4,336 research outputs found
OPE for null Wilson loops and open spin chains
Maximal helicity-violating scattering amplitudes in N=4 supersymmetric
Yang-Mills theory are dual to Wilson loops on closed null polygons. We perform
their operator product expansion analysis in two-dimensional kinematics in the
soft-collinear approximation which corresponds to the case when some light-cone
distances vanish. We construct the expansion in terms of multi-particle
"heavy"-light operators, where the "heavy" fields are identified with the
Wilson lines defining the OPE channel and the light fields emerge from the
curvature of the contour. The correlation function of these define the
remainder function. We study the dilatation operator for these operators at one
loop order and find that it corresponds to a non-compact open spin chain. This
provides an alternative view on elementary excitations propagating on the GKP
string at weak coupling, which now correspond to particles traveling along an
open spin chain. The factorized structure of the Wilson loop in the soft limit
allows one to represent the two-loop correction to the octagon Wilson loop as a
convolution formula and find the corresponding remainder function.Comment: 10 pages, 2 figure
Multi-frequency study of DEM L299 in the Large Magellanic Cloud
We have studied the HII region DEM L299 in the Large Magellanic Cloud to
understand its physical characteristics and morphology in different
wavelengths. We performed a spectral analysis of archived XMM-Newton EPIC data
and studied the morphology of DEM L299 in X-ray, optical, and radio
wavelengths. We used H alpha, [SII], and [OIII] data from the Magellanic Cloud
Emission Line Survey and radio 21 cm line data from the Australia Telescope
Compact Array (ATCA) and the Parkes telescope, and radio continuum data from
ATCA and the Molonglo Synthesis Telescope. Our morphological studies imply
that, in addition to the supernova remnant SNR B0543-68.9 reported in previous
studies, a superbubble also overlaps the SNR in projection. The position of the
SNR is clearly defined through the [SII]/H alpha flux ratio image. Moreover,
the optical images show a shell-like structure that is located farther to the
north and is filled with diffuse X-ray emission, which again indicates the
superbubble. Radio 21 cm line data show a shell around both objects. Radio
continuum data show diffuse emission at the position of DEM L299, which appears
clearly distinguished from the HII region N 164 that lies south-west of it. We
determined the spectral index of SNR B0543-68.9 to be alpha=-0.34, which
indicates the dominance of thermal emission and therefore a rather mature SNR.
We determined the basic properties of the diffuse X-ray emission for the SNR,
the superbubble, and a possible blowout region of the bubble, as suggested by
the optical and X-ray data. We obtained an age of 8.9 (3.5-18.1) kyr for the
SNR and a temperature of 0.64 (0.44-1.37) keV for the hot gas inside the SNR,
and a temperature of the hot gas inside the superbubble of 0.74 (0.44-1.1) keV.
We conclude that DEM L299 consists of a superposition of SNR B0543-68.9 and a
superbubble, which we identified based on optical data.Comment: Accepted for publication in Astronomy and Astrophysics. 17 pages, 16
figure
Velocity Level Approximation of Pressure Field Contact Patches
Pressure Field Contact (PFC) was recently introduced as a method for detailed
modeling of contact interface regions at rates much faster than
elasticity-theory models, while at the same time predicting essential trends
and capturing rich contact behavior. The PFC model was designed to work in
conjunction with error-controlled integration at the acceleration level.
Therefore a vast majority of existent multibody codes using solvers at the
velocity level cannot incorporate PFC in its original form. In this work we
introduce a discrete in time approximation of PFC making it suitable for use
with existent velocity-level time steppers and enabling execution at real-time
rates. We evaluate the accuracy and performance gains of our approach and
demonstrate its effectiveness in simulating relevant manipulation tasks. The
method is available in open source as part of Drake's Hydroelastic Contact
model.Comment: 8 pages, 10 figures. Supplementary video can be found at
https://youtu.be/AdCnTyqqQP
Comments on Correlation Functions of Large Spin Operators and Null Polygonal Wilson Loops
We discuss the relation between correlation functions of twist-two large spin
operators and expectation values of Wilson loops along light-like trajectories.
After presenting some heuristic field theoretical arguments suggesting this
relation, we compute the divergent part of the correlator in the limit of large
't Hooft coupling and large spins, using a semi-classical worldsheet which
asymptotically looks like a GKP rotating string. We show this diverges as
expected from the expectation value of a null Wilson loop, namely, as
, being a cut-off of the theory.Comment: 16+3 pages, 3 figures. Computation in section 4.1 has been clarified.
Some comments on the vertex contributions has been added in section 4.2. Some
other minor corrections. Version to appear in Nucl. Phys.
Rigorous Probabilistic Analysis of Equilibrium Crystal Shapes
The rigorous microscopic theory of equilibrium crystal shapes has made
enormous progress during the last decade. We review here the main results which
have been obtained, both in two and higher dimensions. In particular, we
describe how the phenomenological Wulff and Winterbottom constructions can be
derived from the microscopic description provided by the equilibrium
statistical mechanics of lattice gases. We focus on the main conceptual issues
and describe the central ideas of the existing approaches.Comment: To appear in the March 2000 special issue of Journal of Mathematical
Physics on Probabilistic Methods in Statistical Physic
Isosurface modelling of soft objects in computer graphics.
There are many different modelling techniques used in computer graphics to describe a wide range of objects and phenomena. In this thesis, details of research into the isosurface modelling technique are presented. The
isosurface technique is used in conjunction with more traditional modelling techniques to describe the objects needed in the different scenes of an animation. The isosurface modelling technique allows the description
and animation of objects that would be extremely difficult, or impossible to describe using other methods. The objects suitable for description using isosurface modelling are soft objects. Soft objects merge elegantly with each
other, pull apart, bubble, ripple and exhibit a variety of other effects. The representation was studied in three phases of a computer animation project: modelling of the objects; animation of the objects; and the production of the images. The research clarifies and presents many
algorithms needed to implement the isosurface representation in an animation system. The creation of a hierarchical computer graphics animation system
implementing the isosurface representation is described. The scalar fields defining the isosurfaces are represented using a scalar field description language, created as part of this research, which is automatically generated from the hierarchical description of the scene. This language has many techniques for combining and building the scalar field from a variety of components. Surface attributes of the objects are specified within the graphics system. Techniques are described which allow the handling of
these attributes along with the scalar field calculation. Many animation techniques specific to the isosurface representation are presented. By the conclusion of the research, a graphics system was created which elegantly handles the isosurface representation in a wide variety of
animation situations. This thesis establishes that isosurface modelling of soft objects is a powerful and useful technique which has wide application in the computer graphics community
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