178 research outputs found
Fixed points and vacuum energy of dynamically broken gauge theories
We show that if a gauge theory with dynamical symmetry breaking has
non-trivial fixed points, they will correspond to extrema of the vacuum energy.
This relationship provides a different method to determine fixed points.Comment: 17 pages, uuencoded latex file, 3 figures, uses epsf and epsfig.
Submitted to Mod. Phys. Lett.
Chiral Symmetry Breaking with the Curtis-Pennington Vertex
We study chiral symmetry breaking in quenched QED, using a vertex Ansatz
recently proposed by Curtis and Pennington. Bifurcation analysis is employed to
establish the existence of a critical coupling and to estimate its value. The
main results are in qualitative agreement with the ladder approximation, the
numerical changes being minor.Comment: 19 pages, LaTe
Flavor-Dependence and Higher Orders of Gauge-Independent Solutions in Strong Coupling Gauge Theory
The fermion flavor dependence of non-perturbative solutions in the
strong coupling phase of the gauge theory is reexamined based on the
interrelation between the inversion method and the Schwinger-Dyson equation
approach. Especially we point out that the apparent discrepancy on the value of
the critical coupling in QED will be resolved by taking into account the higher
order corrections which inevitably lead to the flavor-dependence. In the
quenched QED, we conclude that the gauge-independent critical point
obtained by the inversion method to the lowest order will be
reduced to the result of the Schwinger-Dyson equation in the
infinite order limit, but its convergence is quite slow. This is shown by
adding the chiral-invariant four-fermion interaction.Comment: CHIBA-EP-72, 13 pages (including 1 Table), LaTex fil
Patient-specific bronchoscope simulation with pq-space-based 2D/3D registration
Objective: The use of patient-specific models for surgical simulation requires photorealistic rendering of 3D structure and surface properties. For bronchoscope simulation, this requires augmenting virtual bronchoscope views generated from 3D tomographic data with patient-specific bronchoscope videos. To facilitate matching of video images to the geometry extracted from 3D tomographic data, this paper presents a new pq-space-based 2D/3D registration method for camera pose estimation in bronchoscope tracking.
Methods: The proposed technique involves the extraction of surface normals for each pixel of the video images by using a linear local shape-from-shading algorithm derived from the unique camera/lighting constraints of the endoscopes. The resultant pq-vectors are then matched to those of the 3D model by differentiation of the z-buffer. A similarity measure based on angular deviations of the pq-vectors is used to provide a robust 2D/3D registration framework. Localization of tissue deformation is considered by assessing the temporal variation of the pq-vectors between subsequent frames.
Results: The accuracy of the proposed method was assessed by using an electromagnetic tracker and a specially constructed airway phantom. Preliminary in vivo validation of the proposed method was performed on a matched patient bronchoscope video sequence and 3D CT data. Comparison to existing intensity-based techniques was also made.
Conclusion: The proposed method does not involve explicit feature extraction and is relatively immune to illumination changes. The temporal variation of the pq distribution also permits the identification of localized deformation, which offers an effective way of excluding such areas from the registration process
Regularization-independent study of renormalized non-perturbative quenched QED
A recently proposed regularization-independent method is used for the first
time to solve the renormalized fermion Schwinger-Dyson equation numerically in
quenched QED. The Curtis-Pennington vertex is used to illustrate the
technique and to facilitate comparison with previous calculations which used
the alternative regularization schemes of modified ultraviolet cut-off and
dimensional regularization. Our new results are in excellent numerical
agreement with these, and so we can now conclude with confidence that there is
no residual regularization dependence in these results. Moreover, from a
computational point of view the regularization independent method has enormous
advantages, since all integrals are absolutely convergent by construction, and
so do not mix small and arbitrarily large momentum scales. We analytically
predict power law behaviour in the asymptotic region, which is confirmed
numerically with high precision. The successful demonstration of this efficient
new technique opens the way for studies of unquenched QED to be undertaken in
the near future.Comment: 20 pages,5 figure
Chiral Symmetry Breaking in Quenched Massive Strong-Coupling QED
We present results from a study of subtractive renormalization of the fermion
propagator Dyson-Schwinger equation (DSE) in massive strong-coupling quenched
QED. Results are compared for three different fermion-photon proper vertex
{\it Ans\"{a}tze\/}: bare , minimal Ball-Chiu, and
Curtis-Pennington. The procedure is straightforward to implement and
numerically stable. This is the first study in which this technique is used and
it should prove useful in future DSE studies, whenever renormalization is
required in numerical work.Comment: REVTEX 3.0, 15 pages plus 7 uuencoded PostScript figure
On the formation of a Hawking-radiation photosphere around microscopic black holes
We show that once a black hole surpasses some critical temperature
, the emitted Hawking radiation interacts with itself and forms a
nearly thermal photosphere. Using QED, we show that the dominant interactions
are bremsstrahlung and electron-photon pair production, and we estimate
, which when calculated more precisely is
found to be 45 GeV. The formation of the photosphere is
purely a particle physics effect, and not a general relativistic effect, since
the the photosphere forms roughly Schwarzschild radii away from
the black hole. The temperature of the photosphere decreases with distance
from the black hole, and the outer surface is determined by the constraint
(for the QED case), since this is the point at which electrons
and positrons annihilate, and the remaining photons free stream to infinity.
Observational consequences are discussed, and it is found that, although the
QED photosphere will not affect the Page-Hawking limits on primordial black
holes, which is most important for 100MeV black holes, the inclusion of QCD
interactions may significantly effect this limit, since for QCD we estimate
. The photosphere greatly reduces possibility of
observing individual black holes with temperatures greater than ,
since the high energy particles emitted from the black hole are processed
through the photosphere to a lower energy, where the gamma ray background is
much higher. The temperature of the plasma in the photosphere can be extremely
high, and this offers interesting possibilities for processes such as symmetry
restoration.Comment: Latex, 16 pages, 3 postscript figures, submitted to PRD. Also
available at http://fnas08.fnal.gov
Solving Uncalibrated Photometric Stereo using Total Variation
International audienceEstimating the shape and appearance of an object, given one or several images, is still an open and challenging research problem called 3D-reconstruction. Among the different techniques available, photometric stereo (PS) produces highly accurate results when the lighting conditions have been identified. When these conditions are unknown, the problem becomes the so-called uncalibrated PS problem, which is ill-posed. In this paper, we will show how total variation can be used to reduce the ambiguities of uncalibrated PS, and we will study two methods for estimating the parameters of the generalized bas-relief ambiguity. These methods will be evaluated through the 3D-reconstruction of real-world objects
Dyson-Schwinger Equations and the Application to Hadronic Physics
We review the current status of nonperturbative studies of gauge field theory
using the Dyson-Schwinger equation formalism and its application to hadronic
physics. We begin with an introduction to the formalism and a discussion of
renormalisation in this approach. We then review the current status of studies
of Abelian gauge theories [e.g., strong coupling quantum electrodynamics]
before turning our attention to the non-Abelian gauge theory of the strong
interaction, quantum chromodynamics. We discuss confinement, dynamical chiral
symmetry breaking and the application and contribution of these techniques to
our understanding of the strong interactions.Comment: 110 pages, LaTeX. Replaced only to facilitate retrieval. Also
available at /u/ftp/pub/Review.uu via anonymnous-ft
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