23 research outputs found
Nuclear dependence of the coherent eta photoproduction reaction in a relativistic approach
We study the nuclear (or A) dependence of the coherent eta photoproduction
reaction in a relativistic impulse approximation approach. We use a standard
relativistic parameterization of the elementary amplitude, based on a set of
four Lorentz- and gauge-invariant amplitudes, to calculate the coherent
production cross section from He-4, C-12, and Ca-40. In contrast to
nonrelativistic treatments, our approach maintains the full relativistic
structure of the process. The nuclear structure affects the process through the
ground-state tensor density. This density is sensitive to relativistic effects
and depends on A in a different manner than the vector density used in
nonrelativistic approaches. This peculiar dependence results in He-4 having a
cross section significantly smaller than that of C-12---in contrast to existent
nonrelativistic calculations. Distortion effects are incorporated through an
eta-nucleus optical potential that is computed in a simple ``t rho''
approximation.Comment: 4 pages, 3 postscript figures. The replace is due to a misspelling in
the Authors' lis
Lessons to be learned from the coherent photoproduction of pseudoscalar mesons
We study the coherent photoproduction of pseudoscalar mesons---particularly
of neutral pions---placing special emphasis on the various sources that put
into question earlier nonrelativistic-impulse-approximation calculations. These
include: final-state interactions, relativistic effects, off-shell ambiguities,
and violations to the impulse approximation. We establish that, while
distortions play an essential role in the modification of the coherent cross
section, the uncertainty in our results due to the various choices of
optical-potential models is relatively small (of at most 30%). By far the
largest uncertainty emerges from the ambiguity in extending the many
on-shell-equivalent representations of the elementary amplitude off the mass
shell. Indeed, relativistic impulse-approximation calculations that include the
same pionic distortions, the same nuclear-structure model, and two sets of
elementary amplitudes that are identical on-shell, lead to variations in the
magnitude of the coherent cross section by up to factors of five. Finally, we
address qualitatively the assumption of locality implicit in most
impulse-approximation treatments, and suggest that the coherent reaction
probes---in addition to the nuclear density---the polarization structure of the
nucleus.Comment: Manuscript is 27 pages long and includes 11 eps figure
Coherent -photoproduction on He and C in the near-threshold region
Coherent meson photoproduction on He and C is considered in
the near-threshold region. The elementary photoproduction operator
includes contributions from the and resonances as
well as -channel vector meson exchange and the nucleon pole terms. Due to
the suppression of the dominant resonance for spin and isospin
saturated nuclei, the reaction is mainly governed by exchange.
Furthermore, the influence of Fermi motion and of different prescriptions for
the choice of the invariant reaction energy in the elementary
amplitude is studied.Comment: 27 pages revtex including 9 postscript figure
Coherent \pi^\circ photoproduction from ^4He
Differential cross sections and beam asymmetries for coherent \pi^\circ
photoproduction from ^4He in the \Delta energy-range have been measured with
high statistical and systematic precisions using both decay photons for
identifying the process.The experiment was performed at the MAinz MIcrotron
using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon
facility. The differential cross sections are in excellent agreement with
predictions based on the DWIA if an appropriate parametrization of the
\Delta-nuclear interaction is applied. The beam asymmetries are interpreted in
terms of degrees of linear polarization of collimated coherent bremsstrahlung.
The expected increase of the degree of linear polarization with decreasing
collimation angle is confirmed. Agreement with calculations is obtained on a
few-percent level of precision in the maxima of the coherent peaks.Comment: 22 pages, 6 figure