213 research outputs found
Gauge-invariant theory of pion photoproduction with dressed hadrons
Based on an effective field theory of hadrons in which quantum chromodynamics
is assumed to provide the necessary bare cutoff functions, a gauge-invariant
theory of pion photoproduction with fully dressed nucleons is developed. The
formalism provides consistent dynamical descriptions of pi-N --> pi-N
scattering and Gamma-N --> pi-N production mechanisms in terms of nonlinear
integral equations for fully dressed hadrons. Defining electromagnetic currents
via the gauging of hadronic n-point Green's functions, dynamically detailed
currents for dressed nucleons are introduced. The dressed hadron currents and
the pion photoproduction current are explicitly shown to satisfy gauge
invariance in a self-consistent manner. Approximations are discussed that make
the nonlinear formalism manageable in practice and yet preserve gauge
invariance. This is achieved by recasting the gauge conditions for all
contributing interaction currents as continuity equations with ``surface''
terms for the individual particle legs coming into or going out of the hadronic
interaction region. General procedures are given that approximate any type of
(global) interaction current in a gauge-invariance preserving manner as a sum
of single-particle ``surface'' currents. It is argued that these prescriptions
carry over to other reactions, irrespective of the number or type of
contributing hadrons or hadronic systems.Comment: 33 pages, RevTeX; includes 8 postscript figures (requires psfig.sty).
This version corrects some minor errors, etc.; contains updated references.
Accepted for publication in Phys. Rev. C56 (Oct. 97
Transverse QCD Dynamics Near the Light Cone
Starting from the QCD Hamiltonian in near-light cone coordinates, we study
the dynamics of the gluonic zero modes. Euclidean 2+1 dimensional lattice
simulations show that the gap at strong coupling vanishes at intermediate
coupling. This result opens the possibility to synchronize the continuum limit
with the approach to the light cone.Comment: 15 pages, LaTeX, 3 figures (7 PS files
A Gauge Invariant Unitary Theory for Pion Photoproduction
A covariant, unitary and gauge invariant theory for pion photoproduction on a
single nucleon is presented. To achieve gauge invariance at the operator level
one needs to include both the and thresholds. The final
amplitude can be written in terms of a distorted wave in the final
channel provided one includes additional diagrams to the standard Born term in
which the photon is coupled to the final state pion and nucleon. These
additional diagrams are required in order to satisfy gauge invariance.Comment: 4 pages, LaTeX, 1 figure as a separate uuencoded compressed tar fil
Electromagnetic form factors of the bound nucleon
We calculate electromagnetic form factors of the proton bound in specified
orbits for several closed shell nuclei. The quark structure of the nucleon and
the shell structure of the finite nuclei are given by the QMC model. We find
that orbital electromagnetic form factors of the bound nucleon deviate
significantly from those of the free nucleon.Comment: 12 pages including 4 ps figure
Field transformations and simple models illustrating the impossibility of measuring off-shell effects
In the context of simple models illustrating field transformations in
Lagrangian field theories we discuss the impossibility of measuring off-shell
effects in nucleon-nucleon bremsstrahlung, Compton scattering, and related
processes. To that end we introduce a simple phenomenological Lagrangian
describing nucleon-nucleon bremsstrahlung and perform an appropriate change of
variables leading to different off-shell behavior in the nucleon-nucleon
amplitude as well as the photon-nucleon vertex. As a result we obtain a class
of equivalent Lagrangians, generating identical S-matrix elements, of which the
original Lagrangian is but one representative. We make use of this property in
order to show that what appears as an off-shell effect in an S-matrix element
for one Lagrangian may originate in a contact term from an equivalent
Lagrangian. By explicit calculation we demonstrate for the case of
nucleon-nucleon bremsstrahlung as well as nucleon Compton scattering the
equivalence of observables from which we conclude that off-shell effects cannot
in any unambiguous way be extracted from an S-matrix element. Finally, we also
discuss some implications of introducing off-shell effects on a
phenomenological basis, resulting from the requirement that the description of
one process be consistent with that of other processes described by the same
Lagrangian.Comment: 19 pages, Latex, using RevTe
Relativistic Effects in the Electromagnetic Current at GeV Energies
We employ a recent approach to the non-relativistic reduction of the
electromagnetic current operator in calculations of electronuclear reactions.
In contrast to the traditional scheme, where approximations are made for the
transferred momentum, transferred energy and initial momentum of the struck
nucleon in obtaining an on-shell inspired form for the current, we treat the
problem exactly for the transferred energy and transferred momentum. We
calculate response functions for the reaction at CEBAF (TJNAF)
energies and find large relativistic corrections. We also show that in Plane
Wave Impulse Approximation, it is always possible to use the full operator, and
we present a comparison of such a limiting case with the results incorporating
relativistic effects to the first order in the initial momentum of the struck
nucleon.Comment: 31 pages, 8 figures, Revte
Effect of maternal Schistosoma mansoni infection and praziquantel treatment during pregnancy on Schistosoma mansoni infection and immune responsiveness among offspring at age five years.
INTRODUCTION: Offspring of Schistosoma mansoni-infected women in schistosomiasis-endemic areas may be sensitised in-utero. This may influence their immune responsiveness to schistosome infection and schistosomiasis-associated morbidity. Effects of praziquantel treatment of S. mansoni during pregnancy on risk of S. mansoni infection among offspring, and on their immune responsiveness when they become exposed to S. mansoni, are unknown. Here we examined effects of praziquantel treatment of S. mansoni during pregnancy on prevalence of S. mansoni and immune responsiveness among offspring at age five years. METHODS: In a trial in Uganda (ISRCTN32849447, http://www.controlled-trials.com/ISRCTN32849447/elliott), offspring of women treated with praziquantel or placebo during pregnancy were examined for S. mansoni infection and for cytokine and antibody responses to SWA and SEA, as well as for T cell expression of FoxP3, at age five years. RESULTS: Of the 1343 children examined, 32 (2.4%) had S. mansoni infection at age five years based on a single stool sample. Infection prevalence did not differ between children of treated or untreated mothers. Cytokine (IFNγ, IL-5, IL-10 and IL-13) and antibody (IgG1, Ig4 and IgE) responses to SWA and SEA, and FoxP3 expression, were higher among infected than uninfected children. Praziquantel treatment of S. mansoni during pregnancy had no effect on immune responses, with the exception of IL-10 responses to SWA, which was higher in offspring of women that received praziquantel during pregnancy than those who did not. CONCLUSION: We found no evidence that maternal S. mansoni infection and its treatment during pregnancy influence prevalence and intensity of S. mansoni infection or effector immune response to S. mansoni infection among offspring at age five years, but the observed effects on IL-10 responses to SWA suggest that maternal S. mansoni and its treatment during pregnancy may affect immunoregulatory responsiveness in childhood schistosomiasis. This might have implications for pathogenesis of the disease
A calibration method for broad-bandwidth cavity enhanced absorption spectroscopy performed with supercontinuum radiation
An efficient calibration method has been developed for broad-bandwidth cavity enhanced absorption spectroscopy. The calibration is performed using phase shift cavity ring-down spectroscopy, which is conveniently implemented through use of an acousto-optic tunable filter (AOTF). The AOTF permits a narrowband portion of the SC spectrum to be scanned over the full high-reflectivity bandwidth of the cavity mirrors. After calibration the AOTF is switched off and broad-bandwidth CEAS can be performed with the same light source without any loss of alignment to the set-up. We demonstrate the merits of the method by probing transitions of oxygen molecules O-2 and collisional pairs of oxygen molecules (O-2)(2) in the visible spectral range
Virtual Compton scattering off the nucleon at low energies
We investigate the low-energy behavior of the four-point Green's function
describing virtual Compton scattering off the nucleon.
Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive
the leading terms of an expansion of the operator in the four-momenta and
of the initial and final photon, respectively.
The model-independent result is expressed in terms of the electromagnetic
form factors of the free nucleon, i.e., on-shell information which one obtains
from electron-nucleon scattering experiments.
Model-dependent terms appear in the operator at ,
whereas the orders and are
contained in the low-energy theorem for , i.e., no new
parameters appear.
We discuss the leading terms of the matrix element and comment on the use of
on-shell equivalent electromagnetic vertices in the calculation of ``Born
terms'' for virtual Compton scattering.Comment: 26 pages, RevTex, to appear in Phys. Rev.
Baryon magnetic moments in the effective quark Lagrangian approach
An effective quark Lagrangian is derived from first principles through
bilocal gluon field correlators. It is used to write down equations for
baryons, containing both perturbative and nonperturbative fields. As a result
one obtains magnetic moments of octet and decuplet baryons without introduction
of constituent quark masses and using only string tension as an input. Magnetic
moments come out on average in reasonable agreement with experiment, except for
nucleons and . The predictions for the proton and neutron are shown
to be in close agreement with the empirical values once we choose the string
tension such to yield the proper nucleon mass. Pionic corrections to the
nucleon magnetic moments have been estimated. In particular, the total result
of the two-body current contributions are found to be small. Inclusion of the
anomalous magnetic moment contributions from pion and kaon loops leads to an
improvement of the predictions.Comment: 24 pages Revte
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