96 research outputs found
Inelastic nucleon contributions in nuclear response functions
We estimate the contribution of inelastic nucleon excitations to the
inclusive cross section in the CEBAF kinematic range.
Calculations are based upon parameterizations of the nucleon structure
functions measured at SLAC. Nuclear binding effects are included in a
vector-scalar field theory, and are assumed have a minimal effect on the
nucleon excitation spectrum. We find that for q\lsim 1 GeV the elastic and
inelastic nucleon contributions to the nuclear response functions are
comparable, and can be separated, but with roughly a factor of two uncertainty
in the latter from the extrapolation from data. In contrast, for q\rsim 2 GeV
this uncertainty is greatly reduced but the elastic nucleon contribution is
heavily dominated by the inelastic nucleon background.Comment: 20 pages, 7 figures available from the authors at Department of
Physics and Astronomy, University of Rochester, Rochester NY 1462
Two-body Pion Absorption on at Threshold
It is shown that a satisfactory explanation of the ratio of the rates of the
reactions and for stopped pions is obtained
once the effect of the short range two-nucleon components of the axial charge
operator for the nuclear system is taken into account. By employing realistic
models for the nucleon-nucleon interaction in the construction of these
components of the axial charge operator, the predicted ratios agree with the
empirical value to within 10-20\%.Comment: 19, UHPHYDOR-94-
Role of heavy-meson exchange in pion production near threshold
Recent calculations of -wave pion production have severely underestimated
the accurately known \ total cross section near
threshold. In these calculations, only the single-nucleon axial-charge operator
is considered. We have calculated, in addition to the one-body term, the
two-body contributions to this reaction that arise from the exchange of mesons.
We find that the inclusion of the scalar -meson exchange current (and
lesser contributions from other mesons) increases the cross section by about a
factor of five, and leads to excellent agreement with the data. The results are
neither very sensitive to changes in the distorting potential that generates
the wave function, nor to different choices for the meson-nucleon form
factors. We argue that \ data provide direct
experimental evidence for meson-exchange contributions to the axial current.Comment: 28 Pages, IU-NTC #93-0
Dependence of pp->pp pi0 near Threshold on the Spin of the Colliding Nucleons
A polarized internal atomic hydrogen target and a stored, polarized beam are
used to measure the spin-dependent total cross section Delta_sigma_T/sigma_tot,
as well as the polar integrals of the spin correlation coefficient combination
A_xx-A_yy, and the analyzing power A_y for pp-> pp pi0 at four bombarding
energies between 325 and 400 MeV. This experiment is made possible by the use
of a cooled beam in a storage ring. The polarization observables are used to
study the contribution from individual partial waves.Comment: 6 pages, 1 table, 4 figures, corrected equations 2 and
Pion Excess, Nuclear Correlations, and the Interpretation of () Spin Transfer Experiments
Conventional theories of nuclear interactions predict a net increase in the
distribution of virtual pions in nuclei relative to free nucleons. Analysis of
data from several nuclear experiments has led to claims of evidence against
such a pion excess. These conclusions are usually based on a collective theory
(RPA) of the pions, which may be inadequate. The issue is the energy dependence
of the nuclear response, which differs for theories with strong NN correlations
from the RPA predictions. In the present paper, information about the energy
dependence is extracted from sum rules, which are calculated for such a
correlated, noncollective nuclear theory. The results lead to much reduced
sensitivity of nuclear reactions to the correlations that are responsible for
the pion excess. The primary example is spin transfer, for
which the expected effects are found to be smaller than the experimental
uncertainties. The analysis has consequences for Deep Inelastic Scattering
(DIS) experiments as well.Comment: 16 pages, LaTeX, no figures, submitted to Phys. Rev.
Dynamical coupled channel calculation of pion and omega meson production
A dynamical coupled channel approach is used to study and
--meson production induced by pions and photons scattering from the
proton. Six intermediate channels including , , ,
, and are employed to describe unpolarized and
polarized data. Bare parameters in an effective hadronic Lagrangian are
determined in a fit to data for , , , and reactions at center-of-mass
energies from threshold to GeV. The matrix determined in these
fits is used to calculate the photon beam asymmetry for -meson
production and the total cross section and
scattering lengths.Comment: 26 pages, 18 figures, submitted to Phys. Rev. C; corrected typos,
added references, minor revisions, same results/fig
Chiral Perturbation Theory and the pp -> pp pi0 Reaction Near Threshold
A chiral-perturbative consideration of the near-threshold pp -> pp pi0
reaction indicates that the pion-rescattering term has a substantial energy and
momentum dependence. The existing calculations that incorporate this dependence
give pion rescattering contributions significantly larger than those of the
conventional treatment, and this enhanced rescattering term interferes
destructively with the one-body impulse term, leading to theoretical cross
sections that are much smaller than the observed values. However, since the
existing calculations are based on coordinate-space representation, they
involve a number of simplifying assumptions about the energy-momentum flow in
the rescattering diagram, even though the delicate interplay between the
one-body and two-body terms makes it desirable to avoid these kinematical
assumptions. We carry out here a momentum-space calculation that retains the
energy-momentum dependence of the vertices as predicted by chiral perturbation
theory. Our improved treatment increases the rescattering amplitude by a factor
of 3 over the value obtained in the r-space calculations. The pp -> pp pi0
transition amplitude, which is now dominated by the rescattering term, leads to
the cross section much larger than what was reported in the approximate r-space
calculations. Thus, the extremely small cross sections obtained in the previous
chiral perturbative treatments of this reaction should be considered as an
accidental consequence of the approximations employed rather than a general
feature.Comment: 25 pages,REVTEX, 5 ps figure
Spin correlations in pion production near threshold
A first measurement of longitudinal as well as transverse spin correlation
coefficients for the reaction was made using a
polarized proton target and a polarized proton beam. We report kinematically
complete measurements for this reaction at 325, 350, 375 and 400 MeV beam
energy. The spin correlation coefficients and the analyzing power as well as angular
distributions for and the polarization observables
were extracted. Partial wave cross sections for dominant
transition channels were obtained from a partial wave analysis that included
the transitions with final state angular momenta of . The measurements
of the polarization observables are compared
with the predictions from the J\"ulich meson exchange model. The agreement is
very good at 325 MeV, but it deteriorates increasingly for the higher energies.
At all energies agreement with the model is better than for the reaction
.Comment: Preprint, 21 pp, submitted to Phys. Rev. C. Keywords: Mesons,
Polarization, Spin Correlations, Few body system
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