421 research outputs found
Gravitational Scattering in the ADD-model at High and Low Energies
Gravitational scattering in the ADD-model is considered at both sub- and
transplanckian energies using a common formalism. By keeping a physical cut-off
in the KK tower associated with virtual KK exchange, such as the cut-off
implied from a finite brane width, troublesome divergences are removed from the
calculations in both energy ranges. The scattering behavior depends on three
different energy scales: the fundamental Planck mass, the collision energy and
the inverse brane width. The result for energies low compared to the effective
cut-off (inverse brane width) is a contact-like interaction. At high energies
the gravitational scattering associated with the extra dimensional version of
Newton's law is recovered
Orthogonal multiplet bases in SU(Nc) color space
We develop a general recipe for constructing orthogonal bases for the
calculation of color structures appearing in QCD for any number of partons and
arbitrary Nc. The bases are constructed using hermitian gluon projectors onto
irreducible subspaces invariant under SU(Nc). Thus, each basis vector is
associated with an irreducible representation of SU(Nc). The resulting
multiplet bases are not only orthogonal, but also minimal for finite Nc. As a
consequence, for calculations involving many colored particles, the number of
basis vectors is reduced significantly compared to standard approaches
employing overcomplete bases. We exemplify the method by constructing multiplet
bases for all processes involving a total of 6 external colored partons.Comment: 50 pages, 2 figure
Decomposing color structure into multiplet bases
We illustrate how QCD color structure elegantly can be decomposed into
orthogonal multiplet bases corresponding to irreducible representations of
SU(Nc) with the aid of Wigner 3j and 6j coefficients. We also show how to
calculate the relevant 3j and 6j coefficients using multiplet bases and
birdtrack techniques and argue that only a relatively small number of Wigner 3j
and 6j coefficients are required. For up to six gluons plus quark-antiquark
pairs we explicitly calculate all 6j coefficients required for up to NLO
calculations.Comment: Version published in JHE
Tools for calculations in color space
Both the higher energy and the initial state colored partons contribute to
making exact calculations in QCD color space more important at the LHC than at
its predecessors. This is applicable whether the method of assessing QCD is
fixed order calculation, resummation, or parton showers. In this talk we
discuss tools for tackling the problem of performing exact color summed
calculations. We start with theoretical tools in the form of the (standard)
trace bases and the orthogonal multiplet bases (for which a general method of
construction was recently presented). Following this, we focus on two new
packages for performing color structure calculations: one easy to use
Mathematica package, ColorMath, and one C++ package, ColorFull, which is
suitable for more demanding calculations, and for interfacing with event
generators.Comment: 7 pages, to appear in the proceedings of the XXI International
Workshop on Deep-Inelastic Scattering and Related Subjects (DIS2013), 22-26
April 2013, Marseilles, Franc
ColorFull -- a C++ library for calculations in SU(Nc) color space
ColorFull, a C++ package for treating QCD color structure, is presented.
ColorFull, which utilizes the trace basis approach, is intended for interfacing
with event generators, but can also be used as a stand-alone package for
squaring QCD amplitudes, calculating interferences, and describing the effect
of gluon emission and gluon exchange.Comment: Version accepted by EPJC, improved description of trace bases in
section
Multi-windowed defocused speckle photographic system for tilt measurement
Defocused speckle photography has long been used to measure rotations of
rough surfaces. This paper explains how, by adding a suitably perforated mask,
some measurement properties, such as range or lateral resolution, may be
changed at wish. Particularly, the maximum measurable tilt can be significantly
raised, although at the expense of poorer lateral resolution. Advantages over
previously described techniques include independent tuning of speckle size and
optical system aperture and more extended adaptability to different measuring
needs. The benefits and disadvantages of the new and old techniques are
thoroughly compared.Comment: 14 pages, 4 figures, submitted to Appl. Optic
Using the resistivity method for leakage detection in a blind test at the Rossvatn embankment dam test facility in Norway
Internal erosion is a cause of embankment dam failure, thus it is important to develop methods for seepage monitoring and internal erosion detection. In order to evaluate the potential of resistivity monitoring to give an early warning of such leakage/erosion, a test was undertaken on a rockfill embankment dam with a central glacial till core at the Rossvatn test facility in Norway. Three defects, consisting of permeable material, were built into the dam at various depths and locations unknown to the personnel carrying out the measurements and data interpretation. A numerical modelling pre-study was carried out, showing that all the actually constructed defects were too small to be detected by single time investigation. In the final test, repeated measurements were undertaken with different reservoir levels, i.e. a limited monitoring approach. This increased the detection capability, confirming the value of the geophysical approach and that monitoring is superior to single time investigations
Color matrix element corrections for parton showers
We investigate the effects of keeping the full color structure for parton
emissions in parton showers for both LEP and LHC. This is done within the
Herwig 7 dipole shower, and includes gluon emission, gluon splitting, initial
state branching processes, as well as hadronization. The subleading Nc terms
are included as color matrix element corrections to the splitting kernels by
evolving an amplitude-level density operator and correcting the radiation
pattern for each parton multiplicity, up to a fixed number of full color
emissions, after which a standard leading color shower takes over. Our results
are compared to data for a wide range of LEP and LHC observables and show that
the subleading Nc corrections tend to be small for most observables probing
hard, perturbative dynamics, for both LEP and LHC. However, for some of these
observables they exceed 10%. On soft physics we find signs of significantly
larger effects.Comment: 25 pages, 10 figure
The chirality-flow formalism
We take a fresh look at Feynman diagrams in the spinor-helicity formalism.
Focusing on tree-level massless QED and QCD, we develop a new and conceptually
simple graphical method for their calculation. In this pictorial method, which
we dub the chirality-flow formalism, Feynman diagrams are directly represented
in terms of chirality-flow lines corresponding to spinor inner products,
without the need to resort to intermediate algebraic manipulations.Comment: 44 pages, 3 figure
Resistivity monitoring for leakage and internal erosion detection at Hallby embankment dam
Internal erosion is one of the major reasons for embankment dam failures. Despite this, the knowledge of the temporal development of internal erosion in full scale structures is limited. Detection of internal erosion is complicated using conventional methods, and new or improved methods are appreciated. Hallby was the first Swedish embankment to get a permanently installed monitoring system intended for resistivity measurements. Daily measurements started to take place already in late 1996, which make these long term monitoring data unique. This paper includes examples of long term time series from Hallby along with some evaluation and interpretation techniques used when analysing such monitoring data. Time-lapse inversion was used to focus the variation over time and suppress artefacts due to the resistivity structure. Seasonal resistivity variations inside the dam are obvious. Increasing long term resistivity has been noticed in a particular zone in the left embankment. This zone also exhibits larger seasonal variations relative to other parts of the dam, and the variations are increasing. The observations may relate to an ongoing internal erosion process in the dam. The long term change may be indicative of a change in material properties, and the large and increasing variations may reflect higher and increasing seepage levels. In the years 2004 to 2006 the dam was upgraded and the resistivity system reinstalled. The results indicate that resistivity monitoring may have a chance of detecting development of internal erosion at an early stage. (C) 2008 Published by Elsevier B.V
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