1,682 research outputs found
Numerical integration of one-loop Feynman diagrams for N-photon amplitudes
In the calculation of cross sections for infrared-safe observables in high
energy collisions at next-to-leading order, one approach is to perform all of
the integrations, including the virtual loop integration numerically. One would
use a subtraction scheme that removes infrared and collinear divergences from
the integrand in a style similar to that used for real emission graphs. Then
one would perform the loop integration by Monte Carlo integration along with
the integrations over final state momenta. In this paper, we have explored how
one can perform the numerical integration. We have studied the N-photon
scattering amplitude with a massless electron loop in order to have a case with
a singular integrand that is not, however, so singular as to require the
subtractions. We report results for N = 4, N = 5 with left-handed couplings,
and N=6.Comment: 30 pages including 5 figures. This is a revised version that is close
to the published versio
Scattering of Dirac and Majorana Fermions off Domain Walls
We investigate the interaction of fermions having both Dirac and left-handed
and right-handed Majorana mass terms with vacuum domain walls. By solving the
equations of motion in thin-wall approximation, we calculate the reflection and
transmission coefficients for the scattering of fermions off walls.Comment: 6 pages, 1 figure, some typos corrected, one reference added, major
revisions, title changed, version accepted for publication in Phys. Rev.
Massless Three Dimensional Quantum Electrodynamics and Thirring Model Constrained by Large Flavor Number
We explicitly prove that in three dimensional massless quantum
electrodynamics at finite temperature, zero density and large number of flavors
the number of infrared degrees of freedom is never larger than the
corresponding number of ultraviolet. Such a result, strongly dependent on the
asymptotic freedom of the theory, is reversed in three dimensional Thirring
model due to the positive derivative of its running coupling constant
EFFECTS OF SHADOWING IN DOUBLE POMERON EXCHANGE PROCESSES
The effects of shadowing in double Pomeron exchange processes are
investigated within an eikonal approach with a Gaussian input. Damping factors
due to screening are calculated for this process and compared with the factors
obtained for total, elastic and single diffraction cross sections. Our main
conclusion is that counting rate calculations, of various double Pomeron
exchange processes (without screening corrections) such as heavy quark and
Higgs production are reduced by a factor of 5 in the LHC energy range, when
screening corrections are applied.Comment: 9 pages, latex, 5 figures obtainable from author
On the controversy concerning the definition of quark and gluon angular momentum
A major controversy has arisen in QCD as to how to split the total angular
momentum into separate quark and gluon contributions, and as to whether the
gluon angular momentum can itself be split, in a gauge invariant way, into a
spin and orbital part. Several authors have proposed various answers to these
questions and offered a variety of different expressions for the relevant
operators. I argue that none of these is acceptable and suggest that the
canonical expression for the momentum and angular momentum operators is the
correct and physically meaningful one. It is then an inescapable fact that the
gluon angular momentum operator cannot, in general, be split in a gauge
invariant way into a spin and orbital part. However, the projection of the
gluon spin onto its direction of motion i.e. its helicity is gauge invariant
and is measured in deep inelastic scattering on nucleons. The Ji sum rule,
relating the quark angular momentum to generalized parton distributions, though
not based on the canonical operators, is shown to be correct, if interpreted
with due care. I also draw attention to several interesting aspects of QED and
QCD, which, to the best of my knowledge, are not commented upon in the standard
textbooks on Field Theory.Comment: 41 pages; Some incorrect statements have been rectified and a
detailed discussion has been added concerning the momentum carried by quarks
and the Ji sum rule for the angular momentu
The Lamb shift contribution of very light millicharged particles
The leading order vacuum polarization contribution of very light millicharged
fermions and scalar (spin-0) particles with charge \epsilon e and mass \mu to
the Lamb shift of the hydrogen atom is shown to imply universal, i.e.
\mu-independent, upper bounds on \epsilon: \epsilon \lsim 10^{-4} for \mu \lsim
1 keV in the case of fermions, and for scalars this bound is increased by a
factor of 2. This is in contrast to expectations based on the commonly used
approximation to the Uehling potential relevant only for conventionally large
fermion (and scalar) masses.Comment: 10 pages including 3 figures, version to appear in Physical Review D
(Rapid Communications
Heavy-Light Wavefunctions in Lattice QCD
Using a multistate smearing method, Coulomb gauge wave functions of
heavy-light mesons are studied in lattice QCD. Wave functions for the ground
state, the first radially excited S-wave state, and the lowest P-wave states of
a heavy-light meson are calculated in quenched approximation. The results are
found to be in remarkably good agreement with the predictions of a simple
relativistic quark model.Comment: Latex file inputs psfig.tex, and espcrc2.sty, 3 figures in uuencoded
fil
The generalized Fenyes-Nelson model for free scalar field theory
The generalized Fenyes--Nelson model of quantum mechanics is applied to the
free scalar field. The resulting Markov field is equivalent to the Euclidean
Markov field with the times scaled by a common factor which depends on the
diffusion parameter. This result is consistent between Guerra's earlier work on
stochastic quantization of scalar fields. It suggests a deep connection between
Euclidean field theory and the stochastic interpretation of quantum mechanics.
The question of Lorentz covariance is also discussed.Comment: 6 page
Chiral Anomalies via Classical and Quantum Functional Methods
In the quantum path integral formulation of a field theory model an anomaly
arises when the functional measure is not invariant under a symmetry
transformation of the Lagrangian. In this paper, generalizing previous work
done on the point particle, we show that even at the classical level we can
give a path integral formulation for any field theory model. Since classical
mechanics cannot be affected by anomalies, the measure of the classical path
integral of a field theory must be invariant under the symmetry. The classical
path integral measure contains the fields of the quantum one plus some extra
auxiliary ones. So, at the classical level, there must be a sort of
"cancellation" of the quantum anomaly between the original fields and the
auxiliary ones. In this paper we prove in detail how this occurs for the chiral
anomaly.Comment: 26 pages, Latex, misprint fixed, a dedication include
Decoherence in QED at finite temperature
We consider a wave packet of a charged particle passing through a cavity
filled with photons at temperature T and investigate its localization and
interference properties. It is shown that the wave packet becomes localized and
the interference disappears with an exponential speed after a sufficiently long
path through the cavity.Comment: Latex, 10 page
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