179 research outputs found
On velocity structure functions and the spherical vortex model for isotropic turbulence
We investigate a stochastic model for homogeneous, isotropic turbulence based on Hill's spherical vortex. This is an extension of the method of Synge and Lin [Trans. R. Soc. Can. 37, 45 (1943)], to the calculation of higher even-order velocity structure functions. Isotropic turbulence is represented by a homogeneous distribution of eddies, each modeled by a spherical vortex. The cascade process of eddy breakdown is incorporated into the statistical model through an average over an assumed log-normal distribution of vortex radii. We calculate the statistical properties of the model, in particular order-n velocity structure functions defined by rank-n tensors for the ensemble average of a set of incremental differences in velocity components. We define Di[centered ellipsis]s = , where denotes the ensemble average. Specifically Dij, Dijkl, and the longitudinal component of Dijklmn are calculated directly from the spherical vortex ensemble. Matching the longitudinal components of Dij and Dijkl with experimental results fixes two independent model parameters. The lateral and mixed components of Dijkl and the longitudinal component of Dijklmn are then model predictions
Heavy-quark correlations in deep inelastic scattering
We discuss results for heavy quark correlations in next-to-leading order QCD
in deep inelastic electroproduction.Comment: Talk presented by J. Smith. To appear in proceedings of "QCD and QED
in Higher Orders" 1996 Zuethen Workshop on Elementary Particle Theory, April
22-26, 1996. Latex, 5 figures, uses espcrc2.sty (included
The 1999 Center for Simulation of Dynamic Response in Materials Annual Technical Report
Introduction:
This annual report describes research accomplishments for FY 99 of the Center
for Simulation of Dynamic Response of Materials. The Center is constructing a
virtual shock physics facility in which the full three dimensional response of a
variety of target materials can be computed for a wide range of compressive, ten-
sional, and shear loadings, including those produced by detonation of energetic
materials. The goals are to facilitate computation of a variety of experiments
in which strong shock and detonation waves are made to impinge on targets
consisting of various combinations of materials, compute the subsequent dy-
namic response of the target materials, and validate these computations against
experimental data
Collinear Subtractions in Hadroproduction of Heavy Quarks
We present a detailed discussion of the collinear subtraction terms needed to
establish a massive variable-flavour-number scheme for the one-particle
inclusive production of heavy quarks in hadronic collisions. The subtraction
terms are computed by convoluting appropriate partonic cross sections with
perturbative parton distribution and fragmentation functions relying on the
method of mass factorization. We find (with one minor exception) complete
agreement with the subtraction terms obtained in a previous publication by
comparing the zero-mass limit of a fixed-order calculation with the genuine
massles results in the MSbar scheme. This presentation will be useful for
extending the massive variable-flavour-number scheme to other processes.Comment: 29 pages, 17 figures include
Nuclear Parton Distribution Functions
We study nuclear effects of charged current deep inelastic neutrino-iron
scattering in the framework of a chi^2 analysis of parton distribution
functions (PDFs). We extract a set of iron PDFs which are used to compute
x_Bj-dependent and Q^2-dependent nuclear correction factors for iron structure
functions which are required in global analyses of free nucleon PDFs. We
compare our results with nuclear correction factors from neutrino-nucleus
scattering models and correction factors for charged-lepton--iron scattering.
We find that, except for very high x_Bj, our correction factors differ in both
shape and magnitude from the correction factors of the models and
charged-lepton scattering.Comment: 11 pages, 6 figures, to appear in the proceedings of the Ringberg
Workshop "New Trends in HERA Physics 2008
The t W- Mode of Single Top Production
The t W- mode of single top production is proposed as an important means to
study the weak interactions of the top quark. While the rate of this mode is
most likely too small to be observed at Run II of the Fermilab Tevatron, it is
expected to be considerably larger at the CERN LHC. In this article the
inclusive t W- rate is computed, including O(1 / log (m_t^2 / m_b^2))
corrections, and when combined with detailed Monte Carlo simulations including
the top and W decay products, indicate that the t W- single top process may be
extracted from the considerable t tbar and W+ W- j backgrounds at low
luminosity runs of the LHC.Comment: 16 pages, 4 figure
Large- Heavy-Quark Production in Two-Photon Collisions
The next-to-leading-order (NLO) cross section for the production of heavy
quarks at large transverse momenta () in collisions is
calculated with perturbative fragmentation functions (PFF's). This approach
allows for a resummation of terms which
arise in NLO from collinear emission of gluons by heavy quarks at large
or from almost collinear branching of photons or gluons into
heavy-quark pairs. We present single-inclusive distributions in and
rapidity including direct and resolved photons for production of
heavy quarks at colliders and at high-energy colliders.
The results are compared with the fixed-order calculation for finite
including QCD radiative corrections. The two approaches differ in the
definitions and relative contributions of the direct and resolved terms, but
essentially agree in their sum. The resummation of the terms in the PFF approach leads to a softer
distribution and to a reduced sensitivity to the choice of the renormalization
and factorization scales.Comment: 17 pages, Latex, epsf, 7 figures appended as uuencoded file (hardcopy
can be obtained upon request from [email protected]
MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package
MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo
(MC) neutron ray-tracing program that provides researchers with tools for
performing computer modeling and simulations that mirror real neutron
scattering experiments. By adopting modern software engineering practices such
as using composite and visitor design patterns for representing and accessing
neutron scatterers, and using recursive algorithms for multiple scattering,
MCViNE is flexible enough to handle sophisticated neutron scattering problems
including, for example, neutron detection by complex detector systems, and
single and multiple scattering events in a variety of samples and sample
environments. In addition, MCViNE can take advantage of simulation components
in linear-chain-based MC ray tracing packages widely used in instrument design
and optimization, as well as NumPy-based components that make prototypes useful
and easy to develop. These developments have enabled us to carry out detailed
simulations of neutron scattering experiments with non-trivial samples in
time-of-flight inelastic instruments at the Spallation Neutron Source. Examples
of such simulations for powder and single-crystal samples with various
scattering kernels, including kernels for phonon and magnon scattering, are
presented. With simulations that closely reproduce experimental results,
scattering mechanisms can be turned on and off to determine how they contribute
to the measured scattering intensities, improving our understanding of the
underlying physics.Comment: 34 pages, 14 figure
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