10,105 research outputs found
Modeling Lepton-Nucleon Inelastic Scattering from High to Low Momentum Transfer
We present a model for inclusive charged lepton-nucleon and
(anti)neutrino-nucleon cross sections at momentum transfer squared, ,
. We quantify the impact of existing low-Q charged-lepton
deep-inelastic scattering (DIS) data on effects due to high-twist operators and
on the extraction of parton distribution functions (PDFs). No evidence is found
for twist-6 contributions to structure functions (SF), and for a twist-4 term
in the logitudinal SF at . We find that DIS data are consistent
with the NNLO QCD approximation with the target mass and phenomenological high
twist corrections. For , we extend extrapolation of the
operator product expansion, preserving the low- current-conservation
theorems. The procedure yields a good description of data down to . An updated set of PDFs with reduced uncertainty and applicable
down to small momentum transfers in the lepton-nucleon scattering is obtained.Comment: 10 pages, 6 figures, proceedings of the 5th International Workshop on
Neutrino-Nucleus Interactions in the Few-GeV Region (NuInt07), Batavia,
Illinois, 30 May - 3 Jun 200
Linear Theory of Electron-Plasma Waves at Arbitrary Collisionality
The dynamics of electron-plasma waves are described at arbitrary
collisionality by considering the full Coulomb collision operator. The
description is based on a Hermite-Laguerre decomposition of the velocity
dependence of the electron distribution function. The damping rate, frequency,
and eigenmode spectrum of electron-plasma waves are found as functions of the
collision frequency and wavelength. A comparison is made between the
collisionless Landau damping limit, the Lenard-Bernstein and Dougherty
collision operators, and the electron-ion collision operator, finding large
deviations in the damping rates and eigenmode spectra. A purely damped entropy
mode, characteristic of a plasma where pitch-angle scattering effects are
dominant with respect to collisionless effects, is shown to emerge numerically,
and its dispersion relation is analytically derived. It is shown that such a
mode is absent when simplified collision operators are used, and that
like-particle collisions strongly influence the damping rate of the entropy
mode.Comment: 23 pages, 10 figures, accepted for publication on Journal of Plasma
Physic
Fluctuation theorem for stochastic dynamics
The fluctuation theorem of Gallavotti and Cohen holds for finite systems
undergoing Langevin dynamics. In such a context all non-trivial ergodic theory
issues are by-passed, and the theorem takes a particularly simple form.
As a particular case, we obtain a nonlinear fluctuation-dissipation theorem
valid for equilibrium systems perturbed by arbitrarily strong fields.Comment: 15 pages, a section rewritte
Comment on "Two Phase Transitions in the Fully frustrated XY Model"
The conclusions of a recent paper by Olsson (Phys. Rev. Lett. 75, 2758
(1995), cond-mat/9506082) about the fully frustrated XY model in two dimensions
are questioned. In particular, the evidence presented for having two separate
chiral and U(1) phase transitions are critically considered.Comment: One page one table, to Appear in Physical Review Letter
Current-induced vortex dynamics in Josephson-junction arrays: Imaging experiments and model simulations
We study the dynamics of current-biased Josephson-junction arrays with a
magnetic penetration depth smaller than the lattice spacing. We compare the
dynamics imaged by low-temperature scanning electron microscopy to the vortex
dynamics obtained from model calculations based on the resistively-shunted
junction model, in combination with Maxwell's equations. We find three bias
current regions with fundamentally different array dynamics. The first region
is the subcritical region, i.e. below the array critical current I_c. The
second, for currents I above I_c, is a "vortex region", in which the response
is determined by the vortex degrees of freedom. In this region, the dynamics is
characterized by spatial domains where vortices and antivortices move across
the array in opposite directions in adjacent rows and by transverse voltage
fluctuations. In the third, for still higher currents, the dynamics is
dominated by coherent-phase motion, and the current-voltage characteristics are
linear.Comment: 10 pages, with eps figures. To appear in Phys. Rev.
Density Matrix Renormalization Group Study of One-Dimensional Acoustic Phonons
We study the application of the density matrix renormalization group (DMRG)
to systems with one-dimensional acoustic phonons. We show how the use of a
local oscillator basis circumvents the difficulties with the long-range
interactions generated in real space using the normal phonon basis. When
applied to a harmonic atomic chain, we find excellent agreement with the exact
solution even when using a modest number of oscillator and block states (a few
times ten). We discuss the use of this algorithm in more complex cases and
point out its value when other techniques are deficient.Comment: 12 pages. To be published in PRB rapid co
Excursion Sets and Non-Gaussian Void Statistics
Primordial non-Gaussianity (NG) affects the large scale structure (LSS) of
the universe by leaving an imprint on the distribution of matter at late times.
Much attention has been focused on using the distribution of collapsed objects
(i.e. dark matter halos and the galaxies and galaxy clusters that reside in
them) to probe primordial NG. An equally interesting and complementary probe
however is the abundance of extended underdense regions or voids in the LSS.
The calculation of the abundance of voids using the excursion set formalism in
the presence of primordial NG is subject to the same technical issues as the
one for halos, which were discussed e.g. in arXiv:1005.1203. However, unlike
the excursion set problem for halos which involved random walks in the presence
of one barrier , the void excursion set problem involves two barriers
and . This leads to a new complication introduced by what
is called the "void-in-cloud" effect discussed in the literature, which is
unique to the case of voids. We explore a path integral approach which allows
us to carefully account for all these issues, leading to a rigorous derivation
of the effects of primordial NG on void abundances. The void-in-cloud issue in
particular makes the calculation conceptually rather different from the one for
halos. However, we show that its final effect can be described by a simple yet
accurate approximation. Our final void abundance function is valid on larger
scales than the expressions of other authors, while being broadly in agreement
with those expressions on smaller scales.Comment: 28 pages (18+appendices), 7 figures; v2 -- minor changes in sec 3.2,
version published in PR
Online transparency for accountability: one assessing model and two applications
This paper proposes, in the context of Open Government, a model to assess how public sector entities are taking advantage of the internet as an information disclosure tool and a means to promote transparency, specifically in what concerns the use of public resources (input transparency for accountability). The assessment model and resulting Transparency Index gives particular attention to the disclosure of detailed (disaggregated) data according to the principles of Open Government Data, namely by valuing data visibility, adequate format for further processing, and the autonomous presentation of individual information items.
Subsequently, the paper demonstrates the applicability of the proposed model by carrying out two assessment exercises on two subsets of Portuguese and Italian municipalities. Results show that, all in all, the municipalities analysed do not yet disclose enough information useful for accountability processes and they do not take advantage of the internet potential to make the data provided more visible and re-usable by citizens and local stakeholders. Alone, high-level policy directives, governmental requirements and national legislation guaranteeing access to information are not enough to ensure public entities (municipalities in particular) disclose all the relevant data, and therefore specific guidelines are needed
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