519 research outputs found
Perturbative quantum gauge invariance: Where the ghosts come from
A condensed introduction to quantum gauge theories is given in the
perturbative S-matrix framework; path integral methods are used nowhere. This
approach emphasizes the fact that it is not necessary to start from classical
gauge theories which are then subject to quantization, but it is also possible
to recover the classical group structure and coupling properties from purely
quantum mechanical principles. As a main tool we use a free field version of
the Becchi-Rouet-Stora-Tyutin gauge transformation, which contains no
interaction terms related to a coupling constant. This free gauge
transformation can be formulated in an analogous way for quantum
electrodynamics, Yang-Mills theories with massless or massive gauge bosons and
quantum gravity.Comment: 28 pages, LATEX. Some typos corrected, version to be publishe
Causal construction of the massless vertex diagram
The massless one-loop vertex diagram is constructed by exploiting the causal
structure of the diagram in configuration space, which can be translated
directly into dispersive relations in momentum space.Comment: 14 pages, LATEX with style file, corresponds to published versio
Regularization in quantum field theory from the causal point of view
The causal approach to perturbative quantum field theory is presented in
detail, which goes back to a seminal work by Henri Epstein and Vladimir Jurko
Glaser in 1973. Causal perturbation theory is a mathematically rigorous
approach to renormalization theory, which makes it possible to put the
theoretical setup of perturbative quantum field theory on a sound mathematical
basis. Epstein and Glaser solved this problem for a special class of
distributions, the time-ordered products, that fulfill a causality condition,
which itself is a basic requirement in axiomatic quantum field theory. In their
original work, Epstein and Glaser studied only theories involving scalar
particles. In this review, the extension of the method to theories with higher
spin, including gravity, is presented. Furthermore, specific examples are
presented in order to highlight the technical differences between the causal
method and other regularization methods, like, e.g. dimensional regularization.Comment: 75 pages, 8 figures, style file included, some comments and
references adde
Holographic entropy bound from gravitational Fock space truncation
A simplified derivation of Yurtsever's result, which states that the entropy
of a truncated bosonic Fock space is given by a holographic bound when the
energy of the Fock states is constrained gravitationally, is given for
asymptotically flat spacetimes with arbitrary dimension d greater or equal to
four. For this purpose, a scalar field confined to a spherical volume in
d-dimensional spacetime is considered. Imposing an upper bound on the total
energy of the corresponding Fock states which ensures that the system is in a
stable configuration against gravitational collapse and imposing a cutoff on
the maximum energy of the field modes of the order of the Planck energy leads
to an entropy bound of holographic type. A simple derivation of the entropy
bound is also given for the fermionic case.Comment: 5 pages, Latex (incl. style file), minor typos correcte
Electron-positron pair production in the external electromagnetic field of colliding relativistic heavy ions
The results concerning the production in peripheral highly
relativistic heavy-ion collisions presented in a recent paper by Baltz {\em{et
al.}} are rederived in a very straightforward manner. It is shown that the
solution of the Dirac equation directly leads to the multiplicity, i.e. to the
total number of electron-positron pairs produced by the electromagnetic field
of the ions, whereas the calculation of the single pair production probability
is much more involved. A critical observation concerns the unsolved problem of
seemingly absent Coulomb corrections (Bethe-Maximon corrections) in pair
production cross sections. It is shown that neither the inclusion of the
vacuum-vacuum amplitude nor the correct interpretation of the solution of the
Dirac equation concerning the pair multiplicity is able the explain (from a
fundamental point of view) the absence of Coulomb corrections. Therefore the
contradiction has to be accounted to the treatment of the high energy limit.Comment: 6 pages, 4 Postscript figures, uses svjour.cls/svepj.cl
Gauge Independence of the S-Matrix in the Causal Approach
The gauge dependence of the time-ordered products for Yang-Mills theories is
analysed in perturbation theory by means of the causal method of Epstein and
Glaser together with perturbative gauge invariance. This approach allows a
simple inductive proof of the gauge independence of the physical S-matrix.Comment: 19 pages, latex, 1 figur
kGamma distributions in granular packs
It has been recently pointed out that local volume fluctuations in granular
packings follow remarkably well a shifted and rescaled Gamma distribution named
the kGamma distribution [T. Aste, T. Di Matteo, Phys. Rev. E 77 (2008) 021309].
In this paper we confirm, extend and discuss this finding by supporting it with
additional experimental and simulation data.Comment: 10 pages, 5 figure
Entropy Bound with Generalized Uncertainty Principle in General Dimensions
In this letter, the entropy bound for local quantum field theories (LQFT) is
studies in a class of models of the generalized uncertainty principle(GUP)
which predicts a minimal length as a reflection of the quantum gravity effects.
Both bosonic and fermionic fields confined in arbitrary spatial dimension
ball are investigated. It is found that the GUP leads
to the same scaling correction to the entropy bound for
bosons and fermions, although the coefficients of this correction are different
for each case. Based on our calculation, we conclude that the GUP effects can
become manifest at the short distance scale. Some further implications and
speculations of our results are also discussed.Comment: 8 pages, topos corrected and references adde
Resummation of mass terms in perturbative massless quantum field theory
The neutral massless scalar quantum field in four-dimensional
space-time is considered, which is subject to a simple bilinear
self-interaction. Is is well-known from renormalization theory that adding a
term of the form to the Lagrangean has the formal
effect of shifting the particle mass from the original zero value to m after
resummation of all two-leg insertions in the Feynman graphs appearing in the
perturbative expansion of the S-matrix. However, this resummation is
accompanied by some subtleties if done in a proper mathematical manner.
Although the model seems to be almost trivial, is shows many interesting
features which are useful for the understanding of the convergence behavior of
perturbation theory in general. Some important facts in connection with the
basic principles of quantum field theory and distribution theory are
highlighted, and a remark is made on possible generalizations of the
distribution spaces used in local quantum field theory. A short discussion how
one can view the spontaneous breakdown of gauge symmetry in massive gauge
theories within a massless framework is presented.Comment: 15 pages, LaTeX (style files included), one section adde
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