504,044 research outputs found
Towards an unified description of total and diffractive structure functions at HERA in the QCD dipole picture
It is argued that the QCD dipole picture allows to build an unified
theoretical description -based on BFKL dynamics- of the total and diffractive
nucleon structure functions. This description is in qualitative agreement with
the present collection of data obtained by the H1 collaboration. More precise
theoretical estimates, in particular the determination of the normalizations
and proton transverse momentum behaviour of the diffractive components, are
shown to be required in order to reach definite conclusions.Comment: latex file with 5 encapsulated figures, 19 page
Flavourdynamics
These lectures provide an introductory overview of the dynamics of
flavour-changing transitions. The main emphasis is put on present tests of the
quark-mixing matrix structure and the phenomenological determination of its
parameters. The interplay of strong interactions in weak decays and the
important role of flavour symmetries for controlling the size of QCD
corrections to some semileptonic transitions are discussed.Comment: 54 pages, latex, 14 figures (included in a separate .uu file)
Lectures given at the XXIII International Meeting on Fundamental Physics
(Comillas, 22--26 May 1995). 1 reference correcte
Analysis of the stellar population in the central area of the HII region Sh 2-284
There is a lack of state-of-the-art information on very young open clusters,
with implications for determining the structure of the Galaxy. Our main
objective is to study the timing and location of the star formation processes
which yielded the generation of the giant HII region Sh 2-284. The analysis is
based on UBVRcIc CCD measurements and JHKs photometry in the central part of
the HII region, where the cluster Dolidze 25 is located.The determination of
cluster distance, reddening and age is carried out through comparison with
ZAMS, post-MS and PMS isochrones. Reference lines for metallicity Z=0.004 are
used, in agreement with spectroscopic metallicity determination published for
several cluster members. The results are: E(B-V)=0.78+-0.02, M=12.8+-0.2,
LogAge(yr)=6.51+-0.07. A PMS member sequence is proposed, coeval within the
errors with the post-MS cluster age (LogAge(yr)=6.7+-0.2). The mass function
for this PMS population in the mass range above 1.3-3.5 Msun is well fitted by
a Salpeter mass function.The presence of a different star generation in the
cluster with a distinctly older age, around 40 Myr, is suggested. The NIR
results indicate a large number of sources with H-Ks excess, practically
distinct from the optical PMS candidate members.
The distance determined for the cluster is distinctly lower than previously
published values. This result originates in the consistent use of low
metallicity models for ZAMS fitting, applying published metallicity values for
the cluster.Comment: gzipped tar file: 1 aa.cls, 1 tex file, 1 bbl file, 13 figures.
Accepted in Astronomy & Astrophysics (9.11.2009
Analysis of Three-Dimensional Protein Images
A fundamental goal of research in molecular biology is to understand protein
structure. Protein crystallography is currently the most successful method for
determining the three-dimensional (3D) conformation of a protein, yet it
remains labor intensive and relies on an expert's ability to derive and
evaluate a protein scene model. In this paper, the problem of protein structure
determination is formulated as an exercise in scene analysis. A computational
methodology is presented in which a 3D image of a protein is segmented into a
graph of critical points. Bayesian and certainty factor approaches are
described and used to analyze critical point graphs and identify meaningful
substructures, such as alpha-helices and beta-sheets. Results of applying the
methodologies to protein images at low and medium resolution are reported. The
research is related to approaches to representation, segmentation and
classification in vision, as well as to top-down approaches to protein
structure prediction.Comment: See http://www.jair.org/ for any accompanying file
Higher Order Corrections at Zero Recoil
The general structure of the corrections at zero recoil is studied. The
relevant matrix elements are forward matrix elements of local higher
dimensional operators and their time ordered products with higher order terms
from the Lagrangian. These matrix elements may be classified in a simple way
and the analysis at the non recoil point for the form factor of heavy quark
currents simplifies drastically. The second order recoil corrections to the
form factor of the axial vector current, relevant for the
determination from decays, are estimated to be .Comment: LaTeX, 25 pages, one figure, appended after \end{document} as
uu-encoded and compressed eps file, uses epsf, CERN-TH.7162/9
From CFT's to Graphs
In this paper, we pursue the discussion of the connections between rational
conformal field theories (CFT) and graphs. We generalize our recent work on the
relations of operator product algebra (OPA) structure constants of
theories with the Pasquier algebra attached to the graph. We show that in a
variety of CFT built on -- typically conformal embeddings and
orbifolds, similar considerations enable one to write a linear system satisfied
by the matrix elements of the Pasquier algebra in terms of conformal data --
quantum dimensions and fusion coefficients. In some cases, this provides a
sufficient information for the determination of all the eigenvectors of an
adjacency matrix, and hence of a graph.Comment: 44 pages, 6 postscript figures, the whole uuencoded. TEX file, macros
used : harvmac.tex , epsf.tex. Optionally, AMS fonts in amssym.def and
amssym.te
Analytic Approach to Perturbative QCD
The two-loop invariant (running) coupling of QCD is written in terms of the
Lambert W function. The analyticity structure of the coupling in the complex
Q^2-plane is established. The corresponding analytic coupling is reconstructed
via a dispersion relation. We also consider some other approximations to the
QCD beta-function, when the corresponding couplings are solved in terms of the
Lambert function. The Landau gauge gluon propagator has been considered in the
renormalization group invariant analytic approach (IAA). It is shown that there
is a nonperturbative ambiguity in determination of the anomalous dimension
function of the gluon field. Several analytic solutions for the propagator at
the one-loop order are constructed. Properties of the obtained analytical
solutions are discussed.Comment: Latex-file, 19 pages, 2 tables, 51 references, to be published in
Int. J. Mod. Phys.
An integrated software system for geometric correction of LANDSAT MSS imagery
A system for geometrically correcting LANDSAT MSS imagery includes all phases of processing, from receiving a raw computer compatible tape (CCT) to the generation of a corrected CCT (or UTM mosaic). The system comprises modules for: (1) control of the processing flow; (2) calculation of satellite ephemeris and attitude parameters, (3) generation of uncorrected files from raw CCT data; (4) creation, management and maintenance of a ground control point library; (5) determination of the image correction equations, using attitude and ephemeris parameters and existing ground control points; (6) generation of corrected LANDSAT file, using the equations determined beforehand; (7) union of LANDSAT scenes to produce and UTM mosaic; and (8) generation of output tape, in super-structure format
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