96 research outputs found
Virtual-pion and two-photon production in pp scattering
Two-photon production in pp scattering is proposed as a means of studying
virtual-pion emission. Such a process is complementary to real-pion emission in
pp scattering. The virtual-pion signal is embedded in a background of
double-photon bremsstrahlung. We have developed a model to describe this
background process and show that in certain parts of phase space the
virtual-pion signal gives significant contribution. In addition, through
interference with the two-photon bremsstrahlung background, one can determine
the relative phase of the virtual-pion process
Compton scattering on the proton and light nuclei in the Delta-resonance region
Microscopic calculations of Compton scattering on the free proton and light nuclei are presented. For the description of Compton scattering on the proton the conventional K-matrix approach and the "Dressed K-Matrix" model are introduced. The latter approach can be used to calculate polarizabilities as well as Compton scattering for photon energies upto 1 GeV since it obeys the symmetry properties which are appropriate in the different energy regions. In particular, crossing symmetry, gauge invariance and unitarity are satisfied. The extent of violation of analyticity (causality) is used as an expansion parameter. Coherent Compton scattering on light nuclei at 200-300 MeV is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the Delta-resonance. Modifications of the properties of the Delta-resonance due to the nuclear medium are accounted for through the self-energy operator of the Delta. The dominant medium effects such as the Pauli blocking effects in the decay width, effective nucleon mass and particle-hole excitations in the pion propagator axe consistently included in nuclear matter
Consistent off-shell vertex and nucleon self-energy
We present a consistent calculation of half-off-shell form factors in the
pion-nucleon vertex and the nucleon self-energy. Numerical results are
presented. Near the on-shell point the pion-nucleon vertex is dominated by the
pseudovector coupling, while at large nucleon invariant masses we find a
sizable pseudoscalar admixture.Comment: 23 pages, 7 figures, REVTeX, submitted to Phys. Rev. C, replaced with
corrected versio
Polarization Phenomena in Small-Angle Photoproduction of e+e- Pairs and the Gerasimov-Drell-Hearn Sum Rule
Photoproduction of pairs at small angles is investigated as a tool
to determine the functions and entering the real-photon forward
Compton scattering amplitude. The method is based on an interference of the
Bethe-Heitler and the virtual Compton scattering mechanisms, generating an
azimuthal asymmetry in the versus yield. The general case of a
circularly polarized beam and a longitudinally polarized target allows one to
determine both the real and imaginary parts of as well as . The
imaginary part of requires target polarization only. We calculate cross
sections and asymmetries of the reaction , estimate
corrections and backgrounds, and propose suitable kinematical regions to
perform the experiment. Our investigation shows that photoproduction of
-pairs off the proton and light nuclei may serve as a rather sensitive
test of the validity of the Gerasimov-Drell-Hearn sum rule.Comment: 22 pages; revtex; 5 postscript figures included in submission;
submitted to Phys. Rev.
Operator interpretation of resonances generated by some operator matrices
We consider the analytic continuation of the transfer function for a 2x2
matrix Hamiltonian into the unphysical sheets of the energy Riemann surface. We
construct a family of non-selfadjoint operators which reproduce certain parts
of the transfer-function spectrum including resonances situated on the
unphysical sheets neighboring the physical sheet. On this basis, completeness
and basis properties for the root vectors of the transfer function (including
those for the resonances) are proved.Comment: LaTeX, 15 pages, no figures; Contribution to Proceedings of the Mark
Krein International Conference on Operator Theory and Applications, Odessa,
August 18-22, 199
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
Field theory of nucleon to higher-spin baryon transitions
We discuss the nucleon to higher-spin - and -resonance transitions
by pions and photons. The higher-spin baryons are described by Rarita-Schwinger
fields and, as we argue, this imposes a stringent consistency requirement on
the form of the couplings. Popular and couplings
are inconsistent from this point of view. We construct examples of consistent
interactions with the same nonrelativistic limit as the conventional ones.Comment: 5 pages, Revtex, 1 PostScript figure; published versio
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