordance with the Nernst heat theorem. The physical reasons why the Drude
dielectric function, when substituted in the Lifshitz formula, is inconsistent
with electrodynamics are elucidated. The proposed approach is the single one
consistent with all measurements of the Casimir force performed up to date. The
application of this approach to metal-type semiconductors is considered.Comment: 14 pages, 6 figures. Proceedings of QFEXT07, to appear in J. Phys.
Coordinate Singularities in Harmonically-sliced Cosmologies
Harmonic slicing has in recent years become a standard way of prescribing the
lapse function in numerical simulations of general relativity. However, as was
first noticed by Alcubierre (1997), numerical solutions generated using this
slicing condition can show pathological behaviour. In this paper, analytic and
numerical methods are used to examine harmonic slicings of Kasner and Gowdy
cosmological spacetimes. It is shown that in general the slicings are prevented
from covering the whole of the spacetimes by the appearance of coordinate
singularities. As well as limiting the maximum running times of numerical
simulations, the coordinate singularities can lead to features being produced
in numerically evolved solutions which must be distinguished from genuine
physical effects.Comment: 21 pages, REVTeX, 5 figure
Reconstruction of thermally-symmetrized quantum autocorrelation functions from imaginary-time data
In this paper, I propose a technique for recovering quantum dynamical
information from imaginary-time data via the resolution of a one-dimensional
Hamburger moment problem. It is shown that the quantum autocorrelation
functions are uniquely determined by and can be reconstructed from their
sequence of derivatives at origin. A general class of reconstruction algorithms
is then identified, according to Theorem 3. The technique is advocated as
especially effective for a certain class of quantum problems in continuum
space, for which only a few moments are necessary. For such problems, it is
argued that the derivatives at origin can be evaluated by Monte Carlo
simulations via estimators of finite variances in the limit of an infinite
number of path variables. Finally, a maximum entropy inversion algorithm for
the Hamburger moment problem is utilized to compute the quantum rate of
reaction for a one-dimensional symmetric Eckart barrier.Comment: 15 pages, no figures, to appear in Phys. Rev.
- …